+Group: WHATWG +Status: RD +Date: 2025-12-15 +H1: Fetch +Shortname: fetch +Text Macro: TWITTER fetchstandard +Text Macro: LATESTRD 2025-12 +Abstract: The Fetch standard defines requests, responses, and the process that binds them: fetching. +Translation: ja https://triple-underscore.github.io/Fetch-ja.html +Translation: zh-Hans https://htmlspecs.com/fetch/ +Translation: ko https://ko.htmlspecs.com/fetch/ +Markup Shorthands: css off +Translate IDs: typedefdef-bodyinit bodyinit,dictdef-requestinit requestinit,typedefdef-requestinfo requestinfo,enumdef-requestdestination requestdestination,enumdef-requestmode requestmode,enumdef-requestcredentials requestcredentials,enumdef-requestcache requestcache,enumdef-requestredirect requestredirect,dictdef-responseinit responseinit,enumdef-responsetype responsetype ++ +
+urlPrefix:https://httpwg.org/specs/rfc5861.html#;type:dfn;spec:stale-while-revalidate + url:n-the-stale-while-revalidate-cache-control-extension;text:stale-while-revalidate lifetime + +urlPrefix:https://httpwg.org/specs/rfc9651.html#;type:dfn;spec:rfc9651 + url:rfc.section.2;text:structured field value + url:text-serialize;text:serializing structured fields + url:text-parse;text:parsing structured fields + url:;text:structured header + url:token;text:structured field token + +urlPrefix:https://httpwg.org/specs/rfc9110.html#;type:dfn;spec:http + url:method.overview;text:method + url:fields.names;text:field-name + url:fields.values;text:field-value + url:rfc.section.9.2.1;text:unsafe + +urlPrefix:https://httpwg.org/specs/rfc9111.html#;type:dfn;spec:http-caching + url:delta-seconds;text:delta-seconds + url:age.calculations;text:current age + url:calculating.freshness.lifetime;text:freshness lifetime + url:response.cacheability;text:Storing Responses in Caches + url:invalidation;text:Invalidating Stored Responses + url:validation.sent;text:Sending a Validation Request + url:constructing.responses.from.caches;text:Constructing Responses from Caches + url:freshening.responses;text:Freshening Stored Responses upon Validation + +urlPrefix:https://httpwg.org/specs/rfc9112.html#;type:dfn;spec:http1 + url:status.line;text:reason-phrase + +url:https://w3c.github.io/resource-timing/#dfn-mark-resource-timing;text:mark resource timing;type:dfn;spec:resource-timing +url:https://w3c.github.io/webappsec-permissions-policy/#algo-define-inherited-policy-in-container;text:define an inherited policy for feature in container;type:dfn + +urlPrefix:https://w3c.github.io/hr-time/#;spec:hr-time + type:dfn + url:dfn-coarsen-time;text:coarsen time + url:dfn-coarsened-shared-current-time;text:coarsened shared current time + url:dfn-unsafe-shared-current-time;text:unsafe shared current time + type:typedef;url:dom-domhighrestimestamp;text:DOMHighResTimeStamp + +urlPrefix:https://tc39.es/ecma262/#;type:dfn;spec:ecma-262 + url:realm;text:realm + url:sec-list-and-record-specification-type;text:Record + url:current-realm;text:current realm + +urlPrefix:https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-layered-cookies#;type:dfn;spec:cookies + url:name-cookie-default-path;text:cookie default path + url:name-cookie-store-and-limits;text:cookie store + url:name-parse-and-store-a-cookie;text:parse and store a cookie + url:name-parse-a-cookie;text:parse a cookie + url:name-store-a-cookie;text:store a cookie + url:name-retrieve-cookies;text:retrieve cookies + url:name-serialize-cookies;text:serialize cookies + url:name-garbage-collect-cookies;text:garbage collect cookies ++ +
+{
+ "COOKIES": {
+ "authors": ["Johann Hofmann", "Anne van Kesteren"],
+ "href": "https://httpwg.org/http-extensions/draft-ietf-httpbis-layered-cookies.html",
+ "title": "Cookies: HTTP State Management Mechanism"
+ },
+ "HTTP": {
+ "aliasOf": "RFC9110"
+ },
+ "HTTP-CACHING": {
+ "aliasOf": "RFC9111"
+ },
+ "HTTP1": {
+ "aliasOf": "RFC9112"
+ },
+ "HTTP3": {
+ "aliasOf": "RFC9114"
+ },
+ "HTTP3-DATAGRAM": {
+ "aliasOf": "RFC9297"
+ },
+ "REFERRER": {
+ "aliasOf": "referrer-policy"
+ },
+ "STALE-WHILE-REVALIDATE": {
+ "aliasOf": "RFC5861"
+ },
+ "SW": {
+ "aliasOf": "service-workers"
+ },
+ "HSTS": {
+ "aliasOf": "RFC6797"
+ },
+ "HTTPVERBSEC1": {
+ "publisher": "US-CERT",
+ "href": "https://www.kb.cert.org/vuls/id/867593",
+ "title": "Multiple vendors' web servers enable HTTP TRACE method by default."
+ },
+ "HTTPVERBSEC2": {
+ "publisher": "US-CERT",
+ "href": "https://www.kb.cert.org/vuls/id/288308",
+ "title": "Microsoft Internet Information Server (IIS) vulnerable to cross-site scripting via HTTP TRACK method."
+ },
+ "HTTPVERBSEC3": {
+ "publisher": "US-CERT",
+ "href": "https://www.kb.cert.org/vuls/id/150227",
+ "title": "HTTP proxy default configurations allow arbitrary TCP connections."
+ },
+ "WEBTRANSPORT-HTTP3": {
+ "authors": ["V. Vasiliev"],
+ "href": "https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3",
+ "publisher": "IETF",
+ "title": "WebTransport over HTTP/3"
+ },
+ "SVCB": {
+ "aliasOf": "RFC9460"
+ }
+}
+
+
++spec:dom; type:dfn; text:element +spec:infra; type:dfn; text:implementation-defined ++ + + +
The goal is to unify fetching across the web platform and provide consistent handling of +everything that involves, including: + +
Referer` [[!REFERRER]]
+To do so it also supersedes the HTTP `Origin` header semantics
+originally defined in The Web Origin Concept. [[ORIGIN]]
+
+
+
+
At a high level, fetching a resource is a fairly simple operation. A request goes in, a +response comes out. The details of that operation are +however quite involved and used to not be written down carefully and differ from one API +to the next. + +
Numerous APIs provide the ability to fetch a resource, e.g. HTML's img and
+script element, CSS' cursor and list-style-image,
+the navigator.sendBeacon() and self.importScripts() JavaScript
+APIs. The Fetch Standard provides a unified architecture for these features so they are
+all consistent when it comes to various aspects of fetching, such as redirects and the
+CORS protocol.
+
+
The Fetch Standard also defines the fetch() JavaScript API, which
+exposes most of the networking functionality at a fairly low level of abstraction.
+
+
+
+
This specification depends on the Infra Standard. [[!INFRA]] + +
This specification uses terminology from ABNF, Encoding, +HTML, HTTP, MIME Sniffing, Streams, +URL, Web IDL, WebSockets, and WebTransport. +[[!ABNF]] +[[!ENCODING]] +[[!HTML]] +[[!HTTP]] +[[!MIMESNIFF]] +[[!STREAMS]] +[[!URL]] +[[!WEBIDL]] +[[!WEBSOCKETS]] +[[!WEBTRANSPORT]] + +
ABNF means ABNF as augmented by HTTP (in particular the addition of #)
+and RFC 7405. [[!RFC7405]]
+
+
Credentials are HTTP cookies, TLS client certificates, and authentication entries (for HTTP authentication). [[!COOKIES]] +[[!TLS]] [[!HTTP]] + +
A fetch params is a struct used as a bookkeeping detail by the +fetch algorithm. It has the following items: + +
pending", or a response.
+A fetch controller is a struct used to enable callers of +fetch to perform certain operations on it after it has started. It has the following +items: + +
ongoing")
+ ongoing", "terminated", or "aborted"
+
+ To report timing for a +fetch controller controller given a global object global: + +
Assert: controller's + report timing steps is non-null. + +
Call controller's report timing steps with + global. +
To process the next manual redirect for a +fetch controller controller: + +
Assert: controller's + next manual redirect steps is non-null. + +
Call controller's next manual redirect steps. +
To +extract full timing info +given a fetch controller controller: + +
Assert: controller's full timing info + is non-null. + +
Return controller's full timing info. +
To abort a fetch controller +controller with an optional error: + +
Set controller's state to "aborted".
+
+
Let fallbackError be an "{{AbortError}}" {{DOMException}}. + +
Set error to fallbackError if it is not given. + +
Let serializedError be [$StructuredSerialize$](error). + If that threw an exception, catch it, and let serializedError be + [$StructuredSerialize$](fallbackError). + +
Set controller's serialized abort reason to + serializedError. +
To deserialize a serialized abort reason, given null or a Record +abortReason and a realm realm: + +
Let fallbackError be an "{{AbortError}}" {{DOMException}}. + +
Let deserializedError be fallbackError. + +
If abortReason is non-null, then set deserializedError to + [$StructuredDeserialize$](abortReason, realm). If that threw an exception or + returned undefined, then set deserializedError to fallbackError. + +
Return deserializedError. +
To terminate a fetch controller
+controller, set controller's state to
+"terminated".
+
A fetch params fetchParams is aborted if
+its controller's state is
+"aborted".
+
+
A fetch params fetchParams is canceled if
+its controller's state is
+"aborted" or "terminated".
+
+
A fetch timing info is a struct used to maintain timing +information needed by Resource Timing and Navigation Timing. It has the +following items: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]] + +
A response body info is a struct used to maintain +information needed by Resource Timing and Navigation Timing. It has the +following items: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]] + +
To +create an opaque timing info, +given a fetch timing info timingInfo, return a new +fetch timing info whose start time and +post-redirect start time are timingInfo's +start time. +
To queue a fetch task, given an algorithm algorithm, a +global object or a parallel queue taskDestination, run these +steps: + +
If taskDestination is a parallel queue, then + enqueue algorithm to + taskDestination. + +
Otherwise, queue a global task on the networking task source with + taskDestination and algorithm. +
To check if the environment settings object environment +is offline: + +
If the user agent assumes it does not have internet connectivity, then return true. + +
Return environment's WebDriver BiDi network is offline. +
To serialize an integer, represent it as a string of the shortest possible decimal +number. + +
This will be replaced by a more descriptive algorithm in Infra. See +infra/201. + + +
A local scheme is "about", "blob", or
+"data".
+
+
A URL is local if its scheme is a +local scheme. + +
This definition is also used by Referrer Policy. [[REFERRER]] + +
An HTTP(S) scheme is "http" or
+"https".
+
+
A fetch scheme is "about", "blob",
+"data", "file", or an HTTP(S) scheme.
+
+
HTTP(S) scheme and fetch scheme are also used by HTML. +[[HTML]] + + +
While fetching encompasses more than just HTTP, it
+borrows a number of concepts from HTTP and applies these to resources obtained via other
+means (e.g., data URLs).
+
+
An HTTP tab or space is U+0009 TAB or U+0020 SPACE. + +
HTTP whitespace is U+000A LF, U+000D CR, or an HTTP tab or space. + +
HTTP whitespace is only useful for specific constructs that are reused outside +the context of HTTP headers (e.g., MIME types). For HTTP header values, using +HTTP tab or space is preferred, and outside that context ASCII whitespace is +preferred. Unlike ASCII whitespace this excludes U+000C FF. + +
An HTTP newline byte is 0x0A (LF) or 0x0D (CR). + +
An HTTP tab or space byte is 0x09 (HT) or 0x20 (SP). + +
An HTTP whitespace byte is an HTTP newline byte or +HTTP tab or space byte. + +
To +collect an HTTP quoted string +from a string input, given a position variable position +and an optional boolean extract-value (default false): + +
Let positionStart be position. + +
Let value be the empty string. + +
Assert: the code point at position within input is + U+0022 ("). + +
Advance position by 1. + +
While true: + +
Append the result of collecting a sequence of code points that are not U+0022 (") + or U+005C (\) from input, given position, to value. + +
If position is past the end of input, then + break. + +
Let quoteOrBackslash be the code point at position within + input. + +
Advance position by 1. + +
If quoteOrBackslash is U+005C (\), then: + +
If position is past the end of input, then append U+005C (\) to + value and break. + +
Append the code point at position within input to + value. + +
Advance position by 1. +
Otherwise: + +
+If extract-value is true, then return value. + +
Return the code points from positionStart to position, + inclusive, within input. +
| Input + | Output + | Output with extract-value set to true + | Final position variable value + |
|---|---|---|---|
""\"
+ | ""\"
+ | "\"
+ | 2 + |
""Hello" World"
+ | ""Hello""
+ | "Hello"
+ | 7 + |
""Hello \\ World\"""
+ | ""Hello \\ World\"""
+ | "Hello \ World""
+ | 18 + |
The position variable always starts at 0 in these examples. +
A method is a byte sequence that matches the +method token production. + +
A CORS-safelisted method is a
+method that is `GET`,
+`HEAD`, or `POST`.
+
+
A forbidden method is a method that is a
+byte-case-insensitive match for `CONNECT`,
+`TRACE`, or `TRACK`.
+[[HTTPVERBSEC1]], [[HTTPVERBSEC2]], [[HTTPVERBSEC3]]
+
+
To normalize a
+method, if it is a byte-case-insensitive
+match for `DELETE`, `GET`,
+`HEAD`, `OPTIONS`, `POST`, or
+`PUT`, byte-uppercase it.
+
+
Normalization is done for backwards compatibility and +consistency across APIs as methods are actually "case-sensitive". + +
Using `patch` is highly likely to result in a
+`405 Method Not Allowed`. `PATCH` is much more likely to
+succeed.
+
+
There are no restrictions on methods. `CHICKEN` is perfectly
+acceptable (and not a misspelling of `CHECKIN`). Other than those that are
+normalized there are no casing restrictions either.
+`Egg` or `eGg` would be fine, though uppercase is encouraged for
+consistency.
+
+
+
HTTP generally refers to a header as a "field" or "header field". The web platform +uses the more colloquial term "header". [[HTTP]] + + +
A header list is a list of zero or more +headers. It is initially « ». + +
A header list is essentially a specialized multimap: an ordered list of
+key-value pairs with potentially duplicate keys. Since headers other than `Set-Cookie`
+are always combined when exposed to client-side JavaScript, implementations could choose a more
+efficient representation, as long as they also support an associated data structure for
+`Set-Cookie` headers.
+
+
To +get a structured field value +given a header name name and a string type from a +header list list, run these steps. They return null or a +structured field value. + +
Assert: type is one of "dictionary",
+ "list", or "item".
+
+
Let value be the result of getting name from + list. + +
If value is null, then return null. + +
Let result be the result of parsing structured fields with + input_string set to value and header_type set to + type. + +
If parsing failed, then return null. + +
Return result. +
Get a structured field value intentionally does not distinguish between a +header not being present and its value failing to parse as a +structured field value. This ensures uniform processing across the web platform. +
To +set a structured field value +given a tuple (header name name, structured field value +structuredValue), in a header list list: + +
Let serializedValue be the result of executing the + serializing structured fields algorithm on structuredValue. + +
Set (name, serializedValue) in + list. +
Structured field values are defined as objects which HTTP can (eventually) +serialize in interesting and efficient ways. For the moment, Fetch only supports +header values as byte sequences, which means that these objects can be set in +header lists only via serialization, and they can be obtained from +header lists only by parsing. In the future the fact that they are objects might be +preserved end-to-end. [[!RFC9651]] +
A header list list +contains a +header name name if list contains a +header whose name is a byte-case-insensitive match for +name. +
To get a header name +name from a header list list, run these steps. They return null +or a header value. + +
If list does not contain name, then return + null. + +
Return the values of all headers in list + whose name is a byte-case-insensitive match for name, separated + from each other by 0x2C 0x20, in order. +
To +get, decode, and split +a header name name from header list list, run these +steps. They return null or a list of strings. + + +
Let value be the result of getting name from + list. + +
If value is null, then return null. + +
Return the result of getting, decoding, and splitting + value. +
This is how get, decode, and split functions in practice with
+ `A` as the name argument:
+
+
| Headers (as on the network) + | Output + |
|---|---|
+ | « "nosniff", "" »
+ |
+ | |
+ | « "" » + |
+ | null + |
+ | « "text/html;", x/x" »
+ |
+ | |
+ | « "x/x;test="hi"", "y/y" »
+ |
+ | |
+ | « "x / x", "", "", "1" »
+ |
+ | |
+ | « ""1,2"", "3" »
+ |
+ |
To +get, decode, and split +a header value value, run these steps. They return a list of +strings. + + +
Let input be the result of isomorphic decoding value. + +
Let position be a position variable for input, + initially pointing at the start of input. + +
Let temporaryValue be the empty string. + +
While true: + +
Append the result of collecting a sequence of code points that are not U+0022 (") or + U+002C (,) from input, given position, to temporaryValue. + +
The result might be the empty string. + +
If position is not past the end of input and the + code point at position within input is U+0022 ("): + +
Append the result of collecting an HTTP quoted string from input, + given position, to temporaryValue. + +
Remove all HTTP tab or space from the start and end of temporaryValue. + +
Append temporaryValue to values. + +
Set temporaryValue to the empty string. + +
If position is past the end of input, then return values. + +
Assert: the code point at position within + input is U+002C (,). + +
Advance position by 1. +
Except for blessed call sites, the algorithm directly above is not to be invoked +directly. Use get, decode, and split instead. +
To append a header +(name, value) to a header list list: + +
+To delete a +header name name from a header list list, +remove all headers whose name is a +byte-case-insensitive match for name from list. +
To set a header +(name, value) in a header list list: + +
+To combine a +header (name, value) in a header list +list: + +
If list contains name, then set the + value of the first such header to its value, + followed by 0x2C 0x20, followed by value. + +
Otherwise, append (name, value) to list. +
Combine is used by {{XMLHttpRequest}} and the +WebSocket protocol handshake. +
To convert header names to a sorted-lowercase set, given a list of +names headerNames, run these steps. They return an +ordered set of header names. + +
Let headerNamesSet be a new ordered set. + +
For each name of headerNames, append + the result of byte-lowercasing name to + headerNamesSet. + +
Return the result of sorting headerNamesSet in ascending order + with byte less than. +
To sort and combine a +header list list, run these steps. They return a header list. + +
Let headers be a header list. + +
Let names be the result of + convert header names to a sorted-lowercase set with all the names + of the headers in list. + +
For each name of names: + +
If name is `set-cookie`, then:
+
+
Otherwise: + +
+Return headers. +
A header is a tuple that consists of a +name (a header name) and +value (a header value). + +
A header name is a byte sequence that matches the +field-name token production. + +
A header value is a byte sequence that matches the following +conditions: + +
Has no leading or trailing HTTP tab or space bytes. +
Contains no 0x00 (NUL) or HTTP newline bytes. +
The definition of header value is not defined in terms of the +field-value token production as it is +not compatible with deployed content. + +
To normalize a +byte sequence potentialValue, remove any leading and trailing +HTTP whitespace bytes from potentialValue. +
To determine whether a header (name, value) +is a CORS-safelisted request-header, run these steps: + +
If value's length is greater than 128, then return + false. + +
Byte-lowercase name and switch on the result: + +
accept`
+ If value contains a CORS-unsafe request-header byte, then return false. + +
accept-language`
+ content-language`
+ If value contains a byte that is not in the range 0x30 (0) to 0x39 (9), + inclusive, is not in the range 0x41 (A) to 0x5A (Z), inclusive, is not in the range 0x61 (a) to + 0x7A (z), inclusive, and is not 0x20 (SP), 0x2A (*), 0x2C (,), 0x2D (-), 0x2E (.), 0x3B (;), or + 0x3D (=), then return false. + + +
content-type`
+ If value contains a CORS-unsafe request-header byte, then return + false. + +
Let mimeType be the result of parsing the + result of isomorphic decoding value. + +
If mimeType is failure, then return false. + +
If mimeType's essence is not
+ "application/x-www-form-urlencoded", "multipart/form-data", or
+ "text/plain", then return false.
+
This intentionally does not use extract a MIME type as that algorithm is + rather forgiving and servers are not expected to implement it. + +
If extract a MIME type were used the following request would not result in a CORS + preflight and a naïve parser on the server might treat the request body as JSON: + +
+fetch("https://victim.example/naïve-endpoint", {
+ method: "POST",
+ headers: [
+ ["Content-Type", "application/json"],
+ ["Content-Type", "text/plain"]
+ ],
+ credentials: "include",
+ body: JSON.stringify(exerciseForTheReader)
+});
+range`
+ Let rangeValue be the result of parsing a single range header value + given value and false. + +
If rangeValue is failure, then return false. + +
If rangeValue[0] is null, then return false. + +
As web browsers have historically not emitted ranges such as
+ `bytes=-500` this algorithm does not safelist them.
+
Return false. +
Return true. +
There are limited exceptions to the `Content-Type` header safelist, as
+documented in CORS protocol exceptions.
+
A CORS-unsafe request-header byte is a byte byte for which one of the +following is true: + +
byte is less than 0x20 and is not 0x09 HT +
byte is 0x22 ("), 0x28 (left parenthesis), 0x29 (right parenthesis), 0x3A (:), + 0x3C (<), 0x3E (>), 0x3F (?), 0x40 (@), 0x5B ([), 0x5C (\), 0x5D (]), 0x7B ({), 0x7D (}), or + 0x7F DEL. + +
The CORS-unsafe request-header names, given a header list +headers, are determined as follows: + +
Let unsafeNames be a new list. + +
Let potentiallyUnsafeNames be a new list. + +
Let safelistValueSize be 0. + +
For each header of headers: + +
+ +If safelistValueSize is greater than 1024, then for each + name of potentiallyUnsafeNames, append name to + unsafeNames. + +
Return the result of convert header names to a sorted-lowercase set with + unsafeNames. +
A CORS non-wildcard request-header name is a header name that is a
+byte-case-insensitive match for `Authorization`.
+
+
A privileged no-CORS request-header name is a header name that is +a byte-case-insensitive match for one of + +
Range`.
+These are headers that can be set by privileged APIs, and will be preserved if their associated + request object is copied, but will be removed if the request is modified by unprivileged APIs. + +
`Range` headers are commonly used by downloads
+ and media fetches.
+
+
A helper is provided to add a range header to a particular request. +
A CORS-safelisted response-header name, given a list of +header names list, is a header name that is a +byte-case-insensitive match for one of + +
Cache-Control`
+ Content-Language`
+ Content-Length`
+ Content-Type`
+ Expires`
+ Last-Modified`
+ Pragma`
+ A no-CORS-safelisted request-header name is a header name that +is a byte-case-insensitive match for one of + +
Accept`
+ Accept-Language`
+ Content-Language`
+ Content-Type`
+To determine whether a header (name, value) is a +no-CORS-safelisted request-header, run these steps: + +
If name is not a no-CORS-safelisted request-header name, then return + false. + +
Return whether (name, value) is a + CORS-safelisted request-header. +
A header (name, value) is +forbidden request-header if these steps return true: + +
If name is a byte-case-insensitive match for one of: + +
Accept-Charset`
+ Accept-Encoding`
+ Access-Control-Request-Headers`
+ Access-Control-Request-Method`
+ Connection`
+ Content-Length`
+ Cookie`
+ Cookie2`
+ Date`
+ DNT`
+ Expect`
+ Host`
+ Keep-Alive`
+ Origin`
+ Referer`
+ Set-Cookie`
+ TE`
+ Trailer`
+ Transfer-Encoding`
+ Upgrade`
+ Via`
+ then return true. + +
If name when byte-lowercased starts with
+ `proxy-` or `sec-`, then return true.
+
+
If name is a byte-case-insensitive match for one of: + +
X-HTTP-Method`
+ X-HTTP-Method-Override`
+ X-Method-Override`
+ then: + +
Let parsedValues be the result of + getting, decoding, and splitting value. + +
For each method of parsedValues: if the + isomorphic encoding of method is a forbidden method, then return true. +
Return false. +
These are forbidden so the user agent remains in full control over them. + +
Header names starting with `Sec-` are reserved to allow new
+ headers to be minted that are safe from APIs using fetch that allow
+ control over headers by developers, such as {{XMLHttpRequest}}. [[XHR]]
+
+
The `Set-Cookie` header is semantically a response header, so it is not useful on
+ requests. Because `Set-Cookie` headers cannot be combined, they require more complex
+ handling in the {{Headers}} object. It is forbidden here to avoid leaking this complexity into
+ requests.
+
A forbidden response-header name is a header name that is a +byte-case-insensitive match for one of: + +
Set-Cookie`
+ Set-Cookie2`
+A request-body-header name is a header name that is a +byte-case-insensitive match for one of: + +
Content-Encoding`
+ Content-Language`
+ Content-Location`
+ Content-Type`
+To extract header values +given a header header, run these steps: + +
+To +extract header list values +given a header name name and a header list list, +run these steps: + +
If list does not contain name, then return + null. + +
If the ABNF for name allows a single header and list + contains more than one, then return failure. + +
If different error handling is needed, extract the desired header + first. + +
Let values be an empty list. + +
For each header header list + contains whose name is name: + +
Let extract be the result of extracting header values from + header. + +
If extract is failure, then return failure. + +
Append each value in extract, in order, to values. +
Return values. +
To build a content range given an integer rangeStart, an integer +rangeEnd, and an integer fullLength, run these steps: + +
Let contentRange be `bytes `.
+
+
Append rangeStart, serialized and + isomorphic encoded, to contentRange. + +
Append 0x2D (-) to contentRange. + +
Append rangeEnd, serialized and + isomorphic encoded to contentRange. + +
Append 0x2F (/) to contentRange. + +
Append fullLength, serialized and + isomorphic encoded to contentRange. + +
Return contentRange. +
To parse a single range header value from a +byte sequence value and a boolean allowWhitespace, run these steps: + +
Let data be the isomorphic decoding of value. + +
If data does not start with "bytes", then return
+ failure.
+
+
Let position be a position variable for data, initially + pointing at the 5th code point of data. + +
If allowWhitespace is true, collect a sequence of code points that are + HTTP tab or space, from data given position. + +
If the code point at position within data is not U+003D (=), + then return failure. + +
Advance position by 1. + +
If allowWhitespace is true, collect a sequence of code points that are + HTTP tab or space, from data given position. + +
Let rangeStart be the result of collecting a sequence of code points that + are ASCII digits, from data given position. + +
Let rangeStartValue be rangeStart, interpreted as decimal number, if + rangeStart is not the empty string; otherwise null. + +
If allowWhitespace is true, collect a sequence of code points that are + HTTP tab or space, from data given position. + +
If the code point at position within data is not U+002D (-), + then return failure. + +
Advance position by 1. + +
If allowWhitespace is true, collect a sequence of code points that are + HTTP tab or space, from data given position. + +
Let rangeEnd be the result of collecting a sequence of code points that + are ASCII digits, from data given position. + +
Let rangeEndValue be rangeEnd, interpreted as decimal number, if + rangeEnd is not the empty string; otherwise null. + +
If position is not past the end of data, then return failure. + +
If rangeEndValue and rangeStartValue are null, then return failure. + +
If rangeStartValue and rangeEndValue are numbers, and + rangeStartValue is greater than rangeEndValue, then return failure. + +
Return (rangeStartValue, rangeEndValue). + +
The range end or start can be omitted, e.g., `bytes=0-` or
+ `bytes=-500` are valid ranges.
+
Parse a single range header value succeeds for a subset of allowed range header +values, but it is the most common form used by user agents when requesting media or resuming +downloads. This format of range header value can be set using add a range header. +
A default `User-Agent` value is an
+implementation-defined header value for the `User-Agent`
+header.
+
+
For unfortunate web compatibility reasons, web browsers are strongly encouraged to
+have this value start with `Mozilla/5.0 (` and be generally modeled after other web
+browsers.
+
+
To get the environment settings object environment's
+environment default `User-Agent` value:
+
+
Let userAgent be the WebDriver BiDi emulated User-Agent for + environment. + +
If userAgent is non-null, then return userAgent, + isomorphic encoded. + +
Return the default `User-Agent` value.
+
The document `Accept` header value is
+`text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8`.
+
+
+
A status is an integer in the range 0 to 999, inclusive. + +
Various edge cases in mapping HTTP/1's status-code to this concept are
+worked on in issue #1156.
+
+
A null body status is a status that is 101, 103, 204, 205, or 304. + +
An ok status is a status in the range 200 to 299, inclusive. + +
A redirect status is a status that is 301, 302, 303, 307, or 308. + + +
A body consists of: + +
A stream (a {{ReadableStream}} object). + +
A source (null, a + byte sequence, a {{Blob}} object, or a {{FormData}} object), initially null. + +
A length (null or an integer), + initially null. +
To clone a +body body, run these steps: + +
+To get a byte sequence bytes +as a body, return the body of the +result of safely extracting bytes. +
To incrementally read a body body, given an +algorithm processBodyChunk, an algorithm processEndOfBody, an algorithm +processBodyError, and an optional null, parallel queue, or +global object taskDestination (default null), run these steps. +processBodyChunk must be an algorithm accepting a byte sequence. +processEndOfBody must be an algorithm accepting no arguments. processBodyError +must be an algorithm accepting an exception. + +
If taskDestination is null, then set taskDestination to the result of + starting a new parallel queue. + +
Let reader be the result of getting a reader for + body's stream. + +
This operation will not throw an exception. + +
Perform the incrementally-read loop given reader, + taskDestination, processBodyChunk, processEndOfBody, and + processBodyError. +
To perform the incrementally-read loop, given a {{ReadableStreamDefaultReader}} object +reader, parallel queue or global object +taskDestination, algorithm processBodyChunk, algorithm +processEndOfBody, and algorithm processBodyError: + +
Let readRequest be the following read request: + +
Let continueAlgorithm be null. + +
If chunk is not a {{Uint8Array}} object, then set + continueAlgorithm to this step: run processBodyError given a + {{TypeError}}. + +
Otherwise: + +
Let bytes be a copy of + chunk. + +
Implementations are strongly encouraged to use an implementation strategy that + avoids this copy where possible. + +
Set continueAlgorithm to these steps: + +
Run processBodyChunk given bytes. + +
Perform the incrementally-read loop given reader, + taskDestination, processBodyChunk, processEndOfBody, and + processBodyError. +
Queue a fetch task given continueAlgorithm and + taskDestination. +
Queue a fetch task given processEndOfBody and + taskDestination.
Queue a fetch task to run processBodyError given e, + with taskDestination.
Read a chunk from reader given + readRequest. +
To fully read a body body, given an algorithm +processBody, an algorithm processBodyError, and an optional null, +parallel queue, or global object taskDestination (default +null), run these steps. processBody must be an algorithm accepting a +byte sequence. processBodyError must be an algorithm optionally accepting an +exception. + +
If taskDestination is null, then set taskDestination to the result of + starting a new parallel queue. + +
Let successSteps given a byte sequence bytes be to + queue a fetch task to run processBody given bytes, with + taskDestination. + +
Let errorSteps optionally given an exception exception be + to queue a fetch task to run processBodyError given exception, with + taskDestination. + +
Let reader be the result of getting a reader for + body's stream. If that threw an exception, then run + errorSteps with that exception and return. + +
Read all bytes from + reader, given successSteps and errorSteps. +
A body with type is a tuple that consists of a +body (a body) and a +type (a header value or null). + +
To handle content codings given codings and bytes, run +these steps: + +
If codings are not supported, then return bytes. + +
Return the result of decoding bytes with codings as explained in HTTP, + if decoding does not result in an error, and failure otherwise. [[!HTTP]] +
This section documents how requests work in detail. To get started, see +[[#fetch-elsewhere-request]]. + +
The input to fetch is a +request. + +
A request has an associated
+method (a
+method). Unless stated otherwise it is
+`GET`.
+
+
This can be updated during redirects to `GET` as described in
+HTTP fetch.
+
+
A request has an associated URL +(a URL). + +
Implementations are encouraged to make this a pointer to the first URL in +request's URL list. It is provided as a distinct field solely for +the convenience of other standards hooking into Fetch. + +
A request has an associated +local-URLs-only flag. Unless stated otherwise it is +unset. + +
A request has an associated +header list (a +header list). Unless stated otherwise it is « ». + +
A request has an associated +unsafe-request flag. Unless stated otherwise it +is unset. + +
The unsafe-request flag is set by APIs such as
+fetch() and {{XMLHttpRequest}} to ensure a CORS-preflight fetch
+is done based on the supplied method and header list. It does
+not free an API from outlawing forbidden methods and forbidden request-headers.
+
+
A request has an associated +body (null, a byte sequence, or a +body). Unless stated otherwise it is null. + +
A byte sequence will be safely extracted into a +body early on in fetch. As part of HTTP fetch it is possible for +this field to be set to null due to certain redirects. + +
A request has an associated +client (null or an +environment settings object). + +
A request has an associated +reserved client +(null, an environment, or an +environment settings object). Unless stated otherwise it is null. + +
This is only used by navigation requests and worker requests, but not service +worker requests. It references an environment for a navigation request and an +environment settings object for a worker request. + +
A request has an associated +replaces client id +(a string). Unless stated otherwise it is the empty string. + +
This is only used by navigation requests. It is the id +of the target browsing context's active document's +environment settings object. + +
A request has an associated
+traversable for user prompts, that is
+"no-traversable", "client", or a traversable navigable.
+Unless stated otherwise it is "client".
+
+
This is used to determine whether and where to show necessary UI for the request, such as + authentication prompts or client certificate dialogs. + +
no-traversable"
+ client"
+ no-traversable" or to a
+ traversable navigable derived from the request's client during
+ fetching. This provides a convenient way for standards to not have to
+ explicitly set a request's traversable for user prompts.
+
+ When displaying a user interface associated with a request in that request's +traversable for user prompts, the user agent should update the address bar to +display something derived from the request's current URL (and not, e.g., leave +it at its previous value, derived from the URL of the request's initiator). Additionally, the user +agent should avoid displaying content from the request's initiator in the +traversable for user prompts, especially in the case of cross-origin requests. +Displaying a blank page behind such prompts is a good way to fulfill these requirements. Failing to +follow these guidelines can confuse users as to which origin is responsible for the prompt. + +
A request has an associated boolean +keepalive. Unless stated otherwise it is +false. + +
This can be used to allow the request to outlive the
+environment settings object, e.g., navigator.sendBeacon() and the HTML
+img element use this. Requests with this set to true are subject to additional
+processing requirements.
+
+
A request has an associated
+initiator type, which is null,
+"audio",
+"beacon",
+"body",
+"css",
+"early-hints",
+"embed",
+"fetch",
+"font",
+"frame",
+"iframe",
+"image",
+"img",
+"input",
+"link",
+"object",
+"ping",
+"script",
+"track",
+"video",
+"xmlhttprequest", or
+"other". Unless stated otherwise it is null. [[RESOURCE-TIMING]]
+
+
+
A request has an associated service-workers mode, that
+is "all" or "none". Unless stated otherwise it is "all".
+
+
This determines which service workers will receive a {{fetch!!event}} event for this fetch. + +
all"
+ none"
+ A request has an associated
+initiator, which is
+the empty string,
+"download",
+"imageset",
+"manifest",
+"prefetch",
+"prerender", or
+"xslt". Unless stated otherwise it is the empty string.
+
+
A request's initiator is not particularly granular for +the time being as other specifications do not require it to be. It is primarily a specification +device to assist defining CSP and Mixed Content. It is not exposed to JavaScript. [[!CSP]] [[!MIX]] + +
A destination type is one of:
+the empty string,
+"audio",
+"audioworklet",
+"document",
+"embed",
+"font",
+"frame",
+"iframe",
+"image",
+"json",
+"manifest",
+"object",
+"paintworklet",
+"report",
+"script",
+"serviceworker",
+"sharedworker",
+"style",
+"track",
+"video",
+"webidentity",
+"worker", or
+"xslt".
+
+
A request has an associated +destination, which is +destination type. Unless stated otherwise it is the empty string. + +
These are reflected on {{RequestDestination}} except for "serviceworker"
+and "webidentity" as fetches with those destinations skip service workers.
+
+
+
A request's destination is
+script-like if it is "audioworklet",
+"paintworklet", "script", "serviceworker",
+"sharedworker", or "worker".
+
+
Algorithms that use script-like should also consider
+"xslt" as that too can cause script execution. It is not included in the list as it is
+not always relevant and might require different behavior.
+
+
The following table illustrates the relationship between a request's + initiator, destination, CSP directives, and features. It is + not exhaustive with respect to features. Features need to have the relevant values defined in their + respective standards. + +
| Initiator + | Destination + | CSP directive + | Features + |
|---|---|---|---|
| "" + | "report"
+ | — + | CSP, NEL reports. + |
"document"
+ | HTML's navigate algorithm (top-level only). + | ||
"frame"
+ | child-src
+ | HTML's <frame>
+ | |
"iframe"
+ | child-src
+ | HTML's <iframe>
+ | |
| "" + | connect-src
+ | navigator.sendBeacon(), {{EventSource}},
+ HTML's <a ping=""> and <area ping="">,
+ fetch(), fetchLater(), {{XMLHttpRequest}},
+ {{WebSocket}}, {{WebTransport}}, Cache API
+ | |
"object"
+ | object-src
+ | HTML's <object>
+ | |
"embed"
+ | object-src
+ | HTML's <embed>
+ | |
"audio"
+ | media-src
+ | HTML's <audio>
+ | |
"font"
+ | font-src
+ | CSS' @font-face
+ | |
"image"
+ | img-src
+ | HTML's <img src>, /favicon.ico resource,
+ SVG's <image>, CSS' background-image, CSS'
+ cursor, CSS' list-style-image, …
+ | |
"audioworklet"
+ | script-src
+ | audioWorklet.addModule()
+ | |
"paintworklet"
+ | script-src
+ | CSS.paintWorklet.addModule()
+ | |
"script"
+ | script-src
+ | HTML's <script>, importScripts()
+ | |
"serviceworker"
+ | child-src, script-src, worker-src
+ | navigator.serviceWorker.register()
+ | |
"sharedworker"
+ | child-src, script-src, worker-src
+ | SharedWorker
+ | |
"webidentity"
+ | connect-src
+ | Federated Credential Management requests
+ | |
"worker"
+ | child-src, script-src, worker-src
+ | Worker
+ | |
"json"
+ | connect-src
+ | import "..." with { type: "json" }
+ | |
"style"
+ | style-src
+ | HTML's <link rel=stylesheet>, CSS' @import, import "..." with { type: "css" }
+ | |
"track"
+ | media-src
+ | HTML's <track>
+ | |
"video"
+ | media-src
+ | HTML's <video> element
+ | |
"download"
+ | "" + | — + | HTML's download="", "Save Link As…" UI
+ |
"imageset"
+ | "image"
+ | img-src
+ | HTML's <img srcset> and <picture>
+ |
"manifest"
+ | "manifest"
+ | manifest-src
+ | HTML's <link rel=manifest>
+ |
"prefetch"
+ | "" + | default-src (no specific directive)
+ | HTML's <link rel=prefetch>
+ |
"prerender"
+ | HTML's <link rel=prerender>
+ | ||
"xslt"
+ | "xslt"
+ | script-src
+ | <?xml-stylesheet>
+ |
CSP's form-action needs to be a hook directly in HTML's navigate or form
+ submission algorithm.
+
+
CSP will also need to check request's client's
+ global object's associated Document's
+ ancestor navigables for various CSP directives.
+
A request has an associated
+priority, which is "high", "low", or
+"auto". Unless stated otherwise it is "auto".
+
+
A request has an associated +internal priority (null or an +implementation-defined object). Unless otherwise stated it is null. + +
A request has an associated
+origin, which is
+"client" or an origin. Unless stated otherwise it is
+"client".
+
+
"client" is changed to an origin during
+fetching. It provides a convenient way for standards to not have to set
+request's origin.
+
+
A request has an associated +top-level navigation initiator origin, which is an origin +or null. Unless stated otherwise it is null. + +
A request has an associated
+policy container, which is
+"client" or a policy container. Unless stated otherwise it is
+"client".
+
+
"client" is changed to a policy container during
+fetching. It provides a convenient way for standards to not have to set
+request's policy container.
+
+
A request has an associated
+referrer, which is
+"no-referrer", "client", or a URL. Unless stated otherwise it
+is "client".
+
+
"client" is changed to "no-referrer" or a URL
+during fetching. It provides a convenient way for standards to not have to set
+request's referrer.
+
+
A request has an associated +referrer policy, which is a +referrer policy. Unless stated otherwise it is the empty string. [[!REFERRER]] + +
This can be used to override the referrer policy to be used for this +request. + +
A request has an associated
+mode, which is
+"same-origin", "cors", "no-cors",
+"navigate", "websocket", or "webtransport".
+Unless stated otherwise, it is "no-cors".
+
+
same-origin"
+ cors"
+ cors", makes
+ the request a CORS request — in which case, fetch will return a network error if the
+ requested resource does not understand the CORS protocol, or if the requested resource is
+ one that intentionally does not participate in the CORS protocol.
+
+ no-cors"
+ navigate"
+ websocket"
+ webtransport"
+ Even though the default request mode is "no-cors",
+ standards are highly discouraged from using it for new features. It is rather unsafe.
+
A request has an associated +use-CORS-preflight flag. Unless stated +otherwise, it is unset. + +
The use-CORS-preflight flag being set is one of several conditions that results +in a CORS-preflight request. The use-CORS-preflight flag is set if either one or more +event listeners are registered on an {{XMLHttpRequestUpload}} object or if a {{ReadableStream}} +object is used in a request. + +
A request has an associated
+credentials mode,
+which is "omit", "same-origin", or
+"include". Unless stated otherwise, it is "same-origin".
+
+
omit"
+ same-origin"
+ include"
+ Request's credentials mode controls the flow of
+ credentials during a fetch. When request's
+ mode is "navigate", its credentials mode is
+ assumed to be "include" and fetch does not currently account for other
+ values. If HTML changes here, this standard will need corresponding changes.
+
A request has an associated +use-URL-credentials flag. +Unless stated otherwise, it is unset. + +
When this flag is set, when a request's +URL has a username and password, and there is an +available authentication entry for the request, then the URL's +credentials are preferred over that of the authentication entry. Modern specifications avoid +setting this flag, since putting credentials in URLs is discouraged, but some older +features set it for compatibility reasons. + +
A request has an associated
+cache mode, which is
+"default", "no-store", "reload",
+"no-cache", "force-cache", or
+"only-if-cached". Unless stated otherwise, it is "default".
+
+
default"
+ no-store"
+ reload"
+ no-cache"
+ force-cache"
+ only-if-cached"
+ same-origin". Any cached redirects will be followed assuming
+ request's
+ redirect mode is "follow" and the
+ redirects do not violate request's
+ mode.)
+ If header list contains
+ `If-Modified-Since`,
+ `If-None-Match`,
+ `If-Unmodified-Since`,
+ `If-Match`, or
+ `If-Range`,
+ fetch will set
+ cache mode to "no-store" if it is
+ "default".
+
A request has an associated
+redirect mode, which is
+"follow", "error", or "manual".
+Unless stated otherwise, it is "follow".
+
+
follow"
+ error"
+ manual"
+ A request has associated +integrity metadata +(a string). Unless stated otherwise, it is the empty string. + +
A request has associated +cryptographic nonce metadata +(a string). Unless stated otherwise, it is the empty string. + +
A request has associated
+parser metadata
+which is the empty string, "parser-inserted", or
+"not-parser-inserted". Unless otherwise stated, it is the empty string.
+
+
A request's cryptographic nonce metadata and +parser metadata are generally populated from attributes and flags on the HTML +element responsible for creating a request. They are used by various algorithms in +Content Security Policy to determine whether requests or responses are to be blocked in +a given context. [[!CSP]] + +
A request has an associated +reload-navigation flag. +Unless stated otherwise, it is unset. + +
This flag is for exclusive use by HTML's navigate algorithm. [[!HTML]] + +
A request has an associated +history-navigation flag. +Unless stated otherwise, it is unset. + +
This flag is for exclusive use by HTML's navigate algorithm. [[!HTML]] + +
A request has an associated boolean user-activation. +Unless stated otherwise, it is false. + +
This is for exclusive use by HTML's navigate algorithm. [[!HTML]] + +
A request has an associated boolean render-blocking. +Unless stated otherwise, it is false. + +
This flag is for exclusive use by HTML's render-blocking mechanism. [[!HTML]] + +
A request has an associated +URL list (a list of one or +more URLs). Unless stated otherwise, it is a list containing a copy of +request's URL. + +
A request has an associated +current URL. It is a pointer to the +last URL in request's URL list. + +
A request has an associated +redirect count. +Unless stated otherwise, it is zero. + +
A request has an associated
+response tainting,
+which is "basic", "cors", or "opaque".
+Unless stated otherwise, it is "basic".
+
+
A request has an associated +prevent no-cache cache-control header modification flag. +Unless stated otherwise, it is unset. + +
A request has an associated done flag. +Unless stated otherwise, it is unset. + +
A request has an associated +timing allow failed flag. Unless stated +otherwise, it is unset. + +
A request's URL list, current URL, +redirect count, response tainting, +done flag, and timing allow failed flag are used as +bookkeeping details by the fetch algorithm. + +
A subresource request is a request
+whose destination is "audio", "audioworklet",
+"font", "image", "json", "manifest",
+"paintworklet", "script", "style", "track",
+"video", "xslt", or the empty string.
+
+
A non-subresource request is a request
+whose destination is "document", "embed",
+"frame", "iframe", "object", "report",
+"serviceworker", "sharedworker", or "worker".
+
+
A navigation request is a request whose
+destination is
+"document", "embed", "frame", "iframe",
+or "object".
+
+
See handle fetch for usage of these terms. +[[!SW]] + +
To compute the redirect-taint of a
+request request, perform the following steps. They return
+"same-origin", "same-site", or "cross-site".
+
+
Let lastURL be null. + +
Let taint be "same-origin".
+
+
For each url of request's URL list: + +
If lastURL is null, then set lastURL to url and + continue. + +
If url's origin is not same site with
+ lastURL's origin and request's origin is
+ not same site with lastURL's origin, then return
+ "cross-site".
+
+
If url's origin is not same origin with
+ lastURL's origin and request's origin is
+ not same origin with lastURL's origin, then set
+ taint to "same-site".
+
+
Set lastURL to url. +
Return taint. +
Serializing a request origin, given a request request, is to +run these steps: + +
If request's redirect-taint is not "same-origin",
+ then return "null".
+
+
Return request's origin, + serialized. +
Byte-serializing a request origin, given a request request, +is to return the result of serializing a request origin with request, +isomorphic encoded. +
To clone a +request request, run these steps: + +
+To add a range header to a +request request, with an integer first, and an optional integer +last, run these steps: + +
Assert: last is not given, or first is less than or equal + to last. + +
Let rangeValue be `bytes=`.
+
+
Serialize and isomorphic encode first, + and append the result to rangeValue. + +
Append 0x2D (-) to rangeValue. + +
If last is given, then serialize and + isomorphic encode it, and append the result to rangeValue. + +
Append (`Range`, rangeValue) to
+ request's header list.
+
A range header denotes an inclusive byte range. There a range header where +first is 0 and last is 500, is a range of 501 bytes. + +
Features that combine multiple responses into one logical resource are historically a +source of security bugs. Please seek security review for features that deal with partial responses. +
To serialize a response URL for reporting, given a response +response, run these steps: + +
Assert: response's URL list + is not empty. + +
Let url be a copy of response's URL list[0]. + +
This is not response's URL in order to avoid + leaking information about redirect targets (see + similar considerations for CSP reporting + too). [[CSP]] + +
Set the username given url and the empty string. + +
Set the password given url and the empty string. + +
Return the serialization of url with + exclude fragment set to true. +
To check if Cross-Origin-Embedder-Policy allows credentials, given a +request request, run these steps: + +
If request's mode is not "no-cors", then return
+ true.
+
+
If request's client is null, then return true. + +
If request's client's
+ policy container's
+ embedder policy's value is not
+ "credentialless", then return true.
+
+
If request's origin is same origin with
+ request's current URL's origin and request's
+ redirect-taint is not "same-origin", then return true.
+
+
Return false. +
The result of fetch is a +response. A response +evolves over time. That is, not all its fields are available straight away. + +
A response has an associated
+type which is
+"basic",
+"cors",
+"default",
+"error",
+"opaque", or
+"opaqueredirect".
+Unless stated otherwise, it is "default".
+
+
A response can have an associated +aborted flag, which is initially unset. + +
This indicates that the request was intentionally aborted by the developer or +end-user. + +
A response has an associated +URL. It is a pointer to the last +URL in response's URL list and null if +response's URL list is empty. + +
A response has an associated +URL list (a list of zero or +more URLs). Unless stated otherwise, it is « ». + +
Except for the first and last URL, if any, a response's +URL list is not directly exposed to script as that would violate +atomic HTTP redirect handling. + +
A response has an associated +status, which is a status. +Unless stated otherwise it is 200. + +
A response has an associated +status message. Unless stated +otherwise it is the empty byte sequence. + +
Responses over an HTTP/2 connection will always have the empty byte sequence as status +message as HTTP/2 does not support them. + +
A response has an associated +header list (a +header list). Unless stated otherwise it is « ». + +
A response has an associated +body (null or a +body). Unless stated otherwise it is null. + +
The source and length concepts of a network's +response's body are always null. + +
A response has an associated
+cache state (the empty string,
+"local", or "validated"). Unless stated otherwise, it is the empty
+string.
+
+
This is intended for usage by Service Workers and +Resource Timing. [[SW]] [[RESOURCE-TIMING]] + + +
A response has an associated +CORS-exposed header-name list +(a list of zero or more header +names). The list is empty unless otherwise specified. + +
A response will typically get its
+CORS-exposed header-name list set by extracting header values from the
+`Access-Control-Expose-Headers` header. This list is used by a
+CORS filtered response to determine which headers to expose.
+
+
A response has an associated +range-requested flag, which is +initially unset. + +
This is used to prevent a partial response from an earlier ranged request being +provided to an API that didn't make a range request. See the flag's usage for a detailed description +of the attack. + +
A response has an associated request-includes-credentials +(a boolean), which is initially true. + +
A response has an associated +timing allow passed flag, which is +initially unset. + +
This is used so that the caller to a fetch can determine if sensitive timing data is +allowed on the resource fetched by looking at the flag of the response returned. Because the flag on +the response of a redirect has to be set if it was set for previous responses in the redirect chain, +this is also tracked internally using the request's timing allow failed flag. + +
A response has an associated +body info +(a response body info). Unless stated otherwise, it is a new +response body info. + +
A response has an associated +service worker timing info (null or a +service worker timing info), which is initially null. + +
A response has an associated redirect taint
+("same-origin", "same-site", or "cross-site"), which is
+initially "same-origin".
+
+
A network error is a response whose
+type is "error", status is 0,
+status message is the empty byte sequence,
+header list is « », body is null, and
+body info is a new response body info.
+
+
An aborted network error is a +network error whose aborted flag is set. + +
To create the appropriate network error given fetch params +fetchParams: + +
Return an aborted network error if fetchParams is + aborted; otherwise return a network error. +
A filtered response is a response +that offers a limited view on an associated response. This associated +response can be accessed through filtered response's +internal response (a +response that is neither a network error nor a +filtered response). + +
Unless stated otherwise a filtered response's associated concepts (such as its +body) refer to the associated concepts of its +internal response. (The exceptions to this are listed below as part +of defining the concrete types of filtered responses.) + +
The fetch algorithm by way of processResponse and + equivalent parameters exposes filtered responses to callers to ensure they do not + accidentally leak information. If the information needs to be revealed for legacy reasons, e.g., to + feed image data to a decoder, the associated internal response can + be used by specification algorithms. + +
New specifications ought not to build further on opaque filtered responses or + opaque-redirect filtered responses. Those are legacy constructs and cannot always be + adequately protected given contemporary computer architecture. +
A basic filtered response is a
+filtered response whose
+type is "basic" and
+header list excludes any
+headers in
+internal response's
+header list whose
+name is a
+forbidden response-header name.
+
+
A CORS filtered response is a
+filtered response whose
+type is "cors" and
+header list excludes any
+headers in
+internal response's
+header list whose
+name is not a
+CORS-safelisted response-header name, given
+internal response's
+CORS-exposed header-name list.
+
+
An opaque filtered response is a
+filtered response whose
+type is "opaque",
+URL list is « »,
+status is 0,
+status message is the empty byte sequence,
+header list is « »,
+body is null, and
+body info is a new response body info.
+
+
An
+opaque-redirect filtered response
+is a filtered response whose
+type is "opaqueredirect",
+status is 0,
+status message is the empty byte sequence,
+header list is « »,
+body is null, and
+body info is a new response body info.
+
+
Exposing the URL list for + opaque-redirect filtered responses is harmless since + no redirects are followed. + +
In other words, an opaque filtered response and an + opaque-redirect filtered response are nearly indistinguishable from a network error. + When introducing new APIs, do not use the internal response for + internal specification algorithms as that will leak information. + +
This also means that JavaScript APIs, such as
+ response.ok, will return rather useless results.
+
The type of a response is exposed to script through the + {{Response/type}} getter: + +
+console.log(new Response().type); // "default"
+
+console.log((await fetch("/")).type); // "basic"
+
+console.log((await fetch("https://api.example/status")).type); // "cors"
+
+console.log((await fetch("https://crossorigin.example/image", { mode: "no-cors" })).type); // "opaque"
+
+console.log((await fetch("/surprise-me", { redirect: "manual" })).type); // "opaqueredirect"
+(This assumes that the various resources exist, https://api.example/status has the
+ appropriate CORS headers, and /surprise-me uses a redirect status.)
+
To clone a +response response, run these steps: + +
If response is a filtered response, then return a new identical + filtered response whose internal response is a + clone of response's + internal response. + +
Let newResponse be a copy of response, except for its + body. + +
If response's body is non-null, then set + newResponse's body to the result of cloning + response's body. + +
Return newResponse. +
A fresh response is a response whose +current age is within its freshness lifetime. + +
A stale-while-revalidate response is a +response that is not a fresh response and whose current age is within the +stale-while-revalidate lifetime. [[!HTTP-CACHING]] [[!STALE-WHILE-REVALIDATE]] + +
A stale response is a response that is +not a fresh response or a stale-while-revalidate response. + +
The location URL of a +response response, given null or an ASCII string +requestFragment, is the value returned by the following steps. They return null, failure, +or a URL. + +
If response's status is not a redirect status, then + return null. + +
Let location be the result of extracting header list values given
+ `Location` and response's header list.
+
+
+
If location is a header value, then set location to the + result of parsing location with response's + URL. + +
If response was constructed through the {{Response}} constructor, + response's URL will be null, meaning that location will + only parse successfully if it is an absolute-URL-with-fragment string. + +
If location is a URL whose fragment is null, then set + location's fragment to requestFragment. + +
This ensures that synthetic (indeed, all) responses follow the processing model for + redirects defined by HTTP. [[HTTP]] + +
Return location. +
The location URL algorithm is exclusively used for redirect +handling in this standard and in HTML's navigate algorithm which handles redirects +manually. [[!HTML]] +
A potential destination is
+"fetch" or a destination which is not the empty string.
+
+
To translate a +potential destination potentialDestination, run these steps: + +
If potentialDestination is "fetch", then return the empty string.
+
+
Assert: potentialDestination is a destination. + +
Return potentialDestination. +
An authentication entry and a proxy-authentication entry are +tuples of username, password, and realm, used for HTTP authentication and HTTP proxy authentication, +and associated with one or more requests. +
User agents should allow both to be cleared together with HTTP cookies and similar tracking +functionality. + + +
Further details are defined by HTTP. [[!HTTP]] [[!HTTP-CACHING]] + + +
Each environment settings object has an associated +fetch group, which holds a fetch group. + +
A fetch group holds information about fetches. + +
A fetch group has associated: + +
A fetch record is a struct with the following +items: + +
A deferred fetch record is a struct used to maintain state needed to +invoke a fetch at a later time, e.g., when a document is unloaded or becomes not +fully active. It has the following items: + +
pending")
+ pending", "sent", or "aborted".
+When a fetch group fetchGroup is +terminated: + +
For each fetch record record of + fetchGroup's fetch records, if record's + controller is non-null and record's + request's done flag is unset and keepalive is + false, terminate record's + controller. + +
Process deferred fetches for fetchGroup. +
To +resolve an origin, given a +network partition key key and an origin origin: + + +
If origin's host is an IP address, then return + « origin's host ». + +
If origin's host's public suffix is
+ "localhost" or "localhost.", then return « ::1,
+ 127.0.0.1 ».
+
+
Perform an implementation-defined operation to turn origin into a + set of one or more IP addresses. + +
It is also implementation-defined whether other operations might be performed to get + connection information beyond just IP addresses. For example, if origin's + scheme is an HTTP(S) scheme, the implementation might perform a DNS query + for HTTPS RRs. [[SVCB]] + +
If this operation succeeds, return the set of IP addresses and any + additional implementation-defined information. + +
Return failure. +
The results of resolve an origin may be cached. If they are cached, key should +be used as part of the cache key. + +
Typically this operation would involve DNS and as such caching can happen on DNS servers without + key being taken into account. Depending on the implementation it might also not be + possible to take key into account locally. [[RFC1035]] + +
The order of the IP addresses that the resolve an origin algorithm can return + can differ between invocations. + +
The particulars (apart from the cache key) are not tied down as they are not pertinent to the + system the Fetch Standard establishes. Other documents ought not to build on this primitive without + having a considered discussion with the Fetch Standard community first. +
A user agent has an associated connection pool. A +connection pool is an ordered set of zero or more +connections. Each connection is +identified by an associated key (a network partition key), +origin (an origin), and credentials +(a boolean). + +
Each connection has an associated +timing info (a +connection timing info). + +
A connection timing info is a struct used to maintain timing +information pertaining to the process of obtaining a connection. It has the following +items: + +
To clamp and coarsen connection timing info, given a +connection timing info timingInfo, a {{DOMHighResTimeStamp}} +defaultStartTime, and a boolean crossOriginIsolatedCapability, run these +steps: + +
If timingInfo's connection start time is + less than defaultStartTime, then return a new connection timing info whose + domain lookup start time is defaultStartTime, + domain lookup end time is defaultStartTime, + connection start time is defaultStartTime, + connection end time is defaultStartTime, + secure connection start time is defaultStartTime, + and ALPN negotiated protocol is timingInfo's + ALPN negotiated protocol. + +
Return a new connection timing info whose + domain lookup start time is the result of coarsen time + given timingInfo's domain lookup start time and + crossOriginIsolatedCapability, + domain lookup end time is the result of coarsen time + given timingInfo's domain lookup end time and + crossOriginIsolatedCapability, connection start time + is the result of coarsen time given timingInfo's + connection start time and + crossOriginIsolatedCapability, connection end time + is the result of coarsen time given timingInfo's + connection end time and + crossOriginIsolatedCapability, + secure connection start time is the result of + coarsen time given timingInfo's + connection end time and + crossOriginIsolatedCapability, and + ALPN negotiated protocol is timingInfo's + ALPN negotiated protocol. +
A new connection setting is "no", "yes", or
+"yes-and-dedicated".
+
+
To obtain a connection, given a
+network partition key key, URL url, boolean
+credentials, an optional new connection setting new (default
+"no"), and an optional boolean
+requireUnreliable (default false), run these
+steps:
+
+
+
If new is "no", then:
+
+
Let connections be a set of connections in the user agent's + connection pool whose key is key, + origin is url's origin, and + credentials is credentials. + +
If connections is not empty and requireUnreliable is false, then + return one of connections. + +
If there is a connection capable of supporting unreliable transport in + connections, e.g., HTTP/3, then return that connection. +
Let proxies be the result of finding proxies for url in an
+ implementation-defined manner. If there are no proxies, let proxies be
+ « "DIRECT" ».
+
+
This is where non-standard technology such as
+ Web Proxy Auto-Discovery Protocol (WPAD)
+ and proxy auto-config (PAC) come
+ into play. The "DIRECT" value means to not use a proxy for this particular
+ url.
+
+
Let timingInfo be a new connection timing info. + +
For each proxy of proxies: + +
Set timingInfo's domain lookup start time + to the unsafe shared current time. + +
If proxy is "DIRECT", then set hosts to the result of
+ running resolve an origin given key and url's origin.
+
+
If hosts is failure, then continue. + +
Set timingInfo's domain lookup end time to + the unsafe shared current time. + +
Let connection be the result of running this step: run create a connection + given key, url's origin, credentials, + proxy, an implementation-defined host from hosts, + timingInfo, and requireUnreliable an implementation-defined number + of times, in parallel from each other, and wait for at least 1 to return a value. In an + implementation-defined manner, select a value to return from the returned values and + return it. Any other returned values that are connections may be closed. + +
Essentially this allows an implementation to pick one or more
+ IP addresses from the return value of resolve an origin (assuming
+ proxy is "DIRECT") and race them against each other, favor
+ IPv6 addresses, retry in case of a timeout, etc.
+
+
If connection is failure, then continue. + +
If new is not "yes-and-dedicated", then append
+ connection to the user agent's connection pool.
+
+
Return connection. +
Return failure. +
This is intentionally a little vague as there are a lot of nuances to connection
+management that are best left to the discretion of implementers. Describing this helps explain the
+<link rel=preconnect> feature and clearly stipulates that connections are
+keyed on credentials. The latter clarifies that, e.g., TLS session identifiers are not
+reused across connections whose credentials are false with
+connections whose credentials are true.
+
To create a connection, given a network partition key key, +origin origin, boolean credentials, string proxy, +host host, connection timing info timingInfo, and +boolean requireUnreliable, run these steps: + +
Set timingInfo's connection start time to the + unsafe shared current time. + +
Let connection be a new connection whose key is + key, origin is origin, + credentials is credentials, and timing info + is timingInfo. Record connection timing info given connection + and use connection to establish an HTTP connection to host, taking + proxy and origin into account, with the following caveats: [[!HTTP]] + [[!HTTP1]] [[!TLS]] + +
If requireUnreliable is true, then establish a connection capable of unreliable + transport, e.g., an HTTP/3 connection. [[!HTTP3]] + +
When establishing a connection capable of unreliable transport, enable options that are
+ necessary for WebTransport. For HTTP/3, this means including
+ SETTINGS_ENABLE_WEBTRANSPORT with a value of 1 and
+ H3_DATAGRAM with a value of 1 in the initial SETTINGS
+ frame. [[!WEBTRANSPORT-HTTP3]] [[!HTTP3-DATAGRAM]]
+
+
If credentials is false, then do not send a TLS client certificate. + +
If establishing a connection does not succeed (e.g., a UDP, TCP, or TLS error), then + return failure. +
Set timingInfo's ALPN negotiated protocol to + connection's ALPN Protocol ID, with the following caveats: [[RFC7301]] + +
When a proxy is configured, if a tunnel connection is established then this must be the + ALPN Protocol ID of the tunneled protocol, otherwise it must be the ALPN Protocol ID of the first + hop to the proxy. + +
In case the user agent is using an experimental, non-registered protocol, the user agent must + use the used ALPN Protocol ID, if any. If ALPN was not used for protocol negotiations, the user + agent may use another descriptive string. + +
timingInfo's + ALPN negotiated protocol is intended to identify the network + protocol in use regardless of how it was actually negotiated; that is, even if ALPN is not used + to negotiate the network protocol, this is the ALPN Protocol IDs that indicates the protocol in + use. +
IANA maintains a + list of ALPN Protocol IDs. + +
Return connection. +
To record connection timing info given a connection +connection, let timingInfo be connection's +timing info and observe these requirements: + +
timingInfo's connection end time should be the + unsafe shared current time immediately after establishing the connection to the + server or proxy, as follows: + +
The returned time must include the time interval to establish the transport connection, as + well as other time intervals such as SOCKS authentication. It must include the time interval to + complete enough of the TLS handshake to request the resource. + +
If the user agent used TLS False Start for this connection, this interval must not include + the time needed to receive the server's Finished message. [[RFC7918]] + +
If the user agent sends the request with early data without waiting for the full handshake + to complete, this interval must not include the time needed to receive the server's ServerHello + message. [[RFC8470]] + +
If the user agent waits for full handshake completion to send the request, this interval + includes the full TLS handshake even if other requests were sent using early data on + connection. +
Suppose the user agent establishes an HTTP/2
+ connection over TLS 1.3 to send a GET request and a POST request. It
+ sends the ClientHello at time t1 and then sends the GET request with early
+ data. The POST request is not safe ([[HTTP]], section 9.2.1), so the user
+ agent waits to complete the handshake at time t2 before sending it. Although both
+ requests used the same connection, the GET request reports a connection end time of
+ t1, while the POST request reports t2.
+
+
If a secure transport is used, timingInfo's + secure connection start time should be the result of calling + unsafe shared current time immediately before starting the handshake process to + secure connection. [[!TLS]] + +
If connection is an HTTP/3 connection, timingInfo's + connection start time and timingInfo's + secure connection start time must be equal. (In HTTP/3 + the secure transport handshake process is performed as part of the initial connection setup.) + [[!HTTP3]] +
The clamp and coarsen connection timing info algorithm ensures that +details of reused connections are not exposed and time values are coarsened. +
A network partition key is a tuple consisting of a site and null +or an implementation-defined value. + +
To determine the network partition key, given an +environment environment: + +
Let topLevelOrigin be environment's + top-level origin. + +
If topLevelOrigin is null, then set topLevelOrigin to + environment's top-level creation URL's origin. + +
Let topLevelSite be the result of obtaining a site, + given topLevelOrigin. + +
Let secondKey be null or an implementation-defined value. + +
The second key is intentionally a little vague as the finer points are still + evolving. See issue #1035. + +
Return (topLevelSite, secondKey). +
To determine the network partition key, given a request +request: + +
If request's reserved client is non-null, then return the + result of determining the network partition key given request's + reserved client. + +
If request's client is non-null, then return the + result of determining the network partition key given request's + client. + +
Return null. +
To determine the HTTP cache partition, given a request request: + +
Let key be the result of determining the network partition key + given request. + +
If key is null, then return null. + +
Return the unique HTTP cache associated with key. [[!HTTP-CACHING]] +
New protocols can avoid the need for blocking ports by negotiating the protocol +through TLS using ALPN. The protocol cannot be spoofed through HTTP requests in that case. +[[RFC7301]] + +
To determine whether fetching a request request +should be blocked due to a bad port: + +
Let url be request's current URL. + +
If url's scheme is an HTTP(S) scheme and url's + port is a bad port, then return blocked. + +
Return allowed. +
A port is a +bad port if it is listed in the first column of the following table. + +
| Port | Typical service + |
|---|---|
| 0 | — + |
| 1 | tcpmux + |
| 7 | echo + |
| 9 | discard + |
| 11 | systat + |
| 13 | daytime + |
| 15 | netstat + |
| 17 | qotd + |
| 19 | chargen + |
| 20 | ftp-data + |
| 21 | ftp + |
| 22 | ssh + |
| 23 | telnet + |
| 25 | smtp + |
| 37 | time + |
| 42 | name + |
| 43 | nicname + |
| 53 | domain + |
| 69 | tftp + |
| 77 | — + |
| 79 | finger + |
| 87 | — + |
| 95 | supdup + |
| 101 | hostname + |
| 102 | iso-tsap + |
| 103 | gppitnp + |
| 104 | acr-nema + |
| 109 | pop2 + |
| 110 | pop3 + |
| 111 | sunrpc + |
| 113 | auth + |
| 115 | sftp + |
| 117 | uucp-path + |
| 119 | nntp + |
| 123 | ntp + |
| 135 | epmap + |
| 137 | netbios-ns + |
| 139 | netbios-ssn + |
| 143 | imap + |
| 161 | snmp + |
| 179 | bgp + |
| 389 | ldap + |
| 427 | svrloc + |
| 465 | submissions + |
| 512 | exec + |
| 513 | login + |
| 514 | shell + |
| 515 | printer + |
| 526 | tempo + |
| 530 | courier + |
| 531 | chat + |
| 532 | netnews + |
| 540 | uucp + |
| 548 | afp + |
| 554 | rtsp + |
| 556 | remotefs + |
| 563 | nntps + |
| 587 | submission + |
| 601 | syslog-conn + |
| 636 | ldaps + |
| 989 | ftps-data + |
| 990 | ftps + |
| 993 | imaps + |
| 995 | pop3s + |
| 1719 | h323gatestat + |
| 1720 | h323hostcall + |
| 1723 | pptp + |
| 2049 | nfs + |
| 3659 | apple-sasl + |
| 4045 | npp + |
| 4190 | sieve + |
| 5060 | sip + |
| 5061 | sips + |
| 6000 | x11 + |
| 6566 | sane-port + |
| 6665 | ircu + |
| 6666 | ircu + |
| 6667 | ircu + |
| 6668 | ircu + |
| 6669 | ircu + |
| 6679 | osaut + |
| 6697 | ircs-u + |
| 10080 | amanda + |
Run these steps: + +
Let mimeType be the result of extracting a MIME type + from response's header list. + +
If mimeType is failure, then return allowed. + +
Let destination be request's destination. + +
If destination is script-like and one of the + following is true, then return blocked: + +
audio/", "image/", or "video/".
+ text/csv".
+ Return allowed. +
The `Cookie` request header and `Set-Cookie` response headers are
+largely defined in their own specifications. We define additional infrastructure to be able to use
+them conveniently here. [[COOKIES]].
+
+
+
Cookie` headerTo append a request `Cookie` header, given a request
+request:
+
+
If the user agent is configured to disable cookies for request, then it should + return. + +
Let |sameSite| be the result of [=determining the same-site mode=] for request. + +
Let |isSecure| be true if request's current URL's
+ scheme is "https"; otherwise false.
+
+
Let |httpOnlyAllowed| be true. + +
True follows from this being invoked from fetch, as opposed to the
+ document.cookie getter steps for instance.
+
+
Let |cookies| be the result of running retrieve cookies given |isSecure|, + request's current URL's host, request's + current URL's path, |httpOnlyAllowed|, and |sameSite|. + +
The cookie store returns an ordered list of cookies + +
If |cookies| is empty, then return. + +
Let |value| be the result of running serialize cookies given |cookies|. + +
Append (`Cookie`, value) to
+ request's header list.
+
Set-Cookie` headerTo parse and store response `Set-Cookie` headers, given a
+request request and a response response:
+
+
If the user agent is configured to disable cookies for request, then it should + return. + +
Let |allowNonHostOnlyCookieForPublicSuffix| be false. + +
Let |isSecure| be true if request's current URL's
+ scheme is "https"; otherwise false.
+
+
Let |httpOnlyAllowed| be true. + +
True follows from this being invoked from fetch, as opposed to the
+ document.cookie getter steps for instance.
+
+
Let |sameSiteStrictOrLaxAllowed| be true if the result of [=determine the same-site mode=]
+ for |request| is "strict-or-less"; otherwise false.
+
+
For each header of response's + header list: + +
If header's name is not a byte-case-insensitive match
+ for `Set-Cookie`, then continue.
+
+
Parse and store a cookie given header's value, + |isSecure|, request's current URL's host, + request's current URL's path, |httpOnlyAllowed|, + |allowNonHostOnlyCookieForPublicSuffix|, and |sameSiteStrictOrLaxAllowed|. + +
Garbage collect cookies given request's current URL's + host. +
As noted elsewhere the `Set-Cookie` header cannot be combined and
+ therefore each occurrence is processed independently. This is not allowed for any other header.
+
To determine the same-site mode for a given request request: + +
If request's top-level navigation initiator origin is not
+ null and is not same site with request's URL's
+ origin, then return "unset-or-less".
+
+
If request's method is "GET" and
+ request's destination is "document", then return
+ "lax-or-less".
+
+
If request's client's
+ has cross-site ancestor is true, then return
+ "unset-or-less".
+
+
If request's redirect-taint is "cross-site", then
+ return "unset-or-less".
+
+
Return "strict-or-less".
+
To obtain a serialized cookie default path given a URL +url: + +
Let cloneURL be a clone of url. + +
Set cloneURL's path to the cookie default path of + cloneURL's path. + +
Return the URL path serialization of cloneURL. +
Origin` headerThe `Origin`
+request header indicates where a
+fetch originates from.
+
+
The `Origin` header is a version of the
+`Referer` [sic] header that does not reveal a path. It is used for all
+HTTP fetches whose request's
+response tainting is "cors", as well as those where
+request's method is neither `GET` nor
+`HEAD`. Due to compatibility constraints it is not included in all
+fetches.
+
+
+
Its possible values are all the return values of +byte-serializing a request origin, given a request. + +
This supplants the definition in The Web Origin Concept. [[ORIGIN]] + +
To append a request `Origin` header,
+given a request request, run these steps:
+
+
Let serializedOrigin be the result of byte-serializing a request origin + with request. + +
If request's response tainting is "cors" or
+ request's mode is either "websocket" or "webtransport", then
+ append (`Origin`, serializedOrigin) to
+ request's header list.
+
+
Otherwise, if request's method is neither `GET` nor
+ `HEAD`, then:
+
+
If request's mode is not "cors",
+ then switch on request's referrer policy:
+
+
no-referrer"
+ Set serializedOrigin to `null`.
+
+
no-referrer-when-downgrade"
+ strict-origin"
+ strict-origin-when-cross-origin"
+ If request's origin is a tuple origin, its
+ scheme is "https", and request's
+ current URL's scheme is not "https", then set
+ serializedOrigin to `null`.
+
+
same-origin"
+ If request's origin is not same origin with
+ request's current URL's origin, then set
+ serializedOrigin to `null`.
+
+
Append (`Origin`, serializedOrigin) to
+ request's header list.
+
A request's referrer policy is taken into account for +all fetches where the fetcher did not explicitly opt into sharing their origin with the +server, e.g., via using the CORS protocol. +
To allow sharing responses cross-origin and allow for more versatile +fetches than possible with HTML's +<{form}> element, the CORS protocol exists. It +is layered on top of HTTP and allows responses to declare they can be shared with other +origins. + +
It needs to be an opt-in mechanism to prevent leaking data from responses behind a +firewall (intranets). Additionally, for requests including +credentials it needs to be opt-in to prevent leaking potentially-sensitive data. + +
This section explains the CORS protocol as it pertains to server developers. +Requirements for user agents are part of the fetch algorithm, +except for the new HTTP header syntax. + + +
The CORS protocol consists of a set of headers that indicates whether a response can +be shared cross-origin. + +
For requests that are more involved than what is possible with HTML's <{form}> +element, a CORS-preflight request is performed, to ensure request's +current URL supports the CORS protocol. + + +
A CORS request is an HTTP request that includes an
+`Origin` header. It cannot be reliably identified as participating
+in the CORS protocol as the `Origin` header is also included
+for all requests whose method is neither `GET` nor
+`HEAD`.
+
+
A CORS-preflight request is a CORS request
+that checks to see if the CORS protocol is understood. It uses `OPTIONS` as
+method and includes the following header:
+
+
Access-Control-Request-Method`
+ Indicates which method a future + CORS request to the same resource might use. +
A CORS-preflight request can also include the following header: + +
Access-Control-Request-Headers`
+ Indicates which headers a future + CORS request to the same resource might use. +
An HTTP response to a CORS request can include the following +headers: + +
Access-Control-Allow-Origin`
+ Indicates whether the response can be shared, via returning the literal
+ value of the
+ `Origin` request header
+ (which can be `null`) or `*` in a response.
+
+
Access-Control-Allow-Credentials`
+ Indicates whether the response can be shared when request's
+ credentials mode is
+ "include".
+
+
For a CORS-preflight request, request's
+ credentials mode is always "same-origin", i.e., it excludes
+ credentials, but for any subsequent CORS requests it might not be. Support therefore needs
+ to be indicated as part of the HTTP response to the CORS-preflight request as well.
+
An HTTP response to a CORS-preflight request can include the following +headers: + +
Access-Control-Allow-Methods`
+ Indicates which methods are supported by the response's + URL for the purposes of the CORS protocol. + +
The `Allow` header is
+ not relevant for the purposes of the CORS protocol.
+
+
Access-Control-Allow-Headers`
+ Indicates which headers are supported by the response's + URL for the purposes of the CORS protocol. + +
Access-Control-Max-Age`
+ Indicates the number of seconds (5 by default) the information provided by the
+ `Access-Control-Allow-Methods` and
+ `Access-Control-Allow-Headers` headers can be cached.
+
An HTTP response to a CORS request that is not a +CORS-preflight request can also include the following +header: + +
Access-Control-Expose-Headers`
+ Indicates which headers can be exposed as part + of the response by listing their names. +
A successful HTTP response, i.e., one where the server developer intends to share it, to a +CORS request can use any status, as long as it includes the headers +stated above with values matching up with the request. + +
A successful HTTP response to a CORS-preflight request is similar, except it is restricted +to an ok status, e.g., 200 or 204. + +
Any other kind of HTTP response is not successful and will either end up not being shared or fail +the CORS-preflight request. Be aware that any work the server performs might nonetheless leak +through side channels, such as timing. If server developers wish to denote this explicitly, the 403 +status can be used, coupled with omitting the relevant headers. + +
If desired, “failure” could also be shared, but that would make it a successful HTTP +response. That is why for a successful HTTP response to a CORS request that is not a +CORS-preflight request the status can be anything, including 403. + +
Ultimately server developers have a lot of freedom in how they handle HTTP responses and these +tactics can differ between the response to the CORS-preflight request and the +CORS request that follows it: + +
They can provide a static response. This can be helpful when working with caching + intermediaries. A static response can both be successful and not successful depending on the + CORS request. This is okay. + +
They can provide a dynamic response, tuned to CORS request. This can be helpful when + the response body is to be tailored to a specific origin or a response needs to have credentials + and be successful for a set of origins. +
ABNF for the values of the +headers used by the CORS protocol: + +
+Access-Control-Request-Method = method
+Access-Control-Request-Headers = 1#field-name
+
+wildcard = "*"
+Access-Control-Allow-Origin = origin-or-null / wildcard
+Access-Control-Allow-Credentials = %s"true" ; case-sensitive
+Access-Control-Expose-Headers = #field-name
+Access-Control-Max-Age = delta-seconds
+Access-Control-Allow-Methods = #method
+Access-Control-Allow-Headers = #field-name
+For `Access-Control-Expose-Headers`,
+`Access-Control-Allow-Methods`, and `Access-Control-Allow-Headers`
+response headers, the value `*` counts as a wildcard for
+requests without credentials. For such requests there is no
+way to solely match a header name or method that is `*`.
+
+
+
When request's
+credentials mode is "include" it
+has an impact on the functioning of the CORS protocol other than including
+credentials in the fetch.
+
+
In the old days, {{XMLHttpRequest}} could be used to set
+ request's
+ credentials mode to "include":
+
+
+var client = new XMLHttpRequest()
+client.open("GET", "./")
+client.withCredentials = true
+/* … */
+Nowadays, fetch("./", { credentials:"include" }).then(/* … */)
+ suffices.
+
A request's +credentials mode is not necessarily observable +on the server; only when credentials exist for a +request can it be observed by virtue of the +credentials being included. Note that even so, a CORS-preflight request +never includes credentials. + +
The server developer therefore needs to decide whether or not responses "tainted" with +credentials can be shared. And also needs to decide if +requests necessitating a CORS-preflight request can +include credentials. Generally speaking, both sharing responses and allowing requests +with credentials is rather unsafe, and extreme care has to be taken to avoid the +confused deputy problem. + + +
To share responses with credentials, the
+`Access-Control-Allow-Origin` and
+`Access-Control-Allow-Credentials` headers are
+important. The following table serves to illustrate the various legal and illegal combinations for a
+request to https://rabbit.invalid/:
+
+
| Request's credentials mode + | `Access-Control-Allow-Origin`
+ | `Access-Control-Allow-Credentials`
+ | Shared? + | Notes + |
|---|---|---|---|---|
"omit"
+ | `*`
+ | Omitted + | ✅ + | — + |
"omit"
+ | `*`
+ | `true`
+ | ✅ + | If credentials mode is not "include", then
+ `Access-Control-Allow-Credentials` is ignored.
+ |
"omit"
+ | `https://rabbit.invalid/`
+ | Omitted + | ❌ + | A serialized origin has no trailing slash. + |
"omit"
+ | `https://rabbit.invalid`
+ | Omitted + | ✅ + | — + |
"include"
+ | `*`
+ | `true`
+ | ❌ + | If credentials mode is "include", then
+ `Access-Control-Allow-Origin` cannot be
+ `*`.
+ |
"include"
+ | `https://rabbit.invalid`
+ | `true`
+ | ✅ + | — + |
"include"
+ | `https://rabbit.invalid`
+ | `True`
+ | ❌ + | `true` is (byte) case-sensitive.
+ |
Similarly, `Access-Control-Expose-Headers`,
+`Access-Control-Allow-Methods`, and
+`Access-Control-Allow-Headers` response headers can only use
+`*` as value when request's credentials mode is not
+"include".
+
+
+
A script at https://foo.invalid/ wants to fetch some data from
+ https://bar.invalid/. (Neither credentials nor response header access is
+ important.)
+
+
+var url = "https://bar.invalid/api?key=730d67a37d7f3d802e96396d00280768773813fbe726d116944d814422fc1a45&data=about:unicorn";
+fetch(url).then(success, failure)
+This will use the CORS protocol, though this is entirely transparent to the
+ developer from foo.invalid. As part of the CORS protocol, the user agent
+ will include the `Origin` header in the request:
+
+
+Origin: https://foo.invalid
+Upon receiving a response from bar.invalid, the user agent will verify the
+ `Access-Control-Allow-Origin` response header. If its value is
+ either `https://foo.invalid` or `*`, the user agent will invoke the
+ success callback. If it has any other value, or is missing, the user agent will invoke
+ the failure callback.
+
The developer of foo.invalid is back, and now wants to fetch some data from
+ bar.invalid while also accessing a response header.
+
+
+fetch(url).then(response => {
+ var hsts = response.headers.get("strict-transport-security"),
+ csp = response.headers.get("content-security-policy")
+ log(hsts, csp)
+})
+bar.invalid provides a correct
+ `Access-Control-Allow-Origin` response header per the earlier
+ example. The values of hsts and csp will depend on the
+ `Access-Control-Expose-Headers` response header. For example, if
+ the response included the following headers
+
+
+Content-Security-Policy: default-src 'self'
+Strict-Transport-Security: max-age=31536000; includeSubdomains; preload
+Access-Control-Expose-Headers: Content-Security-Policy
+then hsts would be null and csp would be
+ "default-src 'self'", even though the response did include both headers. This is
+ because bar.invalid needs to explicitly share each header by listing their names in
+ the `Access-Control-Expose-Headers` response header.
+
+
Alternatively, if bar.invalid wanted to share all its response headers, for
+ requests that do not include credentials, it could use `*` as value for
+ the `Access-Control-Expose-Headers` response header. If the request
+ would have included credentials, the response header names would have to be listed
+ explicitly and `*` could not be used.
+
The developer of foo.invalid returns, now fetching some data from
+ bar.invalid while including credentials. This time around the
+ CORS protocol is no longer transparent to the developer as credentials
+ require an explicit opt-in:
+
+
+fetch(url, { credentials:"include" }).then(success, failure)
+This also makes any `Set-Cookie` response headers bar.invalid
+ includes fully functional (they are ignored otherwise).
+
+
The user agent will make sure to include any relevant credentials in the request.
+ It will also put stricter requirements on the response. Not only will bar.invalid need
+ to list `https://foo.invalid` as value for the
+ `Access-Control-Allow-Origin` header (`*` is not
+ allowed when credentials are involved), the
+ `Access-Control-Allow-Credentials` header has to be present too:
+
+
+Access-Control-Allow-Origin: https://foo.invalid
+Access-Control-Allow-Credentials: true
+If the response does not include those two headers with those values, the failure
+ callback will be invoked. However, any `Set-Cookie` response headers will be
+ respected.
+
Specifications have allowed limited exceptions to the CORS safelist for non-safelisted
+`Content-Type` header values. These exceptions are made for requests that can be
+triggered by web content but whose headers and bodies can be only minimally controlled by the web
+content. Therefore, servers should expect cross-origin web content to be allowed to trigger
+non-preflighted requests with the following non-safelisted `Content-Type` header
+values:
+
+
application/csp-report` [[CSP]]
+ application/expect-ct-report+json` [[RFC9163]]
+ application/xss-auditor-report`
+ application/ocsp-request` [[RFC6960]]
+Specifications should avoid introducing new exceptions and should only do so with careful +consideration for the security consequences. New exceptions can be proposed by +filing an issue. + + +
Content-Length` headerThe `Content-Length` header is largely defined in HTTP. Its processing model is
+defined here as the model defined in HTTP is not compatible with web content. [[HTTP]]
+
+
To extract a length +from a header list headers, run these steps: + +
Let values be the result of
+ getting, decoding, and splitting `Content-Length` from
+ headers.
+
+
If values is null, then return null. + +
Let candidateValue be null. + +
For each value of values: + +
If candidateValue is null, then set candidateValue to + value. + +
Otherwise, if value is not candidateValue, return failure. +
If candidateValue is the empty string or has a code point that is + not an ASCII digit, then return null. + +
Return candidateValue, interpreted as decimal number. +
Content-Type` headerThe `Content-Type` header is largely defined in HTTP. Its processing model is
+defined here as the model defined in HTTP is not compatible with web content. [[HTTP]]
+
+
To +extract a MIME type +from a header list headers, run these steps. They return failure or a +MIME type. + +
Let charset be null. + +
Let essence be null. + +
Let mimeType be null. + +
Let values be the result of
+ getting, decoding, and splitting `Content-Type` from
+ headers.
+
+
If values is null, then return failure. + +
For each value of values: + +
Let temporaryMimeType be the result of parsing + value. + +
If temporaryMimeType is failure or its essence is
+ "*/*", then continue.
+
+
Set mimeType to temporaryMimeType. + +
If mimeType's essence is not essence, then: + +
Set charset to null. + +
If mimeType's parameters["charset"]
+ exists, then set charset to mimeType's
+ parameters["charset"].
+
+
Set essence to mimeType's essence. +
Otherwise, if mimeType's
+ parameters["charset"] does not exist, and
+ charset is non-null, set mimeType's
+ parameters["charset"] to charset.
+
If mimeType is null, then return failure. + +
Return mimeType. +
When extract a MIME type returns failure or a MIME type whose +essence is incorrect for a given format, treat this as a fatal error. +Existing web platform features have not always followed this pattern, which has been a major source +of security vulnerabilities in those features over the years. In contrast, a +MIME type's parameters can typically be safely ignored. + +
This is how extract a MIME type functions in practice: + +
| Headers (as on the network) + | Output (serialized) + |
|---|---|
+ | text/html
+ |
+ | text/html;x=y;charset=gbk
+ |
+ | |
+ | text/html;x=y
+ |
+ | text/html
+ |
+ | |
+ |
To legacy extract an encoding given failure or a MIME type +mimeType and an encoding fallbackEncoding, run these steps: + +
If mimeType is failure, then return fallbackEncoding. + +
If mimeType["charset"] does not exist, then return
+ fallbackEncoding.
+
+
Let tentativeEncoding be the result of getting an encoding from
+ mimeType["charset"].
+
+
If tentativeEncoding is failure, then return fallbackEncoding. + +
Return tentativeEncoding. +
This algorithm allows mimeType to be failure so it can be more easily combined with + extract a MIME type. + +
It is denoted as legacy as modern formats are to exclusively use UTF-8. +
X-Content-Type-Options` headerThe
+`X-Content-Type-Options`
+response header can be used to require checking of a response's
+`Content-Type` header against the destination of a
+request.
+
+
To determine nosniff, given a header list list, run +these steps: + +
Let values be the result of
+ getting, decoding, and splitting
+ `X-Content-Type-Options` from list.
+
+
If values is null, then return false. + +
If values[0] is an ASCII case-insensitive match for
+ "nosniff", then return true.
+
+
Return false. +
Web developers and conformance checkers must use the following value
+ABNF for `X-Content-Type-Options`:
+
+
+X-Content-Type-Options = "nosniff" ; case-insensitive
+Run these steps: + +
If determine nosniff with response's header list is + false, then return allowed. + +
Let mimeType be the result of extracting a MIME type + from response's header list. + +
Let destination be request's destination. + +
If destination is script-like and + mimeType is failure or is not a JavaScript MIME type, then return blocked. + +
If destination is "style" and mimeType is failure or its
+ essence is not "text/css", then return blocked.
+
+
Return allowed. +
Only request destinations that are
+script-like or "style" are considered as any exploits
+pertain to them. Also, considering "image" was not compatible with deployed content.
+
Cross-Origin-Resource-Policy` headerThe
+`Cross-Origin-Resource-Policy`
+response header can be used to require checking a request's
+current URL's origin against a request's
+origin when request's mode is
+"no-cors".
+
+
+Cross-Origin-Resource-Policy = %s"same-origin" / %s"same-site" / %s"cross-origin" ; case-sensitive
+To perform a cross-origin resource policy check, given an origin +origin, an environment settings object settingsObject, a string +destination, a response response, and an optional boolean +forNavigation, run these steps: + +
Set forNavigation to false if it is not given. + +
Let embedderPolicy be settingsObject's + policy container's + embedder policy. + +
If the cross-origin resource policy internal check with origin,
+ "unsafe-none", response, and
+ forNavigation returns blocked, then return blocked.
+
+
This step is needed because we don't want to report violations not related to + Cross-Origin Embedder Policy below. + +
If the cross-origin resource policy internal check with origin, + embedderPolicy's report only value, response, + and forNavigation returns blocked, then + queue a cross-origin embedder policy CORP violation report with response, + settingsObject, destination, and true. + +
If the cross-origin resource policy internal check with origin, + embedderPolicy's value, response, and + forNavigation returns allowed, then return allowed. + +
Queue a cross-origin embedder policy CORP violation report with response, + settingsObject, destination, and false. + +
Return blocked. +
Only HTML's navigate algorithm uses this check with forNavigation set to +true, and it's always for nested navigations. Otherwise, response is either the +internal response of an opaque filtered response or a +response which will be the internal response of an +opaque filtered response. [[HTML]] +
To perform a cross-origin resource policy internal check, given an +origin origin, an embedder policy value +embedderPolicyValue, a response response, and a boolean +forNavigation, run these steps: + +
If forNavigation is true and embedderPolicyValue is
+ "unsafe-none", then return allowed.
+
+
Let policy be the result of getting
+ `Cross-Origin-Resource-Policy` from response's
+ header list.
+
+
This means that `Cross-Origin-Resource-Policy: same-site, same-origin`
+ ends up as allowed below as it will never match anything, as long as
+ embedderPolicyValue is "unsafe-none".
+ Two or more `Cross-Origin-Resource-Policy` headers will have the
+ same effect.
+
+
If policy is neither `same-origin`, `same-site`, nor
+ `cross-origin`, then set policy to null.
+
+
If policy is null, then switch on embedderPolicyValue:
+ +unsafe-none"
+ Do nothing. + +
credentialless"
+ Set policy to `same-origin` if:
+
+
require-corp"
+ Set policy to `same-origin`.
+
Switch on policy: + +
cross-origin`
+ Return allowed. + +
same-origin`
+ If origin is same origin with response's URL's + origin, then return allowed. + +
Otherwise, return blocked. + +
same-site`
+ If all of the following are true + +
origin is schemelessly same site with response's + URL's origin + +
origin's scheme is "https" or
+ response's URL's scheme is not
+ "https"
+
then return allowed. + +
Otherwise, return blocked. + +
`Cross-Origin-Resource-Policy: same-site` does not consider a
+ response delivered via a secure transport to match a non-secure requesting origin,
+ even if their hosts are otherwise same site. Securely-transported responses will only
+ match a securely-transported initiator.
+
To queue a cross-origin embedder policy CORP violation report, given a +response response, an environment settings object +settingsObject, a string destination, and a boolean reportOnly, +run these steps: + +
Let endpoint be settingsObject's + policy container's + embedder policy's + report only reporting endpoint if reportOnly is true and + settingsObject's policy container's + embedder policy's + reporting endpoint otherwise. + +
Let serializedURL be the result of + serializing a response URL for reporting with + response. + +
Let disposition be "reporting" if reportOnly is true;
+ otherwise "enforce".
+
+
Let body be a new object containing the following properties: + +
| key + | value + |
|---|---|
"type"
+ | "corp"
+ |
"blockedURL"
+ | serializedURL + |
"destination"
+ | destination + |
"disposition"
+ | disposition + |
Generate and queue a report for settingsObject's
+ global object given the
+ "coep" report type, endpoint, and body. [[!REPORTING]]
+
Sec-Purpose` headerThe `Sec-Purpose` HTTP request
+header specifies that the request serves one or more purposes other than requesting the resource for
+immediate use by the user.
+
+
The `Sec-Purpose` header field is a structured header
+whose value must be a token.
+
+
The sole token defined is prefetch. It
+indicates the request’s purpose is to fetch a resource that is anticipated to be needed shortly.
+
+
The server can use this to adjust the caching expiry for prefetches, to disallow the +prefetch, or to treat it differently when counting page visits. + + + +
The algorithm below defines fetching. In broad strokes, it takes +a request and one or more algorithms to run at various points during the operation. A +response is passed to the last two algorithms listed below. The first two algorithms +can be used to capture uploads. + +
To fetch, given a request request, an +optional algorithm +processRequestBodyChunkLength, an +optional algorithm +processRequestEndOfBody, +an optional algorithm processEarlyHintsResponse, an optional +algorithm processResponse, an optional +algorithm processResponseEndOfBody, an optional algorithm +processResponseConsumeBody, +and an optional boolean useParallelQueue (default false), run +the steps below. If given, processRequestBodyChunkLength must be an algorithm accepting +an integer representing the number of bytes transmitted. If given, +processRequestEndOfBody must be an algorithm accepting no arguments. If given, +processEarlyHintsResponse must be an algorithm accepting a response. If +given, processResponse must be an algorithm accepting a response. If given, +processResponseEndOfBody must be an algorithm accepting a response. If +given, processResponseConsumeBody must be an algorithm accepting a response +and null, failure, or a byte sequence. + +
The user agent may be asked to +suspend the ongoing fetch. +The user agent may either accept or ignore the suspension request. The suspended fetch can be +resumed. The user agent should +ignore the suspension request if the ongoing fetch is updating the response in the HTTP cache for +the request. + +
The user agent does not update the entry in the HTTP cache for a request
+if request's cache mode is "no-store" or a `Cache-Control: no-store` header appears in
+the response. [[!HTTP-CACHING]]
+
+
Assert: request's mode is "navigate" or
+ processEarlyHintsResponse is null.
+
+
Processing of early hints (responses whose status + is 103) is only vetted for navigations. + +
Let taskDestination be null. + +
Let crossOriginIsolatedCapability be false. + +
Populate request from client given request. + +
If request's client is non-null, then: + +
Set taskDestination to request's client's + global object. + +
Set crossOriginIsolatedCapability to request's + client's + cross-origin isolated capability. +
If useParallelQueue is true, then set taskDestination to the result of + starting a new parallel queue. + + + +
Let timingInfo be a new fetch timing info whose + start time and + post-redirect start time are the + coarsened shared current time given crossOriginIsolatedCapability, and + render-blocking is set to request's + render-blocking. + +
Let fetchParams be a new fetch params whose + request is request, + timing info is timingInfo, + process request body chunk length is + processRequestBodyChunkLength, + process request end-of-body is processRequestEndOfBody, + process early hints response is processEarlyHintsResponse, + process response is processResponse, + process response consume body is processResponseConsumeBody, + process response end-of-body is processResponseEndOfBody, + task destination is taskDestination, and + cross-origin isolated capability is + crossOriginIsolatedCapability. + +
If request's body is a byte sequence, then set + request's body to request's body + as a body. + +
If all of the following conditions are true: + +
request's URL's scheme is an + HTTP(S) scheme + +
request's mode is "same-origin",
+ "cors", or "no-cors"
+
+
request's client is not null, and request's + client's global object is a + {{Window}} object + +
request's method is `GET`
+
+
request's unsafe-request flag is not set or + request's header list is empty +
then: + +
Assert: request's origin is same origin + with request's client's + origin. + +
Let onPreloadedResponseAvailable be an algorithm that runs the following + step given a response response: set fetchParams's + preloaded response candidate to response. + +
Let foundPreloadedResource be the result of invoking + consume a preloaded resource for request's client, given + request's URL, request's destination, + request's mode, request's + credentials mode, request's integrity metadata, + and onPreloadedResponseAvailable. + +
If foundPreloadedResource is true and fetchParams's
+ preloaded response candidate is null, then set fetchParams's
+ preloaded response candidate to "pending".
+
If request's header list
+ does not contain `Accept`, then:
+
+
Let value be `*/*`.
+
+
If request's initiator is "prefetch", then set
+ value to the document `Accept` header value.
+
+
Otherwise, the user agent should set value to the first matching statement, if + any, switching on request's destination: + + +
document"
+ frame"
+ iframe"
+ Accept` header value
+
+ image"
+ image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5`
+
+ json"
+ application/json,*/*;q=0.5`
+
+ style"
+ text/css,*/*;q=0.1`
+ Append (`Accept`, value) to
+ request's header list.
+
If request's header list
+ does not contain `Accept-Language` and request's
+ client is non-null:
+
+
Let emulatedLanguage be the WebDriver BiDi emulated language for + request's client. + +
If emulatedLanguage is non-null: + +
Let encodedEmulatedLanguage be emulatedLanguage, + isomorphic encoded. + +
Append
+ (`Accept-Language`, encodedEmulatedLanguage) to request's
+ header list.
+
If request's header list
+ does not contain `Accept-Language`, then user agents should
+ append (`Accept-Language, an appropriate
+ header value) to request's header list.
+
+
If request's internal priority is null, then use + request's priority, initiator, + destination, and render-blocking in an + implementation-defined manner to set request's + internal priority to an implementation-defined object. + +
The implementation-defined object could encompass stream weight and + dependency for HTTP/2, priorities used in Extensible Prioritization Scheme for HTTP + for transports where it applies (including HTTP/3), and equivalent information used to prioritize + dispatch and processing of HTTP/1 fetches. [[!RFC9218]] + +
If request is a subresource request: + +
Let record be a new fetch record whose + request is request and controller + is fetchParams's controller. + +
Append record to request's + client's fetch group's + fetch records. +
Run main fetch given fetchParams. + +
Return fetchParams's controller. +
To populate request from client given a request request: + +
If request's traversable for user prompts is "client":
+
+
Set request's traversable for user prompts to
+ "no-traversable".
+
+
If request's client is non-null: + +
Let global be request's client's + global object. + +
If global is a {{Window}} object and global's + navigable is not null, then set request's + traversable for user prompts to global's + navigable's traversable navigable. +
If request's origin is "client":
+
+
If request's policy container is "client":
+
+
If request's client is non-null, then set + request's policy container to a + clone of request's client's + policy container. [[!HTML]] + +
Otherwise, set request's policy container to a new + policy container. +
To main fetch, given a fetch params +fetchParams and an optional boolean recursive (default false), run these +steps: + +
Let request be fetchParams's request. + +
Let response be null. + +
If request's local-URLs-only flag is set and request's + current URL is not local, then set response to a + network error. + +
Run report Content Security Policy violations for request. + +
Upgrade request to a potentially trustworthy URL, if appropriate. + +
Upgrade a mixed content request to a potentially trustworthy URL, if appropriate. + +
If should request be blocked due to a bad port, + should fetching request be blocked as mixed content, + should request be blocked by Content Security Policy, or + should request be blocked by Integrity Policy Policy + returns blocked, then set response to a network error. + +
If request's referrer policy is the empty string, then set + request's referrer policy to request's + policy container's referrer policy. + +
If request's referrer is not "no-referrer", then set
+ request's referrer to the result of invoking
+ determine request's referrer. [[!REFERRER]]
+
+
As stated in Referrer Policy, user agents can provide the end user with
+ options to override request's referrer to "no-referrer"
+ or have it expose less sensitive information.
+
+
Set request's current URL's scheme to
+ "https" if all of the following conditions are true:
+
+
http"
+ localhost" or "localhost."
+ includeSubDomains directive
+ or a congruent match (with or without an asserted includeSubDomains directive) [[!HSTS]]; or
+ DNS resolution for the request finds a matching HTTPS RR per
+ section 9.5
+ of [[!SVCB]].
+ [[!HSTS]] [[!SVCB]]
+ As all DNS operations are generally implementation-defined, how it is + determined that DNS resolution contains an HTTPS RR is also implementation-defined. As DNS + operations are not traditionally performed until attempting to obtain a connection, user + agents might need to perform DNS operations earlier, consult local DNS caches, or wait until later + in the fetch algorithm and potentially unwind logic on discovering the need to change + request's current URL's scheme. + +
If recursive is false, then run the remaining steps in parallel. + +
If response is null, then set response to the result of running the steps + corresponding to the first matching statement: + +
Wait until fetchParams's
+ preloaded response candidate is not "pending".
+
+
Assert: fetchParams's + preloaded response candidate is a response. + +
Return fetchParams's preloaded response candidate. +
basic"
+ data"
+ navigate", "websocket" or "webtransport"
+ Set request's
+ response tainting to "basic".
+
+
Return the result of running override fetch given "scheme-fetch"
+ and fetchParams.
+
HTML assigns any documents and workers created from URLs whose
+ scheme is "data" a unique opaque origin. Service workers can
+ only be created from URLs whose scheme is an HTTP(S) scheme.
+ [[!HTML]] [[!SW]]
+
+
same-origin"
+ Return a network error. + +
no-cors"
+ If request's redirect mode is not "follow",
+ then return a network error.
+
+
Set request's response tainting to "opaque".
+
+
Return the result of running override fetch given "scheme-fetch" and
+ fetchParams.
+
+
Return a network error. + +
Set request's
+ response tainting to
+ "cors".
+
+
Let corsWithPreflightResponse be the result of running override fetch
+ given "http-fetch", fetchParams, and true.
+
+
If corsWithPreflightResponse is a network error, then + clear cache entries using request. + +
Return corsWithPreflightResponse. +
Set request's
+ response tainting to
+ "cors".
+
+
Return the result of running override fetch given "http-fetch" and
+ fetchParams.
+
If recursive is true, then return response. + +
If response is not a network error and response is not a + filtered response, then: + +
If request's response tainting is "cors", then:
+
+
Let headerNames be the result of extracting header list values given
+ `Access-Control-Expose-Headers` and response's
+ header list.
+
+
If request's credentials mode is not
+ "include" and headerNames contains `*`, then set
+ response's CORS-exposed header-name list to all unique
+ header names in response's
+ header list.
+
+
Otherwise, if headerNames is non-null or failure, then set response's + CORS-exposed header-name list to headerNames. + +
One of the headerNames can still be `*` at this point,
+ but will only match a header whose name is `*`.
+
Set response to the following filtered response with response as + its internal response, depending on request's + response tainting: + +
basic"
+ cors"
+ opaque"
+ Let internalResponse be response, if response is a + network error; otherwise response's + internal response. + +
If internalResponse's URL list is empty, then + set it to a clone of request's URL list. + +
A response's URL list can be empty, e.g., when
+ fetching an about: URL.
+
+
+
Set internalResponse's redirect taint to request's + redirect-taint. + +
If request's timing allow failed flag is unset, then set + internalResponse's timing allow passed flag. + +
If response is not a network error and any of the following returns + blocked + +
should internalResponse to request be blocked as mixed content + +
should internalResponse to request be blocked by Content Security Policy + +
should internalResponse to request be blocked due to its MIME type + +
should internalResponse to request be blocked due to nosniff +
then set response and internalResponse to a network error. + +
If response's type is "opaque",
+ internalResponse's status is 206, internalResponse's
+ range-requested flag is set, and request's
+ header list does not contain `Range`,
+ then set response and internalResponse to a network error.
+
+
Traditionally, APIs accept a ranged response even if a range was not requested. This prevents
+ a partial response from an earlier ranged request being provided to an API that did not make a
+ range request.
+
+ The above steps prevent the following attack:
+
+ A media element is used to request a range of a cross-origin HTML resource. Although this is
+ invalid media, a reference to a clone of the response can be retained in a service worker. This
+ can later be used as the response to a script element's fetch. If the partial response is valid
+ JavaScript (even though the whole resource is not), executing it would leak private data.
+ Further details
+
+
If response is not a network error and
+ either request's method is
+ `HEAD` or `CONNECT`, or internalResponse's
+ status is a null body status,
+ set internalResponse's body to
+ null and disregard any enqueuing toward it (if any).
+
+
This standardizes the error handling for servers that violate HTTP. + +
If request's integrity metadata is not the empty string, then: + +
Let processBodyError be this step: run fetch response handover given + fetchParams and a network error. + +
If response's body is null, then run + processBodyError and abort these steps. + +
Let processBody given bytes be these steps: + +
If bytes do not match + request's integrity metadata, then run + processBodyError and abort these steps. [[!SRI]] + +
Run fetch response handover given fetchParams and response. +
Fully read response's body given + processBody and processBodyError. +
Otherwise, run fetch response handover given fetchParams and + response. +
The fetch response handover, given a fetch params +fetchParams and a response response, run these steps: + +
Let timingInfo be fetchParams's + timing info. + +
If response is not a network error and fetchParams's
+ request's client is a secure context, then set
+ timingInfo's server-timing headers to the
+ result of getting, decoding, and splitting `Server-Timing`
+ from response's internal response's
+ header list.
+
+
Using _response_'s internal response is safe as
+ exposing `Server-Timing` header data is guarded through the
+ `Timing-Allow-Origin` header.
+
+
The user agent may decide to expose `Server-Timing` headers to non-secure contexts
+ requests as well.
+
+
If fetchParams's request's
+ destination is "document", then set fetchParams's
+ controller's full timing info to
+ fetchParams's timing info.
+
+
Let processResponseEndOfBody be the following steps: + +
Let unsafeEndTime be the unsafe shared current time. + +
Set fetchParams's controller's + report timing steps to the following steps given a + global object global: + +
If fetchParams's request's URL's + scheme is not an HTTP(S) scheme, then return. + +
Set timingInfo's end time to the + relative high resolution time given unsafeEndTime and + global. + +
Let cacheState be response's cache state. + +
Let bodyInfo be response's body info. + +
If response's timing allow passed flag is not set, + then set timingInfo to the result of creating an opaque timing info for + timingInfo and set cacheState to the empty string. + +
This covers the case of response being a network error. + +
Let responseStatus be 0. + +
If fetchParams's request's mode is
+ not "navigate" or response's
+ redirect taint is "same-origin":
+
+
Set responseStatus to response's status. + +
Let mimeType be the result of + extracting a MIME type from response's + header list. + +
If mimeType is not failure, then set bodyInfo's + content type to the result of + minimizing a supported MIME type given mimeType. +
If fetchParams's request's + initiator type is non-null, then mark resource timing given + timingInfo, fetchParams's request's + URL, fetchParams's request's + initiator type, global, cacheState, + bodyInfo, and responseStatus. +
Let processResponseEndOfBodyTask be the following steps: + +
If fetchParams's process response end-of-body is + non-null, then run fetchParams's + process response end-of-body given response. + +
If fetchParams's request's + initiator type is non-null and fetchParams's + request's client's + global object is fetchParams's + task destination, then run fetchParams's + controller's report timing steps given + fetchParams's request's client's + global object. +
Queue a fetch task to run processResponseEndOfBodyTask with + fetchParams's task destination. +
If fetchParams's process response is non-null, then + queue a fetch task to run fetchParams's + process response given response, with fetchParams's + task destination. + +
Let internalResponse be response, if response is a + network error; otherwise response's + internal response. + +
If internalResponse's body is null, then run + processResponseEndOfBody. + +
Otherwise:
+ +Let transformStream be a new {{TransformStream}}. + +
Let identityTransformAlgorithm be an algorithm which, given chunk, + enqueues chunk in transformStream. + +
Set up transformStream with + transformAlgorithm set to + identityTransformAlgorithm and + flushAlgorithm set to + processResponseEndOfBody. + +
Set internalResponse's body's stream to the + result of internalResponse's body's stream + piped through transformStream. +
This {{TransformStream}} is needed for the purpose of receiving a notification when + the stream reaches its end, and is otherwise an identity transform stream. + +
If fetchParams's process response consume body is + non-null, then: + +
Let processBody given nullOrBytes be this step: run + fetchParams's process response consume body given + response and nullOrBytes. + +
Let processBodyError be this step: run fetchParams's + process response consume body given response and failure. + +
If internalResponse's body is null, then + queue a fetch task to run processBody given null, with fetchParams's + task destination. + +
Otherwise, fully read internalResponse's + body given processBody, processBodyError, and + fetchParams's task destination. +
To override fetch, given "scheme-fetch" or
+"http-fetch" type, a fetch params fetchParams, and
+an optional boolean makeCORSPreflight (default false):
+
+
Let request be fetchParams' request. + +
Let response be the result of executing + potentially override response for a request on request. + +
If response is non-null, then return response. + +
Switch on type and run the associated step: + +
scheme fetch"
+ Set response be the result of running scheme fetch given + fetchParams. + +
HTTP fetch"
+ Set response be the result of running HTTP fetch given + fetchParams and makeCORSPreflight. +
Return response. +
The potentially override response for a request algorithm takes a request +request, and returns either a response or null. Its behavior is +implementation-defined, allowing user agents to intervene on the request by +returning a response directly, or allowing the request to proceed by returning null. + +
By default, the algorithm has the following trivial implementation: + +
Return null. +
User agents will generally override this default implementation with a somewhat more complex + set of behaviors. For example, a user agent might decide that its users' safety is best preserved + by generally blocking requests to `https://unsafe.example/`, while synthesizing a shim for the + widely-used resource `https://unsafe.example/widget.js` to avoid breakage. That implementation + might look like the following: + +
If request's current url's host's
+ registrable domain is "unsafe.example":
+
+
If request's current url's path is
+ « "widget.js" »:
+
+
Return a network error. +
Return null. +
To scheme fetch, given a +fetch params fetchParams: + +
If fetchParams is canceled, then return the + appropriate network error for fetchParams. + +
Let request be fetchParams's request. + +
Switch on request's current URL's scheme and run + the associated steps: + +
about"
+ If request's current URL's path is the string
+ "blank", then return a new response whose
+ status message is `OK`, header list is «
+ (`Content-Type`, `text/html;charset=utf-8`) », and
+ body is the empty byte sequence as a body.
+
+
URLs such as "about:config" are handled during
+ navigation and result in a network error in the context of
+ fetching.
+
+
blob"
+ Let blobURLEntry be request's current URL's + blob URL entry. + +
If request's method is not `GET` or
+ blobURLEntry is null, then return a network error. [[!FILEAPI]]
+
+
The `GET` method restriction serves no useful purpose
+ other than being interoperable.
+
+
Let requestEnvironment be the result of + determining the environment given request. + +
Let isTopLevelSelfFetch be false. + +
If request's client is non-null: + +
Let global be request's client's + global object. + +
If all of the following conditions are true: + +
global is a {{Window}} object; + +
global's navigable is not null; + +
requestEnvironment's creation URL + equals request's current URL, +
then set isTopLevelSelfFetch to true. +
Let stringOrEnvironment be the result of these steps: + +
If request's destination is "document",
+ then return "top-level-navigation".
+
+
If isTopLevelSelfFetch is true, then return
+ "top-level-self-fetch".
+
+
Return requestEnvironment. +
Let blob be the result of obtaining a blob object given + blobURLEntry and stringOrEnvironment. + +
If blob is not a {{Blob}} object, then return a network error. + +
Let response be a new response. + +
Let fullLength be blob's {{Blob/size}}. + +
Let serializedFullLength be fullLength, + serialized and isomorphic encoded. + +
Let type be blob's {{Blob/type}}. + +
If request's header list
+ does not contain `Range`:
+
+
Let bodyWithType be the result of safely extracting + blob. + +
Set response's status message to `OK`.
+
+
Set response's header list to «
+ (`Content-Length`, serializedFullLength),
+ (`Content-Type`, type) ».
+
Otherwise: + +
Set response's range-requested flag. + +
Let rangeHeader be the result of getting
+ `Range` from request's header list.
+
+
+
Let rangeValue be the result of parsing a single range header value + given rangeHeader and true. + +
If rangeValue is failure, then return a network error. + +
Let (rangeStart, rangeEnd) be rangeValue. + +
If rangeStart is null: + +
Set rangeStart to fullLength − rangeEnd. + +
Set rangeEnd to rangeStart + rangeEnd − 1. +
Otherwise: + +
If rangeStart is greater than or equal to fullLength, then + return a network error. + +
If rangeEnd is null or rangeEnd is greater than or equal to + fullLength, then set rangeEnd to fullLength − 1. +
Let slicedBlob be the result of invoking slice blob given + blob, rangeStart, rangeEnd + 1, and type. + +
A range header denotes an inclusive byte range, while the slice blob + algorithm input range does not. To use the slice blob algorithm, we have to increment + rangeEnd. + +
Let slicedBodyWithType be the result of + safely extracting slicedBlob. + +
Let serializedSlicedLength be slicedBlob's {{Blob/size}}, + serialized and isomorphic encoded. + +
Let contentRange be the result of invoking build a content range + given rangeStart, rangeEnd, and fullLength. + +
Set response's status to 206. + +
Set response's status message to
+ `Partial Content`.
+
+
Set response's header list to «
+ (`Content-Length`, serializedSlicedLength),
+ (`Content-Type`, type), (`Content-Range`,
+ contentRange) ».
+
Return response. +
data"
+ Let dataURLStruct be the result of running the
+ data: URL processor on request's current URL.
+
+
If dataURLStruct is failure, then return a network error. + +
Let mimeType be dataURLStruct's + MIME type, serialized. + +
Return a new response whose status message is
+ `OK`, header list is « (`Content-Type`,
+ mimeType) », and body is dataURLStruct's
+ body as a body.
+
file"
+ For now, unfortunate as it is, file: URLs are left as an exercise
+ for the reader.
+
+
When in doubt, return a network error. + +
Return the result of running HTTP fetch given fetchParams. +
Return a network error. +
To determine the environment, given a request +request: + +
If request's reserved client is non-null, then return + request's reserved client. + +
If request's client is non-null, then return + request's client. + +
Return null. +
To HTTP fetch, given a fetch params +fetchParams and an optional boolean makeCORSPreflight (default false), run +these steps: + + +
Let request be fetchParams's request. + +
Let response and internalResponse be null. + +
If request's service-workers mode is "all", then:
+
+
Let requestForServiceWorker be a clone of + request. + +
If requestForServiceWorker's body is non-null, then: + +
Let transformStream be a new {{TransformStream}}. + +
Let transformAlgorithm given chunk be these steps: + +
If fetchParams is canceled, then abort these + steps. + +
If chunk is not a {{Uint8Array}} object, then + terminate fetchParams's + controller. + +
Otherwise, enqueue chunk in + transformStream. The user agent may split the chunk into + implementation-defined practical sizes and enqueue each of + them. The user agent also may concatenate the chunks into an implementation-defined + practical size and enqueue it. +
Set up transformStream with + transformAlgorithm set to + transformAlgorithm. + +
Set requestForServiceWorker's body's stream to + the result of requestForServiceWorker's body's stream + piped through transformStream. +
Let serviceWorkerStartTime be the coarsened shared current time + given fetchParams's cross-origin isolated capability. + +
Let fetchResponse be the result of invoking handle fetch for + requestForServiceWorker, with fetchParams's + controller and fetchParams's + cross-origin isolated capability. [[!HTML]] [[!SW]] + +
If fetchResponse is a response: + +
Set response to fetchResponse. + +
Set fetchParams's timing info's + final service worker start time to + serviceWorkerStartTime. + +
Set fetchParams's timing info's + service worker timing info to response's + service worker timing info. + +
If request's body is non-null, then + cancel request's body with undefined. + +
Set internalResponse to response, if response is not a + filtered response; otherwise to response's + internal response. + +
If one of the following is true + +
response's type is "error"
+
+
request's mode is "same-origin" and
+ response's type is "cors"
+
+
request's mode is not "no-cors" and
+ response's type is "opaque"
+
+
manual" and
+ response's type is "opaqueredirect"
+
+ follow" and
+ response's URL list has more than one item
+ then return a network error. +
Otherwise, if fetchResponse is a service worker timing info, + then set fetchParams's timing info's + service worker timing info to fetchResponse. +
If response is null, then: + +
If makeCORSPreflight is true and one of these conditions is true: + +
There is no method cache entry match for request's + method using request, and either request's + method is not a CORS-safelisted method or request's + use-CORS-preflight flag is set. + +
Then: + +
Let preflightResponse be the result of running CORS-preflight fetch + given request. + +
If preflightResponse is a network error, then return + preflightResponse. +
This step checks the CORS-preflight cache and if there is no suitable entry + it performs a CORS-preflight fetch which, if successful, populates the cache. The purpose + of the CORS-preflight fetch is to ensure the fetched resource is + familiar with the CORS protocol. The cache is there to minimize the number of + CORS-preflight fetches. + +
If request's redirect mode is "follow", then set
+ request's service-workers mode to "none".
+
+
Redirects coming from the network (as opposed to from a service worker) are not to + be exposed to a service worker. + +
Set response and internalResponse to the result of running + HTTP-network-or-cache fetch given fetchParams. + +
If request's response tainting is "cors" and a
+ CORS check for request and response returns failure, then return a
+ network error.
+
+
As the CORS check is not to be applied to responses whose + status is 304 or 407, or responses from a service worker for + that matter, it is applied here. + +
If the TAO check for request and response returns failure, + then set request's timing allow failed flag. +
If either request's response tainting or response's
+ type is "opaque", and the
+ cross-origin resource policy check with request's origin,
+ request's client, request's
+ destination, and internalResponse returns blocked, then
+ return a network error.
+
+
The cross-origin resource policy check runs for responses coming from the + network and responses coming from the service worker. This is different from the + CORS check, as request's client and the service worker can + have different embedder policies. + +
If internalResponse's status is a redirect status: + +
If internalResponse's status is not 303, request's
+ body is non-null, and the connection uses HTTP/2, then user agents
+ may, and are even encouraged to, transmit an RST_STREAM frame.
+
+
303 is excluded as certain communities ascribe special status to it. + +
Switch on request's + redirect mode: + +
error"
+ Set response to a network error.
manual"
+ If request's mode is "navigate", then set
+ fetchParams's controller's
+ next manual redirect steps to run HTTP-redirect fetch
+ given fetchParams and response.
+
+
Otherwise, set response to an opaque-redirect filtered response + whose internal response is internalResponse. +
follow"
+ Set response to the result of running HTTP-redirect fetch given + fetchParams and response.
Return response. Typically internalResponse's + body's stream is still being enqueued to after + returning. +
To HTTP-redirect fetch, given a +fetch params fetchParams and a response response, +run these steps: + +
Let request be fetchParams's request. + +
Let internalResponse be response, if response is not a + filtered response; otherwise response's + internal response. + +
Let locationURL be internalResponse's location URL + given request's current URL's fragment. + +
If locationURL is null, then return response. + +
If locationURL is failure, then return a network error. + +
If locationURL's scheme is not an HTTP(S) scheme, then + return a network error. + +
If request's redirect count is 20, then return a + network error. + +
Increase request's redirect count by 1. + +
If request's mode is "cors",
+ locationURL includes credentials, and request's
+ origin is not same origin with locationURL's
+ origin, then return a network error.
+
+
If request's response tainting is "cors" and
+ locationURL includes credentials, then return a network error.
+
+
This catches a cross-origin resource redirecting to a same-origin URL. + +
If internalResponse's status is not 303, request's + body is non-null, and request's body's + source is null, then return a network error. + +
If one of the following is true + +
internalResponse's status is 301 or 302 and
+ request's method is `POST`
+
internalResponse's status is 303 and request's
+ method is not `GET` or `HEAD`
+
then: + +
Set request's method to `GET` and
+ request's body to null.
+
+
For each headerName of request-body-header name, + delete headerName from request's + header list. +
If request's current URL's origin is not + same origin with locationURL's origin, then + for each headerName of CORS non-wildcard request-header name, + delete headerName from request's + header list. + +
I.e., the moment another origin is seen after the initial request, the
+ `Authorization` header is removed.
+
+
If request's body is non-null, then set request's + body to the body of the result of + safely extracting request's body's + source. + +
request's body's source's nullity has + already been checked. + +
Let timingInfo be fetchParams's timing info. + +
Set timingInfo's redirect end time and + post-redirect start time to the + coarsened shared current time given fetchParams's + cross-origin isolated capability. + +
If timingInfo's redirect start time is 0, then set + timingInfo's redirect start time to + timingInfo's start time. + +
Invoke set request's referrer policy on redirect on request and + internalResponse. [[!REFERRER]] + +
Let recursive be true. + +
If request's redirect mode is "manual", then:
+
+
Return the result of running main fetch given fetchParams and + recursive. + +
This has to invoke main fetch to get request's + response tainting correct. +
To HTTP-network-or-cache fetch, given a +fetch params fetchParams, an optional boolean +isAuthenticationFetch (default false), and an optional boolean +isNewConnectionFetch (default false), run these steps: + +
Some implementations might support caching of partial content, as per +HTTP Caching. However, this is not widely supported by browser caches. +[[HTTP-CACHING]] + +
Let request be fetchParams's request. + +
Let httpFetchParams be null. + +
Let httpRequest be null. + +
Let response be null. + +
Let storedResponse be null. + +
Let httpCache be null. + +
Let the revalidatingFlag be unset. + +
Run these steps, but abort when fetchParams is + canceled: + +
If request's traversable for user prompts is
+ "no-traversable" and request's redirect mode is
+ "error", then set httpFetchParams to fetchParams and
+ httpRequest to request.
+
+
Otherwise: + +
Set httpRequest to a clone of request. + +
Implementations are encouraged to avoid teeing request's + body's stream when request's + body's source is null as only a single body is needed in + that case. E.g., when request's body's source + is null, redirects and authentication will end up failing the fetch. + +
Set httpFetchParams to a copy of fetchParams. + +
Set httpFetchParams's request to + httpRequest. +
If user prompts or redirects are possible, then the user agent might need to + re-send the request with a new set of headers after the user answers the prompt or the redirect + location is determined. At that time, the original request body might have been partially sent + already, so we need to clone the request (including the body) beforehand so that we have a + spare copy available. + +
Let includeCredentials be true if one of + +
include"
+ same-origin" and request's
+ response tainting is "basic"
+ is true; otherwise false. + +
If Cross-Origin-Embedder-Policy allows credentials with request returns + false, then set includeCredentials to false. + +
Let contentLength be httpRequest's body's + length, if httpRequest's body is non-null; + otherwise null. + +
Let contentLengthHeaderValue be null. + +
If httpRequest's body is null and httpRequest's
+ method is `POST` or `PUT`, then set
+ contentLengthHeaderValue to `0`.
+
+
+
If contentLength is non-null, then set contentLengthHeaderValue to + contentLength, serialized and + isomorphic encoded. + +
If contentLengthHeaderValue is non-null, then append
+ (`Content-Length`, contentLengthHeaderValue) to httpRequest's
+ header list.
+
+
If contentLength is non-null and httpRequest's + keepalive is true, then: + +
Let inflightKeepaliveBytes be 0. + +
Let group be httpRequest's client's + fetch group. + +
Let inflightRecords be the set of fetch records in + group whose request's keepalive is true + and done flag is unset. + +
For each fetchRecord of inflightRecords: + +
+ +If the sum of contentLength and inflightKeepaliveBytes is greater + than 64 kibibytes, then return a network error. +
The above limit ensures that requests that are allowed to outlive the + environment settings object and contain a body, have a bounded size and are not allowed + to stay alive indefinitely. + +
If httpRequest's referrer is a URL, then: + +
Let referrerValue be httpRequest's referrer, + serialized and isomorphic encoded. + +
Append (`Referer`, referrerValue) to
+ httpRequest's header list.
+
Append a request `Origin` header for httpRequest.
+
+
Append the Fetch metadata headers for httpRequest. + [[!FETCH-METADATA]] + +
If httpRequest's initiator is "prefetch", then
+ set a structured field value given (`Sec-Purpose`,
+ the token prefetch) in
+ httpRequest's header list.
+
+
If httpRequest's header list does not contain `User-Agent`, then user agents should:
+
+
Let userAgent be httpRequest's client's
+ environment default `User-Agent` value.
+
+
Append (`User-Agent`, userAgent) to
+ httpRequest's header list.
+
If httpRequest's cache mode is "default" and
+ httpRequest's header list contains
+ `If-Modified-Since`,
+ `If-None-Match`,
+ `If-Unmodified-Since`,
+ `If-Match`, or
+ `If-Range`, then set httpRequest's
+ cache mode to "no-store".
+
+
If httpRequest's cache mode is "no-cache",
+ httpRequest's prevent no-cache cache-control header modification flag
+ is unset, and httpRequest's header list
+ does not contain `Cache-Control`, then
+ append (`Cache-Control`, `max-age=0`) to
+ httpRequest's header list.
+
+
If httpRequest's cache mode is "no-store" or
+ "reload", then:
+
+
If httpRequest's header list
+ does not contain `Pragma`, then
+ append (`Pragma`, `no-cache`) to
+ httpRequest's header list.
+
+
If httpRequest's header list
+ does not contain `Cache-Control`, then
+ append (`Cache-Control`, `no-cache`) to
+ httpRequest's header list.
+
+
If httpRequest's header list contains
+ `Range`, then append (`Accept-Encoding`,
+ `identity`) to httpRequest's header list.
+
+
This avoids a failure when handling content codings with + a part of an encoded response. + +
Additionally,
+ many servers
+ mistakenly ignore `Range` headers if a non-identity encoding is accepted.
+
Modify httpRequest's header list per HTTP. Do not + append a given header if httpRequest's + header list contains that header's + name. + +
It would be great if we could make this more normative somehow. At this point
+ headers such as
+ `Accept-Encoding`,
+ `Connection`,
+ `DNT`, and
+ `Host`,
+ are to be appended if necessary.
+
+
`Accept`,
+ `Accept-Charset`, and
+ `Accept-Language` must not be included at this point.
+
+
`Accept` and `Accept-Language` are already included
+ (unless fetch() is used, which does not include the latter by
+ default), and `Accept-Charset` is a waste of bytes. See
+ HTTP header layer division for more details.
+
+
If includeCredentials is true, then: + +
Append a request `Cookie` header for httpRequest.
+
+
If httpRequest's header list
+ does not contain `Authorization`, then:
+
+
+
Let authorizationValue be null. + +
If there's an authentication entry for httpRequest and either + httpRequest's use-URL-credentials flag is unset or + httpRequest's current URL does not include credentials, + then set authorizationValue to authentication entry. + + +
Otherwise, if httpRequest's current URL does
+ include credentials and isAuthenticationFetch is true, set
+ authorizationValue to httpRequest's current URL,
+ converted to an `Authorization` value.
+
+
If authorizationValue is non-null, then append
+ (`Authorization`, authorizationValue) to httpRequest's
+ header list.
+
If there's a proxy-authentication entry, use it as appropriate. + +
This intentionally does not depend on httpRequest's + credentials mode. + +
Set httpCache to the result of determining the HTTP cache partition, + given httpRequest. + +
If httpCache is null, then set httpRequest's
+ cache mode to "no-store".
+
+
If httpRequest's cache mode is neither "no-store"
+ nor "reload", then:
+
+
Set storedResponse to the result of selecting a response from the + httpCache, possibly needing validation, as per the + "Constructing Responses from Caches" chapter of HTTP Caching, if any. + [[!HTTP-CACHING]] + +
As mandated by HTTP, this still takes the `Vary`
+ header into account.
+
+
If storedResponse is non-null, then: + + +
If cache mode is "default", storedResponse
+ is a stale-while-revalidate response, and httpRequest's
+ client is non-null, then:
+
+
Set response to storedResponse. + +
Set response's cache state to "local".
+
+
Let revalidateRequest be a clone of + request. + +
Set revalidateRequest's cache mode set to
+ "no-cache".
+
+
Set revalidateRequest's + prevent no-cache cache-control header modification flag. + +
Set revalidateRequest's service-workers mode set to
+ "none".
+
+
In parallel, run main fetch given a new fetch params whose + request is revalidateRequest. + +
This fetch is only meant to update the state of httpCache + and the response will be unused until another cache access. The stale response will be used + as the response to current request. This fetch is issued in the context of a client so if + it goes away the request will be terminated. +
Otherwise: + +
If storedResponse is a stale response, then set the + revalidatingFlag. + +
If the revalidatingFlag is set and httpRequest's
+ cache mode is neither "force-cache" nor
+ "only-if-cached", then:
+
+
If storedResponse's header list
+ contains `ETag`, then
+ append (`If-None-Match`, `ETag`'s
+ value) to httpRequest's header list.
+
+
If storedResponse's header list
+ contains `Last-Modified`, then
+ append (`If-Modified-Since`,
+ `Last-Modified`'s value) to httpRequest's
+ header list.
+
See also the "Sending a Validation Request" chapter of + HTTP Caching. [[!HTTP-CACHING]] + +
Otherwise, set response to storedResponse and set
+ response's cache state to "local".
+
If aborted, then return the appropriate network error for + fetchParams. + + + +
If response is null, then: + +
If httpRequest's cache mode is
+ "only-if-cached", then return a network error.
+
+
Let forwardResponse be the result of running HTTP-network fetch given + httpFetchParams, includeCredentials, and isNewConnectionFetch. + +
If httpRequest's method is unsafe and + forwardResponse's status is in the range 200 to 399, inclusive, + invalidate appropriate stored responses in httpCache, as per the + "Invalidating Stored Responses" chapter of HTTP Caching, and set + storedResponse to null. [[!HTTP-CACHING]] + +
If the revalidatingFlag is set and forwardResponse's + status is 304, then: + +
Update storedResponse's header list using + forwardResponse's header list, as per the + "Freshening Stored Responses upon Validation" chapter of HTTP Caching. + [[!HTTP-CACHING]] + +
This updates the stored response in cache as well. + +
Set response to storedResponse. + +
Set response's cache state to "validated".
+
If response is null, then: + +
Set response to forwardResponse. + +
Store httpRequest and forwardResponse in httpCache, as per + the "Storing Responses in Caches" chapter of HTTP Caching. + [[!HTTP-CACHING]] + +
If forwardResponse is a network error, this effectively caches + the network error, which is sometimes known as "negative caching". + +
The associated body info is stored in the cache + alongside the response. +
Set response's URL list to a clone of + httpRequest's URL list. + +
If httpRequest's header list contains
+ `Range`, then set response's range-requested flag.
+
+
Set response's request-includes-credentials to + includeCredentials. + +
If response's status is 401, httpRequest's
+ response tainting is not "cors", includeCredentials is
+ true, and request's traversable for user prompts is a
+ traversable navigable:
+
+
Needs testing: multiple `WWW-Authenticate` headers, missing,
+ parsing issues.
+
+
If request's body is non-null, then: + +
If request's body's source is null, + then return a network error. + +
Set request's body to the body + of the result of safely extracting request's + body's source. +
If request's use-URL-credentials flag is unset or + isAuthenticationFetch is true, then: + +
If fetchParams is canceled, then return the + appropriate network error for fetchParams. + +
Let username and password be the result of prompting the end user + for a username and password, respectively, in request's + traversable for user prompts. + +
Set the username given request's current URL and + username. + +
Set the password given request's current URL and + password. +
Set response to the result of running HTTP-network-or-cache fetch given + fetchParams and true. +
If response's status is 407, then: + +
If request's traversable for user prompts is
+ "no-traversable", then return a network error.
+
+
Needs testing: multiple `Proxy-Authenticate` headers, missing,
+ parsing issues.
+
+
If fetchParams is canceled, then return the + appropriate network error for fetchParams. + +
Prompt the end user as appropriate in request's + traversable for user prompts and store the result as a + proxy-authentication entry. [[!HTTP]] + +
Remaining details surrounding proxy authentication are defined by HTTP. + +
Set response to the result of running HTTP-network-or-cache fetch given + fetchParams. +
If all of the following are true + +
response's status is 421 + +
isNewConnectionFetch is false + +
request's body is null, or request's + body is non-null and request's body's + source is non-null +
then: + +
If fetchParams is canceled, then return the + appropriate network error for fetchParams. + +
Set response to the result of running HTTP-network-or-cache fetch given + fetchParams, isAuthenticationFetch, and true. +
If isAuthenticationFetch is true, then create an authentication entry for + request and the given realm. + +
Return response. Typically response's + body's stream is still being enqueued to after + returning. +
To HTTP-network fetch, given a fetch params +fetchParams, an optional boolean includeCredentials (default false), and an +optional boolean forceNewConnection (default false), run these steps: + +
Let request be fetchParams's request. + +
If request's client is offline, then return a + network error. + +
Let response be null. + +
Let timingInfo be fetchParams's timing info. + +
Let networkPartitionKey be the result of + determining the network partition key given request. + +
Let newConnection be "yes" if forceNewConnection is true;
+ otherwise "no".
+
+
Switch on request's mode: + +
websocket"
+ Let connection be the result of + obtaining a WebSocket connection, given + request's current URL. + +
webtransport"
+ Let connection be the result of obtaining a WebTransport connection, + given networkPartitionKey and request. + +
Let connection be the result of + obtaining a connection, given networkPartitionKey, + request's current URL, includeCredentials, and + newConnection. +
Run these steps, but abort when fetchParams is + canceled: + +
If connection is failure, then return a network error. + +
Set timingInfo's final connection timing info to + the result of calling clamp and coarsen connection timing info with + connection's timing info, timingInfo's + post-redirect start time, and fetchParams's + cross-origin isolated capability. + +
If connection is an HTTP/1.x connection, request's + body is non-null, and request's body's + source is null, then return a network error. + +
Set response to the result of making an HTTP request over connection + using request with the following caveats: + +
Follow the relevant requirements from HTTP. [[!HTTP]] [[!HTTP-CACHING]] + +
If request's body is non-null, and request's + body's source is null, then the user agent may have a + buffer of up to 64 kibibytes and store a part of request's body + in that buffer. If the user agent reads from request's body + beyond that buffer's size and the user agent needs to resend request, then instead + return a network error. + +
The resending is needed when the connection is timed out, for example. + +
The buffer is not needed when request's body's + source is non-null, because request's body can + be recreated from it. + +
When request's body's source is null, it + means body is created from a {{ReadableStream}} object, which means + body cannot be recreated and that is why the buffer is needed. +
While true: + +
Set timingInfo's + final network-response start time to the + coarsened shared current time given fetchParams's + cross-origin isolated capability, immediately after the user + agent's HTTP parser receives the first byte of the response (e.g., frame header bytes for + HTTP/2 or response status line for HTTP/1.x). + +
Wait until all the HTTP response headers are transmitted. + +
Let status be the HTTP response's status code. + +
If status is in the range 100 to 199, inclusive: + + +
If timingInfo's + first interim network-response start time is 0, then set + timingInfo's + first interim network-response start time to + timingInfo's final network-response start time. + +
If request's mode is "websocket" and
+ status is 101, then break.
+
+
If status is 103 and fetchParams's + process early hints response is non-null, then + queue a fetch task to run fetchParams's + process early hints response, with response. + +
Continue. +
These kind of HTTP responses are eventually followed by a "final" HTTP + response. + +
Break. +
The exact layering between Fetch and HTTP still needs to be sorted through and + therefore response represents both a response and + an HTTP response here. + +
If the HTTP request results in a TLS client certificate dialog, then: + +
If request's traversable for user prompts is a + traversable navigable, then make the dialog available in request's + traversable for user prompts. + +
Otherwise, return a network error. +
To transmit request's body body, run these steps: + +
If body is null and fetchParams's + process request end-of-body is non-null, then + queue a fetch task given fetchParams's + process request end-of-body and fetchParams's + task destination. + +
Otherwise, if body is non-null: + +
Let processBodyChunk given bytes be these steps: + +
If fetchParams is canceled, then abort these + steps. + +
Run this step in parallel: transmit bytes. + +
If fetchParams's + process request body chunk length is non-null, then run + fetchParams's process request body chunk length given + bytes's length. +
Let processEndOfBody be these steps: + +
If fetchParams is canceled, then abort these + steps. + +
If fetchParams's process request end-of-body is + non-null, then run fetchParams's + process request end-of-body. +
Let processBodyError given e be these steps: + +
If fetchParams is canceled, then abort these + steps. + +
If e is an "AbortError" {{DOMException}},
+ then abort fetchParams's
+ controller.
+
+
Otherwise, terminate fetchParams's + controller. +
Incrementally read request's body given + processBodyChunk, processEndOfBody, processBodyError, and + fetchParams's task destination. + +
If aborted, then: + +
If connection uses HTTP/2, then transmit an RST_STREAM frame.
+
+
Return the appropriate network error for fetchParams. +
Let |buffer| be an empty [=byte sequence=]. + +
This represents an internal buffer inside the network layer of the user agent. + +
Let stream be a new {{ReadableStream}}. + +
Let |pullAlgorithm| be the following steps: + +
Let |promise| be [=a new promise=]. + +
Run the following steps [=in parallel=]: + +
If the size of |buffer| is smaller than a lower limit chosen by the user agent and the + ongoing fetch is [=fetch/suspend|suspended=], [=fetch/resumed|resume=] the fetch. + +
Wait until |buffer| is not empty. + +
[=Queue a fetch task=] to run the following steps, with |fetchParams|'s + [=task destination=]. + +
[=ReadableStream/Pull from bytes=] |buffer| into |stream|. + +
If |stream| is [=ReadableStream/errored=], then [=fetch controller/terminate=] + |fetchParams|'s [=fetch params/controller=]. + +
[=/Resolve=] |promise| with undefined. +
Return |promise|. +
Let cancelAlgorithm be an algorithm that aborts + fetchParams's controller with reason, given + reason. + +
[=ReadableStream/set up with byte reading support|Set up=] |stream| with byte reading + support with [=ReadableStream/set up/pullAlgorithm=] set to |pullAlgorithm|, + [=ReadableStream/set up/cancelAlgorithm=] set to |cancelAlgorithm|. + +
Set response's body to a new body whose + stream is stream. + +
If includeCredentials is true, then the user agent should
+ parse and store response `Set-Cookie` headers given request and
+ response.
+
+
Run these steps in parallel: + +
Run these steps, but abort when fetchParams is + canceled: + +
While true: + +
If one or more bytes have been transmitted from response's message body, then: + +
Let bytes be the transmitted bytes. + +
Let codings be the result of extracting header list values given
+ `Content-Encoding` and response's header list.
+
+
Let filteredCoding be "@unknown".
+
+
If codings is null or failure, then set filteredCoding to + the empty string. + +
Otherwise, if codings's size is greater than 1, then set
+ filteredCoding to "multiple".
+
+
Otherwise, if codings[0] is the empty string, or it is supported by the + user agent, and is a byte-case-insensitive match for an entry listed in the + HTTP Content Coding Registry, then set filteredCoding to the result + of byte-lowercasing codings[0]. [[!IANA-HTTP-PARAMS]] + +
Set response's body info's + content encoding to filteredCoding. + +
Increase response's body info's + encoded size by bytes's + length. + +
Set bytes to the result of handling content + codings given codings and bytes. + +
This makes the `Content-Length` header unreliable
+ to the extent that it was reliable to begin with.
+
+
Increase response's body info's + decoded size by + bytes's length. + +
If bytes is failure, then terminate + fetchParams's controller. + +
Append |bytes| to |buffer|. + +
If the size of |buffer| is larger than an upper limit chosen by the user agent, ask + the user agent to [=fetch/suspend=] the ongoing fetch. +
Otherwise, if the bytes transmission for response's message body is done + normally and stream is readable, then + close stream, and abort these in-parallel steps. +
If aborted, then: + +
If fetchParams is aborted, then: + +
Set response's aborted flag. + +
If stream is readable, then + error stream with the result of + deserialize a serialized abort reason given fetchParams's + controller's serialized abort reason + and an implementation-defined realm. + +
Otherwise, if stream is readable, + error stream with a {{TypeError}}. + +
If connection uses HTTP/2, then transmit an RST_STREAM frame.
+
+
Otherwise, the user agent should close connection unless it would be bad for + performance to do so. + +
For instance, the user agent could keep the connection open if it knows there's + only a few bytes of transfer remaining on a reusable connection. In this case it could be + worse to close the connection and go through the handshake process again for the next fetch. +
These are run in parallel as at this point it is unclear whether + response's body is relevant (response might be a + redirect). + +
Return response. Typically response's + body's stream is still being enqueued to after + returning. +
This is effectively the user agent implementation of the check to see if the +CORS protocol is understood. The so-called CORS-preflight request. If successful it +populates the CORS-preflight cache to minimize the number of these +fetches. + +
To CORS-preflight fetch, given a request +request, run these steps: + +
Let preflight be a new request whose
+ method is `OPTIONS`,
+ URL list is a clone of request's
+ URL list,
+ initiator is request's initiator,
+ destination is request's destination,
+ origin is request's origin,
+ referrer is request's referrer,
+ referrer policy is request's referrer policy,
+ mode is "cors", and
+ response tainting is "cors".
+
+
The service-workers mode of preflight does not matter + as this algorithm uses HTTP-network-or-cache fetch rather than HTTP fetch. + +
Append (`Accept`, `*/*`) to
+ preflight's header list.
+
+
Append
+ (`Access-Control-Request-Method`, request's
+ method) to preflight's header list.
+
+
Let headers be the CORS-unsafe request-header names with + request's header list. + +
If headers is not empty, then: + +
Let value be the items in headers separated from each other by
+ `,`.
+
+
Append
+ (`Access-Control-Request-Headers`, value) to
+ preflight's header list.
+
This intentionally does not use combine, as 0x20 following + 0x2C is not the way this was implemented, for better or worse. + +
Let response be the result of running HTTP-network-or-cache fetch given + a new fetch params whose request is preflight. + +
If a CORS check for request and response returns success and + response's status is an ok status, then: + + +
The CORS check is done on request rather than preflight + to ensure the correct credentials mode is used. + +
Let methods be the result of extracting header list values given
+ `Access-Control-Allow-Methods` and response's
+ header list.
+
+
Let headerNames be the result of extracting header list values given
+ `Access-Control-Allow-Headers` and response's
+ header list.
+
+
If either methods or headerNames is failure, + return a network error. + +
If methods is null and request's use-CORS-preflight flag + is set, then set methods to a new list containing request's + method. + +
This ensures that a CORS-preflight fetch that happened due to + request's use-CORS-preflight flag being set is + cached. + +
If request's method is not in methods,
+ request's method is not a CORS-safelisted method, and
+ request's credentials mode is "include" or
+ methods does not contain `*`, then return a network error.
+
+
If one of request's header list's + names is a CORS non-wildcard request-header name and is not a + byte-case-insensitive match for an item in headerNames, then + return a network error. + +
For each unsafeName of the
+ CORS-unsafe request-header names with request's
+ header list, if unsafeName is not a byte-case-insensitive
+ match for an item in headerNames and request's
+ credentials mode is "include" or headerNames does not
+ contain `*`, return a network error.
+
+
Let max-age be the result of extracting header list values given
+ `Access-Control-Max-Age` and response's
+ header list.
+
+
If max-age is failure or null, then set max-age to 5. + +
If max-age is greater than an imposed limit on + max-age, then set max-age to the imposed limit. + +
If the user agent does not provide for a cache, then + return response. + +
For each method in methods for which there is a + method cache entry match using request, set matching entry's + max-age to max-age. + +
For each method in methods for which there is no + method cache entry match using request, create a new cache entry with + request, max-age, method, and null. + +
For each headerName in headerNames for which there is a + header-name cache entry match using request, set matching entry's + max-age to max-age. + +
For each headerName in headerNames for which there is no + header-name cache entry match using request, create a new cache entry + with request, max-age, null, and headerName. + +
Return response. +
Otherwise, return a network error. +
A user agent has an associated CORS-preflight cache. A +CORS-preflight cache is a list of cache entries. + +
A cache entry consists of: + +
*`, or a
+ method)
+ *`,
+ or a header name)
+Cache entries must be removed after the seconds specified in their +max-age field have passed since storing the entry. Cache entries may +be removed before that moment arrives. + +
To create a new cache entry, given request, +max-age, method, and headerName, run these steps: + +
Let entry be a cache entry, initialized as follows: + +
The result of determining the network partition key given + request + +
The result of byte-serializing a request origin with request + +
request's current URL + +
max-age + +
True if request's credentials mode is
+ "include", and false otherwise
+
+
method + +
headerName +
Append entry to the user agent's CORS-preflight cache. +
To clear cache entries, given a request, +remove any cache entries in the user agent's CORS-preflight cache +whose key is the result of +determining the network partition key given request, +byte-serialized origin is the result of +byte-serializing a request origin with request, and URL +is request's current URL. + +
There is a cache entry match for a cache entry +entry with request if entry's key is the +result of determining the network partition key given request, +entry's byte-serialized origin is the result of +byte-serializing a request origin with request, entry's +URL is request's current URL, and one of + +
include".
+is true. + +
There is a method cache entry match for
+method using request when there is a cache entry in the user agent's
+CORS-preflight cache for which there is a cache entry match with request
+and its method is method or `*`.
+
+
There is a header-name cache entry match for +headerName using request when there is a cache entry in the user +agent's CORS-preflight cache for which there is a cache entry match with +request and one of + +
*` and headerName is not
+ a CORS non-wildcard request-header name
+is true. + + +
To perform a CORS check for a request and +response, run these steps: + +
Let origin be the result of getting
+ `Access-Control-Allow-Origin` from response's
+ header list.
+
+
If origin is null, then return failure. + +
Null is not `null`.
+
+
If request's credentials mode is not "include"
+ and origin is `*`, then return success.
+
+
If the result of byte-serializing a request origin with request is not + origin, then return failure. + +
If request's credentials mode is not "include",
+ then return success.
+
+
Let credentials be the result of getting
+ `Access-Control-Allow-Credentials` from response's
+ header list.
+
+
If credentials is `true`, then return success.
+
+
Return failure. +
To perform a TAO check for a request and +response, run these steps: + +
If request's timing allow failed flag is set, then return + failure. + +
Let values be the result of
+ getting, decoding, and splitting `Timing-Allow-Origin` from
+ response's header list.
+
+
If values contains "*", then return success.
+
+
If values contains the result of + serializing a request origin with request, then return success. + +
If request's mode is "navigate" and
+ request's current URL's origin is not
+ same origin with request's origin, then return failure.
+
+
This is necessary for navigations of a nested navigable. There, + request's origin would be the container document's + origin and the TAO check would return failure. Since navigation timing + never validates the results of the TAO check, the nested document would still have access + to the full timing information, but the container document would not. + +
If request's response tainting is "basic", then
+ return success.
+
+
Return failure. +
Deferred fetching allows callers to request that a fetch is invoked at the latest possible +moment, i.e., when a fetch group is terminated, or after a +timeout. + +
The deferred fetch task source is a task source used to update the result of a
+deferred fetch. User agents must prioritize tasks in this task source before other task
+sources, specifically task sources that can result in running scripts such as the
+DOM manipulation task source, to reflect the most recent state of a
+fetchLater() call before running any scripts that might depend on it.
+
+
To queue a deferred fetch given a request request, a null or +{{DOMHighResTimeStamp}} activateAfter, and onActivatedWithoutTermination, +which is an algorithm that takes no arguments: + +
Populate request from client given request. + +
Set request's service-workers mode to "none".
+
+
Set request's keepalive to true. + +
Let deferredRecord be a new deferred fetch record whose + request is request, and whose + notify invoked is + onActivatedWithoutTermination. + +
Append deferredRecord to request's + client's fetch group's + deferred fetch records. + +
If activateAfter is non-null, then run the following steps in parallel: + +
The user agent should wait until any of the following conditions is met: + +
At least activateAfter milliseconds have passed. + +
The user agent has a reason to believe that it is about to lose the opportunity to
+ execute scripts, e.g., when the browser is moved to the background, or when
+ request's client's
+ global object is a {{Window}} object whose
+ associated document had a "hidden" visibility state for
+ a long period of time.
+
Process deferredRecord. +
Return deferredRecord. +
To compute the total request length of a request request: + +
Let totalRequestLength be the length of request's + URL, serialized with + exclude fragment set to true. + +
Increment totalRequestLength by the length of + request's referrer, serialized. + +
For each (name, value) of request's + header list, increment totalRequestLength by name's + length + value's length. + +
Increment totalRequestLength by request's body's + length. + +
Return totalRequestLength. +
To process deferred fetches given a fetch group fetchGroup: + +
For each deferred fetch record + deferredRecord of fetchGroup's + deferred fetch records, process a deferred fetch + deferredRecord. +
To process a deferred fetch deferredRecord: +
If deferredRecord's invoke state is not
+ "pending", then return.
+
+
Set deferredRecord's invoke state to
+ "sent".
+
+
Queue a global task on the deferred fetch task source with + deferredRecord's request's + client's global object to run + deferredRecord's notify invoked. +
This section is non-normative. + +
The deferred-fetch quota is allocated to a top-level traversable (a "tab"), +amounting to 640 kibibytes. The top-level document and its same-origin directly nested documents can +use this quota to queue deferred fetches, or delegate some of it to cross-origin nested documents, +using permissions policy. + +
By default, 128 kibibytes out of these 640 kibibytes are allocated to delegating the quota to +cross-origin nested documents, each reserving 8 kibibytes. + +
The top-level document, and subsequently its nested documents, can control how much +of their quota is delegates to cross-origin child documents, using permissions policy. By default, +the "{{PermissionsPolicy/deferred-fetch-minimal}}" policy is enabled for any origin, while +"{{PermissionsPolicy/deferred-fetch}}" is enabled for the top-level document's origin only. By +relaxing the "{{PermissionsPolicy/deferred-fetch}}" policy for particular origins and nested +documents, the top-level document can allocate 64 kibibytes to those nested documents. Similarly, by +restricting the "{{PermissionsPolicy/deferred-fetch-minimal}}" policy for a particular origin or +nested document, the document can prevent the document from reserving the 8 kibibytes it would +receive by default. By disabling the "{{PermissionsPolicy/deferred-fetch-minimal}}" policy for the +top-level document itself, the entire 128 kibibytes delegated quota is collected back into the main +pool of 640 kibibytes. + +
Out of the allocated quota for a document, only 64 kibibytes can be used +concurrently for the same reporting origin (the request's URL's +origin). This prevents a situation where particular third-party libraries would reserve +quota opportunistically, before they have data to send. + +
Any of the following calls to fetchLater() would throw due to
+ the request itself exceeding the 64 kibibytes quota allocated to a reporting origin. Note that the
+ size of the request includes the URL itself, the body, the
+ header list, and the referrer.
+
+
+fetchLater(a_72_kb_url);
+fetchLater("https://origin.example.com", {headers: headers_exceeding_64kb});
+fetchLater(a_32_kb_url, {headers: headers_exceeding_32kb});
+fetchLater("https://origin.example.com", {method: "POST", body: body_exceeding_64_kb});
+fetchLater(a_62_kb_url /* with a 3kb referrer */);
+In the following sequence, the first two requests would succeed, but the third one would throw.
+ That's because the overall 640 kibibytes quota was not exceeded in the first two calls, however the
+ 3rd request exceeds the reporting-origin quota for https://a.example.com, and would
+ throw.
+
+
+fetchLater("https://a.example.com", {method: "POST", body: a_64kb_body});
+fetchLater("https://b.example.com", {method: "POST", body: a_64kb_body});
+fetchLater("https://a.example.com");
+Same-origin nested documents share the quota of their parent. However, cross-origin or + cross-agent iframes only receive 8kb of quota by default. So in the following example, the first + three calls would succeed and the last one would throw. + +
+// In main page
+fetchLater("https://a.example.com", {method: "POST", body: a_64kb_body});
+
+// In same-origin nested document
+fetchLater("https://b.example.com", {method: "POST", body: a_64kb_body});
+
+// In cross-origin nested document at https://fratop.example.com
+fetchLater("https://a.example.com", {body: a_5kb_body});
+fetchLater("https://a.example.com", {body: a_12kb_body});
+To make the previous example not throw, the top-level document can delegate some of its quota
+ to https://fratop.example.com, for example by serving the following header:
+
+
Permissions-Policy: deferred-fetch=(self "https://fratop.example.com")Each nested document reserves its own quota. So the following would work, because each frame + reserve 8 kibibytes: + +
+// In cross-origin nested document at https://fratop.example.com/frame-1
+fetchLater("https://a.example.com", {body: a_6kb_body});
+
+// In cross-origin nested document at https://fratop.example.com/frame-2
+fetchLater("https://a.example.com", {body: a_6kb_body});
+The following tree illustrates how quota is distributed to different nested documents in a tree: + +
https://top.example.com, with permissions policy set to
+ Permissions-policy: deferred-fetch=(self "https://ok.example.com")
+
https://top.example.com/frame: shares quota with the top-level traversable, as
+ they are same origin.
+
+
https://x.example.com: receives 8 kibibytes.
https://x.example.com: receives 8 kibibytes.
+
https://top.example.com: 0. Even though it's same origin with the
+ top-level traversable, it does not automatically share its quota as they are separated by a
+ cross-origin intermediary.
https://ok.example.com/good: receives 64 kibibytes, granted via the
+ "{{PermissionsPolicy/deferred-fetch}}" policy.
+
+
https://x.example.com: receives no quota. Only documents with the same
+ origin as the top-level traversable can grant the 8 kibibytes based on the
+ "{{PermissionsPolicy/deferred-fetch-minimal}}" policy.
https://ok.example.com/redirect, navigated to
+ https://x.example.com: receives no quota. The reserved 64 kibibytes for
+ https://ok.example.com are not available for
+ https://x.example.com.
+
+
https://ok.example.com/back, navigated to
+ https://top.example.com: shares quota with the top-level traversable, as they're
+ same origin.
+
In the above example, the top-level traversable and its same origin + descendants share a quota of 384 kibibytes. That value is computed as such: +
640 kibibytes are initially granted to the top-level traversable. + +
128 kibibytes are reserved for the "{{PermissionsPolicy/deferred-fetch-minimal}}" policy. + +
64 kibibytes are reserved for the container navigating to
+ https://ok.example/good.
+
+
64 kibibytes are reserved for the container navigating to
+ https://ok.example/redirect, and lost when it navigates away.
+
+
https://ok.example.com/back did not reserve 64 kibibytes, because it navigated
+ back to top-level traversable's origin.
+
+ 640 − 128 − 64 − 64 = 384 kibibytes. +
This specification defines a policy-controlled feature identified by the string
+"deferred-fetch". Its
+default allowlist is "self".
+
+
This specification defines a policy-controlled feature identified by the string
+"deferred-fetch-minimal". Its
+default allowlist is "*".
+
+
The quota reserved for deferred-fetch-minimal is 128 kibibytes.
+
+
Each navigable container has an associated number +reserved deferred-fetch quota. Its possible values are +minimal quota, which is 8 kibibytes, and +normal quota, which is 0 or 64 kibibytes. Unless +stated otherwise, it is 0. + +
To get the available deferred-fetch quota given a document +document and an origin-or-null origin: + +
Let controlDocument be document's + deferred-fetch control document. + +
Let navigable be controlDocument's node navigable. + +
Let isTopLevel be true if controlDocument's node navigable is a + top-level traversable; otherwise false. + +
Let deferredFetchAllowed be true if controlDocument is + allowed to use the policy-controlled feature + "{{PermissionsPolicy/deferred-fetch}}"; otherwise false. + +
Let deferredFetchMinimalAllowed be true if controlDocument is + allowed to use the policy-controlled feature + "{{PermissionsPolicy/deferred-fetch-minimal}}"; otherwise false. + +
Let quota be the result of the first matching statement: + +
640 kibibytes +
640kb should be enough for everyone. + +
512 kibibytes +
The default of 640 kibibytes, decremented By
+ quota reserved for deferred-fetch-minimal)
+
+
Let quotaForRequestOrigin be 64 kibibytes. + +
For each navigable in controlDocument's + node navigable's inclusive descendant navigables whose + active document's deferred-fetch control document is + controlDocument: + +
For each container in navigable's + active document's shadow-including inclusive descendants which is a + navigable container, decrement quota by container's + reserved deferred-fetch quota. + +
For each deferred fetch record deferredRecord of + navigable's active document's relevant settings object's + fetch group's + deferred fetch records: + +
Let requestLength be the total request length of + deferredRecord's request. + +
Decrement quota by requestLength. + +
If deferredRecord's request's + URL's origin is same origin with origin, + then decrement quotaForRequestOrigin by requestLength. +
If quota is equal or less than 0, then return 0. + +
If quota is less than quotaForRequestOrigin, then return + quota. + +
Return quotaForRequestOrigin. +
To reserve deferred-fetch quota for a navigable container +container given an origin originToNavigateTo: + +
This is called on navigation, when the source document of the navigation is the +navigable's parent document. It potentially reserves either 64kb or 8kb of quota for +the container and its navigable, if allowed by permissions policy. It is not observable to the +cotnainer document whether the reserved quota was used in practice. This algorithm assumes that the +container's document might delegate quota to the navigated container, and the reserved quota would +only apply in that case, and would be ignored if it ends up being shared. If quota was reserved and +the document ends up being same origin with its parent, the quota would be +freed. + +
Set container's reserved deferred-fetch quota to 0. + +
Let controlDocument be container's node document's + deferred-fetch control document. + +
If the inherited policy
+ for "{{PermissionsPolicy/deferred-fetch}}", container and originToNavigateTo
+ is "Enabled", and the available deferred-fetch quota for
+ controlDocument is equal or greater than
+ normal quota, then set container's
+ reserved deferred-fetch quota to normal quota and
+ return.
+
+
If all of the following conditions are true: + +
controlDocument's node navigable is a top-level traversable; + +
the inherited policy
+ for "{{PermissionsPolicy/deferred-fetch-minimal}}", container and
+ originToNavigateTo is "Enabled"; and
+
+
the size of controlDocument's node navigable's
+ descendant navigables, removing any navigable
+ whose navigable container's reserved deferred-fetch quota is not
+ minimal quota, is less than
+ quota reserved for deferred-fetch-minimal /
+ minimal quota,
+
then set container's reserved deferred-fetch quota to + minimal quota. +
To potentially free deferred-fetch quota for a document +document, if document's node navigable's container document is +not null, and its origin is same origin with document, then +set document's node navigable's navigable container's +reserved deferred-fetch quota to 0. + +
This is called when a document is created. It ensures that same-origin +nested documents don't reserve quota, as they anyway share their parent quota. It can only be called +upon document creation, as the origin of the document is only known +after redirects are handled. +
To get the deferred-fetch control document of a document +document: + +
If document' node navigable's container document is null or a + document whose origin is not same origin with + document, then return document; otherwise, return the + deferred-fetch control document given document's node navigable's + container document. +
The fetch() method is relatively low-level API for
+fetching resources. It covers slightly more ground than {{XMLHttpRequest}},
+although it is currently lacking when it comes to request progression (not response progression).
+
+
The fetch() method makes it quite straightforward to
+ fetch a resource and extract its contents as a {{Blob}}:
+
+
+fetch("/music/pk/altes-kamuffel.flac")
+ .then(res => res.blob()).then(playBlob)
+If you just care to log a particular response header: + +
+fetch("/", {method:"HEAD"})
+ .then(res => log(res.headers.get("strict-transport-security")))
+If you want to check a particular response header and then process the response of a + cross-origin resource: + +
+fetch("https://pk.example/berlin-calling.json", {mode:"cors"})
+ .then(res => {
+ if(res.headers.get("content-type") &&
+ res.headers.get("content-type").toLowerCase().indexOf("application/json") >= 0) {
+ return res.json()
+ } else {
+ throw new TypeError()
+ }
+ }).then(processJSON)
+If you want to work with URL query parameters: + +
+var url = new URL("https://geo.example.org/api"),
+ params = {lat:35.696233, long:139.570431}
+Object.keys(params).forEach(key => url.searchParams.append(key, params[key]))
+fetch(url).then(/* … */)
+If you want to receive the body data progressively: + +
+function consume(reader) {
+ var total = 0
+ return pump()
+ function pump() {
+ return reader.read().then(({done, value}) => {
+ if (done) {
+ return
+ }
+ total += value.byteLength
+ log(`received ${value.byteLength} bytes (${total} bytes in total)`)
+ return pump()
+ })
+ }
+}
+
+fetch("/music/pk/altes-kamuffel.flac")
+ .then(res => consume(res.body.getReader()))
+ .then(() => log("consumed the entire body without keeping the whole thing in memory!"))
+ .catch(e => log("something went wrong: " + e))
+
+typedef (sequence<sequence<ByteString>> or record<ByteString, ByteString>) HeadersInit;
+
+[Exposed=(Window,Worker)]
+interface Headers {
+ constructor(optional HeadersInit init);
+
+ undefined append(ByteString name, ByteString value);
+ undefined delete(ByteString name);
+ ByteString? get(ByteString name);
+ sequence<ByteString> getSetCookie();
+ boolean has(ByteString name);
+ undefined set(ByteString name, ByteString value);
+ iterable<ByteString, ByteString>;
+};
+
+
+A {{Headers}} object has an associated +header list (a +header list), which is initially empty. This +can be a pointer to the header list of something else, e.g., +of a request as demonstrated by {{Request}} +objects. + +
A {{Headers}} object also has an associated
+guard, which is a headers guard. A
+headers guard is "immutable", "request",
+"request-no-cors", "response" or "none".
+
+
headers = new Headers([init])
+ Creates a new {{Headers}} object. init can be used to fill its internal header list, + as per the example below. + +
+const meta = { "Content-Type": "text/xml", "Breaking-Bad": "<3" };
+new Headers(meta);
+
+// The above is equivalent to
+const meta2 = [
+ [ "Content-Type", "text/xml" ],
+ [ "Breaking-Bad", "<3" ]
+];
+new Headers(meta2);
+headers . append(name, value)
+ Appends a header to headers. + +
headers . delete(name)
+ Removes a header from headers. + +
headers . get(name)
+ Returns as a string the values of all headers whose name is name, separated by a + comma and a space. + +
headers . getSetCookie()
+ Returns a list of the values for all headers whose name is `Set-Cookie`.
+
+
headers . has(name)
+ Returns whether there is a header whose name is name. + +
headers . set(name, value)
+ Replaces the value of the first header whose name is name with value + and removes any remaining headers whose name is name. + +
for(const [name, value] of headers)
+ headers can be iterated over. +
To validate a header (name, value) for +a {{Headers}} object headers: + +
If name is not a header name or value is not a + header value, then throw a {{TypeError}}. + +
If headers's guard is "immutable", then
+ throw a {{TypeError}}.
+
+
If headers's guard is "request" and
+ (name, value) is a forbidden request-header, then return false.
+
+
If headers's guard is "response" and
+ name is a forbidden response-header name, then return false.
+
+
Return true. +
Steps for "request-no-cors" are not shared as you cannot have a fake
+value (for {{Headers/delete()}}) that always succeeds in CORS-safelisted request-header.
+
+
To append a header +(name, value) to a {{Headers}} object headers, run these steps: + +
Normalize value. + +
If validating (name, value) for headers + returns false, then return. + +
If headers's guard is "request-no-cors":
+
+
Let temporaryValue be the result of getting + name from headers's header list. + +
If temporaryValue is null, then set temporaryValue to + value. + +
Otherwise, set temporaryValue to temporaryValue, followed by + 0x2C 0x20, followed by value. + +
If (name, temporaryValue) is not a + no-CORS-safelisted request-header, then return. +
Append (name, value) to headers's + header list. + +
If headers's guard is "request-no-cors", then
+ remove privileged no-CORS request-headers from headers.
+
To fill a {{Headers}} object +headers with a given object object, run these steps: + +
+To +remove privileged no-CORS request-headers +from a {{Headers}} object (headers), run these steps: + +
For each headerName of + privileged no-CORS request-header names: + +
Delete headerName from headers's + header list. +
This is called when headers are modified by unprivileged code. +
The
+new Headers(init)
+constructor steps are:
+
+
The delete(name) method steps are:
+
+
If validating (name, ``) for this returns false, then + return. + +
Passing a dummy header value ought not to have any negative repercussions. + +
If this's guard is "request-no-cors", name
+ is not a no-CORS-safelisted request-header name, and name is not a
+ privileged no-CORS request-header name, then return.
+
+
If this's header list does not contain + name, then return. + + +
Delete name from this's + header list. + +
If this's guard is "request-no-cors", then
+ remove privileged no-CORS request-headers from this.
+
The get(name) method steps are:
+
+
If name is not a header name, then throw a {{TypeError}}. + +
Return the result of getting name from this's + header list. +
The getSetCookie() method steps are:
+
+
If this's header list does not contain
+ `Set-Cookie`, then return « ».
+
+
Return the values of all headers in this's
+ header list whose name is a byte-case-insensitive match
+ for `Set-Cookie`, in order.
+
The has(name) method steps are:
+
+
If name is not a header name, then throw a {{TypeError}}. + +
Return true if this's header list + contains name; otherwise false. +
The set(name, value)
+method steps are:
+
+
Normalize value. + +
If validating (name, value) for this returns + false, then return. + +
If this's guard is "request-no-cors" and
+ (name, value) is not a no-CORS-safelisted request-header, then return.
+
+
Set (name, value) in this's + header list. + +
If this's guard is "request-no-cors", then
+ remove privileged no-CORS request-headers from this.
+
The value pairs to iterate over are the return value of running +sort and combine with this's header list. + + +
+typedef (Blob or BufferSource or FormData or URLSearchParams or USVString) XMLHttpRequestBodyInit; + +typedef (ReadableStream or XMLHttpRequestBodyInit) BodyInit;+ +
To safely extract a body with type from a +byte sequence or {{BodyInit}} object object, run these steps: + +
If object is a {{ReadableStream}} object, then: + +
+ +Return the result of extracting object. +
The safely extract operation is a subset of the +extract operation that is guaranteed to not throw an exception. + +
To extract a +body with type from a byte sequence or {{BodyInit}} object +object, with an optional boolean +keepalive (default false), run these +steps: + +
Let stream be null. + +
If object is a {{ReadableStream}} object, then set stream to + object. + +
Otherwise, if object is a {{Blob}} object, set stream to the result of + running object's get stream. + +
Otherwise, set stream to a new {{ReadableStream}} object, and + [=ReadableStream/set up with byte reading support|set up=] stream with byte reading + support. + +
Assert: stream is a {{ReadableStream}} object. + +
Let action be null. + +
Let source be null. + +
Let length be null. + +
Let type be null. + +
Switch on object: + +
Set source to object. + +
Set length to object's {{Blob/size}}. + +
If object's {{Blob/type}} + attribute is not the empty byte sequence, set type to its value. + + +
Set source to object. + +
Set source to a copy of the bytes + held by object. + +
Set action to this step: run the
+ multipart/form-data encoding algorithm, with object's
+ entry list and UTF-8.
+
+
Set source to object. + +
Set length to unclear, see + html/6424 for improving this. + +
Set type to `multipart/form-data; boundary=`, followed by the
+ multipart/form-data boundary string generated by the
+ multipart/form-data encoding algorithm.
+
+
Set source to the result of running the
+ application/x-www-form-urlencoded serializer with
+ object's list.
+
+
Set type to
+ `application/x-www-form-urlencoded;charset=UTF-8`.
+
+
Set source to the UTF-8 encoding of object. + +
Set type to `text/plain;charset=UTF-8`.
+
+
If keepalive is true, then throw a {{TypeError}}. + +
If object is disturbed or + locked, then throw a {{TypeError}}. +
If source is a byte sequence, then set action to a step + that returns source and length to source's + length. + +
If action is non-null, then run these steps in parallel: + +
+ +Let body be a body whose stream is + stream, source is source, and length is + length. + +
Return (body, type). +
+interface mixin Body {
+ readonly attribute ReadableStream? body;
+ readonly attribute boolean bodyUsed;
+ [NewObject] Promise<ArrayBuffer> arrayBuffer();
+ [NewObject] Promise<Blob> blob();
+ [NewObject] Promise<Uint8Array> bytes();
+ [NewObject] Promise<FormData> formData();
+ [NewObject] Promise<any> json();
+ [NewObject] Promise<USVString> text();
+};
+
+Formats you would not want a network layer to be dependent upon, such as +HTML, will likely not be exposed here. Rather, an HTML parser API might accept a stream in +due course. + + +
Objects including the {{Body}} interface mixin have an associated +body (null or a body). + +
An object including the {{Body}} interface mixin is said to be +unusable if its body is non-null and its +body's stream is disturbed or +locked. + +
requestOrResponse . body
+ Returns requestOrResponse's body as {{ReadableStream}}. + +
requestOrResponse . bodyUsed
+ Returns whether requestOrResponse's body has been read from. + +
requestOrResponse . arrayBuffer()
+ Returns a promise fulfilled with requestOrResponse's body as {{ArrayBuffer}}. + +
requestOrResponse . blob()
+ Returns a promise fulfilled with requestOrResponse's body as {{Blob}}. + +
requestOrResponse . bytes()
+ Returns a promise fulfilled with requestOrResponse's body as {{Uint8Array}}. + +
requestOrResponse . formData()
+ Returns a promise fulfilled with requestOrResponse's body as {{FormData}}. + +
requestOrResponse . json()
+ Returns a promise fulfilled with requestOrResponse's body parsed as JSON. + +
requestOrResponse . text()
+ Returns a promise fulfilled with requestOrResponse's body as string. +
To get the MIME type, given a {{Request}} or +{{Response}} object requestOrResponse: + +
Let headers be null. + +
If requestOrResponse is a {{Request}} object, then set headers to + requestOrResponse's request's header list. + +
Otherwise, set headers to requestOrResponse's + response's header list. + +
Let mimeType be the result of extracting a MIME type + from headers. + +
If mimeType is failure, then return null. + +
Return mimeType. +
The body getter steps are to return null if
+this's body is null; otherwise this's body's
+stream.
+
The bodyUsed getter steps are to return true if
+this's body is non-null and this's body's
+stream is disturbed; otherwise false.
+
The consume body +algorithm, given an object that includes {{Body}} object and an algorithm that takes a +byte sequence and returns a JavaScript value or throws an exception +convertBytesToJSValue, runs these steps: + +
If object is unusable, then return a promise rejected with + a {{TypeError}}. + +
Let promise be a new promise. + +
If object's body is null, then run successSteps + with an empty byte sequence. + +
Otherwise, fully read object's body given + successSteps, errorSteps, and object's + relevant global object. + +
Return promise. +
The arrayBuffer() method steps are to return the result
+of running consume body with this and the following step given a
+byte sequence bytes: return the result of [=ArrayBuffer/creating=] an
+{{ArrayBuffer}} from bytes in this's relevant realm.
+
+
The above method can reject with a {{RangeError}}. +
The blob() method steps are to return the result
+of running consume body with this and the following step given a
+byte sequence bytes: return a {{Blob}} whose contents are bytes
+and whose {{Blob/type}} attribute is the result of get the MIME type with
+this.
+
+
The bytes() method steps are to return the result
+of running consume body with this and the following step given a
+byte sequence bytes: return the result of [=ArrayBufferView/create|creating=] a
+{{Uint8Array}} from bytes in this's relevant realm.
+
+
The above method can reject with a {{RangeError}}. +
The formData() method steps are to return the result of
+running consume body with this and the following steps given a
+byte sequence bytes:
+
+
Let mimeType be the result of get the MIME type with this. + +
If mimeType is non-null, then switch on mimeType's + essence and run the corresponding steps: + +
multipart/form-data"
+ Parse bytes, using the value of the `boundary` parameter from
+ mimeType, per the rules set forth in
+ Returning Values from Forms: multipart/form-data. [[!RFC7578]]
+
+
Each part whose `Content-Disposition` header contains a
+ `filename` parameter must be parsed into an entry whose
+ value is a {{File}} object whose contents are the contents of the part. The {{File/name}}
+ attribute of the {{File}} object must have the value of the `filename` parameter
+ of the part. The {{Blob/type}} attribute of the {{File}} object must have the value of the
+ `Content-Type` header of the part if the part has such header, and
+ `text/plain` (the default defined by [[!RFC7578]] section 4.4) otherwise.
+
+
Each part whose `Content-Disposition` header does not contain a
+ `filename` parameter must be parsed into an entry whose
+ value is the UTF-8 decoded without BOM content of the
+ part. This is done regardless of the presence or the value of a
+ `Content-Type` header and regardless of the presence or the value of a
+ `charset` parameter.
+
+
A part whose `Content-Disposition` header contains a
+ `name` parameter whose value is `_charset_` is parsed like any other
+ part. It does not change the encoding.
+
+
If that fails for some reason, then throw a {{TypeError}}. + +
Return a new {{FormData}} object, appending each entry, + resulting from the parsing operation, to its entry list. +
The above is a rough approximation of what is needed for
+ `multipart/form-data`, a more detailed parsing specification is to be written.
+ Volunteers welcome.
+
+
application/x-www-form-urlencoded"
+ Let entries be the result of parsing + bytes. + +
Return a new {{FormData}} object whose entry list is + entries. +
Throw a {{TypeError}}. +
The json() method steps are to return the result
+of running consume body with this and parse JSON from bytes.
+
+
The above method can reject with a {{SyntaxError}}. +
The text() method steps are to return the result
+of running consume body with this and UTF-8 decode.
+
+typedef (Request or USVString) RequestInfo;
+
+[Exposed=(Window,Worker)]
+interface Request {
+ constructor(RequestInfo input, optional RequestInit init = {});
+
+ readonly attribute ByteString method;
+ readonly attribute USVString url;
+ [SameObject] readonly attribute Headers headers;
+
+ readonly attribute RequestDestination destination;
+ readonly attribute USVString referrer;
+ readonly attribute ReferrerPolicy referrerPolicy;
+ readonly attribute RequestMode mode;
+ readonly attribute RequestCredentials credentials;
+ readonly attribute RequestCache cache;
+ readonly attribute RequestRedirect redirect;
+ readonly attribute DOMString integrity;
+ readonly attribute boolean keepalive;
+ readonly attribute boolean isReloadNavigation;
+ readonly attribute boolean isHistoryNavigation;
+ readonly attribute AbortSignal signal;
+ readonly attribute RequestDuplex duplex;
+
+ [NewObject] Request clone();
+};
+Request includes Body;
+
+dictionary RequestInit {
+ ByteString method;
+ HeadersInit headers;
+ BodyInit? body;
+ USVString referrer;
+ ReferrerPolicy referrerPolicy;
+ RequestMode mode;
+ RequestCredentials credentials;
+ RequestCache cache;
+ RequestRedirect redirect;
+ DOMString integrity;
+ boolean keepalive;
+ AbortSignal? signal;
+ RequestDuplex duplex;
+ RequestPriority priority;
+ any window; // can only be set to null
+};
+
+enum RequestDestination { "", "audio", "audioworklet", "document", "embed", "font", "frame", "iframe", "image", "json", "manifest", "object", "paintworklet", "report", "script", "sharedworker", "style", "track", "video", "worker", "xslt" };
+enum RequestMode { "navigate", "same-origin", "no-cors", "cors" };
+enum RequestCredentials { "omit", "same-origin", "include" };
+enum RequestCache { "default", "no-store", "reload", "no-cache", "force-cache", "only-if-cached" };
+enum RequestRedirect { "follow", "error", "manual" };
+enum RequestDuplex { "half" };
+enum RequestPriority { "high", "low", "auto" };
+
+
+"serviceworker" is omitted from
+RequestDestination as it cannot be observed from JavaScript. Implementations
+will still need to support it as a destination. "websocket" and
+"webtransport" are omitted from RequestMode as they cannot be
+used or observed from JavaScript.
+
+
A {{Request}} object has an associated +request (a request). + +
A {{Request}} object also has an associated headers (null or a +{{Headers}} object), initially null. + +
A {{Request}} object has an associated signal (null or an {{AbortSignal}} +object), initially null. + +
A {{Request}} object's body is its +request's +body. + +
request = new Request(input [,
+ init])
+ Returns a new request whose {{Request/url}} property is input if + input is a string, and input's {{Request/url}} if input is a + {{Request}} object. + +
The init argument is an object whose properties can be set as follows:
+ +about:client", or the empty string,
+ to set request's referrer.
+
+ cors".
+
+ same-origin".
+
+ half" is the only valid value and it is for initiating a half-duplex fetch
+ (i.e., the user agent sends the entire request before processing the response).
+ "full" is reserved for future use, for initiating a full-duplex fetch (i.e., the
+ user agent can process the response before sending the entire request). This member needs to be
+ set when {{RequestInit/body}} is a {{ReadableStream}} object. See
+ issue #1254 for defining
+ "full".
+
+ request . method
+ GET" by default.
+
+ request . url
+ request . headers
+ Host" header.
+
+ request . destination
+ document" or
+ "script".
+
+ request . referrer
+ about:client" when defaulting to the global's default. This is used during
+ fetching to determine the value of the `Referer` header of the request being made.
+
+ request . referrerPolicy
+ request . mode
+ request . credentials
+ request . cache
+ request . redirect
+ request . integrity
+ request . keepalive
+ request . isReloadNavigation
+ request . isHistoryNavigation
+ request . signal
+ request . duplex
+ half", meaning the fetch will be half-duplex (i.e., the user agent sends
+ the entire request before processing the response). In future, it could also return
+ "full", meaning the fetch will be full-duplex (i.e., the user agent can process the
+ response before sending the entire request) to indicate that the fetch will be full-duplex.
+ See issue #1254 for
+ defining "full".
+
+ request . clone()
+ Returns a clone of request. +
To create a {{Request}} object, given a +request request, headers guard guard, +{{AbortSignal}} object signal, and realm realm: + +
Let requestObject be a new {{Request}} object with realm. + +
Set requestObject's request to request. + +
Set requestObject's headers to a new {{Headers}} + object with realm, whose headers list is request's + headers list and guard is guard. + +
Set requestObject's signal to signal. + +
Return requestObject. +
The
+new Request(input, init)
+constructor steps are:
+
+
Let request be null. + +
Let fallbackMode be null. + +
Let baseURL be this's relevant settings object's + API base URL. + +
Let signal be null. + +
If input is a string, then: + +
+ +Otherwise: + +
+ +Let origin be this's relevant settings object's + origin. + +
Let traversableForUserPrompts be "client".
+
+
If request's traversable for user prompts + is an environment settings object and its + origin is same origin with + origin, then set traversableForUserPrompts to + request's traversable for user prompts. + +
If init["{{RequestInit/window}}"] exists and is non-null, then + throw a {{TypeError}}. + +
If init["{{RequestInit/window}}"] exists, then set
+ traversableForUserPrompts to "no-traversable".
+
+
Set request to a new request with the following properties: + +
fetch".
+ If init is not empty, then: + +
If request's mode is
+ "navigate", then set it to "same-origin".
+
+
+
Unset request's reload-navigation flag. + +
Unset request's history-navigation flag. + +
Set request's origin to "client".
+
+
Set request's referrer to "client".
+
+
Set request's referrer policy to the empty string. + +
Set request's URL to request's + current URL. + +
This is done to ensure that when a service worker "redirects" a request, e.g., from + an image in a cross-origin style sheet, and makes modifications, it no longer appears to come from + the original source (i.e., the cross-origin style sheet), but instead from the service worker that + "redirected" the request. This is important as the original source might not even be able to + generate the same kind of requests as the service worker. Services that trust the original source + could therefore be exploited were this not done, although that is somewhat farfetched. + +
If init["{{RequestInit/referrer}}"] exists, then: + +
Let referrer be init["{{RequestInit/referrer}}"]. + +
If referrer is the empty string, then set request's
+ referrer to "no-referrer".
+
+
Otherwise: + +
Let parsedReferrer be the result of parsing + referrer with baseURL. + +
If parsedReferrer is failure, then throw a {{TypeError}}. + +
If one of the following is true + +
parsedReferrer's scheme is "about" and
+ path is the string "client"
+
+
parsedReferrer's origin is not same origin with + origin +
then set request's referrer to "client".
+
+
+
Otherwise, set request's referrer to + parsedReferrer. +
If init["{{RequestInit/referrerPolicy}}"] exists, then set + request's referrer policy to it. + +
Let mode be init["{{RequestInit/mode}}"] if it exists, + and fallbackMode otherwise. + +
If mode is "navigate", then throw a {{TypeError}}.
+
+
If mode is non-null, set request's + mode to mode. + +
If init["{{RequestInit/credentials}}"] exists, then set + request's credentials mode to it. + +
If init["{{RequestInit/cache}}"] exists, then set + request's cache mode to it. + +
If request's cache mode is "only-if-cached" and
+ request's mode is not "same-origin", then
+ throw a {{TypeError}}.
+
+
If init["{{RequestInit/redirect}}"] exists, then set + request's redirect mode to it. + +
If init["{{RequestInit/integrity}}"] exists, then set + request's integrity metadata to it. + +
If init["{{RequestInit/keepalive}}"] exists, then set + request's keepalive to it. + +
If init["{{RequestInit/method}}"] exists, then: + +
Let method be init["{{RequestInit/method}}"]. + +
If method is not a method or method is a + forbidden method, then throw a {{TypeError}}. + +
Normalize method. + +
Set request's method to method. +
If init["{{RequestInit/signal}}"] exists, then set + signal to it. + +
If init["{{RequestInit/priority}}"] exists, then: + +
If request's internal priority is not null, then update + request's internal priority in an implementation-defined + manner. + +
Otherwise, set request's priority to + init["{{RequestInit/priority}}"]. +
Let signals be « signal » if signal is non-null; otherwise + « ». + +
Set this's signal to the result of + creating a dependent abort signal from signals, using {{AbortSignal}} and + this's relevant realm. + +
Set this's headers to a new {{Headers}} object with
+ this's relevant realm, whose header list is request's
+ header list and guard is "request".
+
+
If init is not empty, then: + +
The headers are sanitized as they might contain headers that are not allowed by this + mode. Otherwise, they were previously sanitized or are unmodified since they were set by a + privileged API. + +
Let headers be a copy of this's headers and its + associated header list. + +
If init["{{RequestInit/headers}}"] exists, then set + headers to init["{{RequestInit/headers}}"]. + +
Empty this's headers's header list. + +
If headers is a {{Headers}} object, then for each + header of its header list, append + header to this's headers. + +
Let inputBody be input's request's + body if input is a {{Request}} object; otherwise null. + +
If either init["{{RequestInit/body}}"] exists and is non-null or
+ inputBody is non-null, and request's method is
+ `GET` or `HEAD`, then throw a {{TypeError}}.
+
+
Let initBody be null. + +
If init["{{RequestInit/body}}"] exists and is non-null, then: + +
Let bodyWithType be the result of extracting + init["{{RequestInit/body}}"], with keepalive + set to request's keepalive. + +
Set initBody to bodyWithType's body. + +
Let type be bodyWithType's type. + +
If type is non-null and this's headers's
+ header list does not contain
+ `Content-Type`, then append (`Content-Type`,
+ type) to this's headers.
+
Let inputOrInitBody be initBody if it is non-null; otherwise + inputBody. + +
If inputOrInitBody is non-null and inputOrInitBody's + source is null, then: + +
+ +Let finalBody be inputOrInitBody. + +
If initBody is null and inputBody is non-null, then: + +
If inputBody is unusable, then throw a {{TypeError}}. + + + +
Set finalBody to the result of creating a proxy for + inputBody. +
The method getter steps are to return this's
+request's method.
+
+
The url getter steps are to return this's
+request's URL, serialized.
+
+
The headers getter steps are to return
+this's headers.
+
+
The destination getter are to return this's
+request's destination.
+
+
The referrer getter steps are:
+
+
The referrerPolicy getter steps are to return
+this's request's referrer policy.
+
+
The mode getter steps are to return this's
+request's mode.
+
+
The credentials getter steps are to return
+this's request's credentials mode.
+
+
The cache getter steps are to return this's
+request's cache mode.
+
+
The redirect getter steps are to return
+this's request's redirect mode.
+
+
The integrity getter steps are to return
+this's request's integrity metadata.
+
+
The keepalive getter steps are to return
+this's request's keepalive.
+
+
The isReloadNavigation getter steps are to return
+true if this's request's reload-navigation flag is set;
+otherwise false.
+
+
The isHistoryNavigation getter steps are to return
+true if this's request's history-navigation flag is
+set; otherwise false.
+
+
The signal getter steps are to return this's
+signal.
+
+
This's signal is always initialized in the +constructor and when +cloning. + +
The duplex getter steps are to return
+"half".
+
+
The clone() method steps are:
+
+
Let clonedRequest be the result of cloning + this's request. + +
Let clonedSignal be the result of creating a dependent abort signal from + « this's signal », using {{AbortSignal}} and this's + relevant realm. + +
Let clonedRequestObject be the result of creating a + {{Request}} object, given clonedRequest, this's headers's + guard, clonedSignal and this's relevant realm. + +
Return clonedRequestObject. +
[Exposed=(Window,Worker)]
+interface Response {
+ constructor(optional BodyInit? body = null, optional ResponseInit init = {});
+
+ [NewObject] static Response error();
+ [NewObject] static Response redirect(USVString url, optional unsigned short status = 302);
+ [NewObject] static Response json(any data, optional ResponseInit init = {});
+
+ readonly attribute ResponseType type;
+
+ readonly attribute USVString url;
+ readonly attribute boolean redirected;
+ readonly attribute unsigned short status;
+ readonly attribute boolean ok;
+ readonly attribute ByteString statusText;
+ [SameObject] readonly attribute Headers headers;
+
+ [NewObject] Response clone();
+};
+Response includes Body;
+
+dictionary ResponseInit {
+ unsigned short status = 200;
+ ByteString statusText = "";
+ HeadersInit headers;
+};
+
+enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredirect" };
+
+
+A {{Response}} object has an associated +response (a +response). + +
A {{Response}} object also has an associated headers (null or a +{{Headers}} object), initially null. + +
A {{Response}} object's body is its +response's body. + +
response = new Response(body = null [, init])
+ Creates a {{Response}} whose body is body, and status, status message, and + headers are provided by init. + +
response = Response . error()
+ Creates network error {{Response}}. + +
response = Response . redirect(url, status = 302)
+ Creates a redirect {{Response}} that redirects to url with status + status. + +
response = Response . json(data [, init])
+ Creates a {{Response}} whose body is the JSON-encoded data, and status, status + message, and headers are provided by init. + +
response . type
+ Returns response's type, e.g., "cors".
+
+
response . url
+ Returns response's URL, if it has one; otherwise the empty string. + +
response . redirected
+ Returns whether response was obtained through a redirect. + +
response . status
+ Returns response's status. + +
response . ok
+ Returns whether response's status is an ok status. + +
response . statusText
+ Returns response's status message. + +
response . headers
+ Returns response's headers as {{Headers}}. + +
response . clone()
+ Returns a clone of response. +
To create a {{Response}} object, given a +response response, headers guard guard, and +realm realm, run these steps: + +
Let responseObject be a new {{Response}} object with + realm. + +
Set responseObject's response to response. + +
Set responseObject's headers to a new + {{Headers}} object with realm, whose headers list is + response's headers list and guard is + guard. + +
Return responseObject. +
To initialize a response, given a {{Response}} object response, +{{ResponseInit}} init, and null or a body with type body: + +
If init["{{ResponseInit/status}}"] is not in the range 200 to 599, inclusive, + then throw a {{RangeError}}. + +
If init["{{ResponseInit/statusText}}"] is not the empty string and does not match + the reason-phrase token production, then throw a {{TypeError}}. + +
Set response's response's status to + init["{{ResponseInit/status}}"]. + +
Set response's response's status message + to init["{{ResponseInit/statusText}}"]. + +
If init["{{ResponseInit/headers}}"] exists, then + fill response's headers with + init["{{ResponseInit/headers}}"]. + +
If body is non-null, then: + +
If response's status is a null body status, then + throw a {{TypeError}}. + +
101 and 103 are included in null body status due to their use elsewhere. + They do not affect this step. + +
If body's type is non-null and
+ response's header list does not contain
+ `Content-Type`, then append (`Content-Type`,
+ body's type) to response's
+ header list.
+
The
+new Response(body, init)
+constructor steps are:
+
+
Set this's headers to a new {{Headers}} object with
+ this's relevant realm, whose header list is this's
+ response's header list and guard is
+ "response".
+
+
Let bodyWithType be null. + +
If body is non-null, then set bodyWithType to the result of + extracting body. + +
Perform initialize a response given this, init, and + bodyWithType. +
The static error() method steps are to return the
+result of creating a {{Response}} object, given a new network error,
+"immutable", and the current realm.
+
+
The static
+redirect(url, status) method steps
+are:
+
+
Let parsedURL be the result of parsing url with + current settings object's API base URL. + +
If parsedURL is failure, then throw a {{TypeError}}. + +
If status is not a redirect status, then throw a {{RangeError}}. + +
Let responseObject be the result of creating a {{Response}}
+ object, given a new response, "immutable", and the current realm.
+
+
Let value be parsedURL, serialized and + isomorphic encoded. + +
Append (`Location`, value) to
+ responseObject's response's header list.
+
+
Return responseObject. +
The static
+json(data, init) method steps
+are:
+
+
Let bytes the result of running serialize a JavaScript value to JSON bytes + on data. + +
Let body be the result of extracting bytes. + +
Let responseObject be the result of creating a {{Response}}
+ object, given a new response, "response", and the current realm.
+
+
Perform initialize a response given responseObject, init, and
+ (body, "application/json").
+
+
Return responseObject. +
The type getter steps are to return this's
+response's type.
+
+
The url getter steps are to return
+the empty string if this's response's URL is null;
+otherwise this's response's URL,
+serialized with exclude fragment set
+to true.
+
+
The redirected getter steps are to return true if
+this's response's URL list's size is
+greater than 1; otherwise false.
+
+
To filter out responses that are the result of a
+redirect, do this directly through the API, e.g., fetch(url, { redirect:"error" }).
+This way a potentially unsafe response cannot accidentally leak.
+
+
The status getter steps are to return
+this's response's status.
+
+
The ok getter steps are to return true if
+this's response's status is an ok status;
+otherwise false.
+
+
The statusText getter steps are to return
+this's response's status message.
+
+
The headers getter steps are to return
+this's headers.
+
+
The clone() method steps are:
+
+
+partial interface mixin WindowOrWorkerGlobalScope {
+ [NewObject] Promise<Response> fetch(RequestInfo input, optional RequestInit init = {});
+};
+
+dictionary DeferredRequestInit : RequestInit {
+ DOMHighResTimeStamp activateAfter;
+};
+
+[Exposed=Window]
+interface FetchLaterResult {
+ readonly attribute boolean activated;
+};
+
+partial interface Window {
+ [NewObject, SecureContext] FetchLaterResult fetchLater(RequestInfo input, optional DeferredRequestInit init = {});
+};
+
+
+The
+fetch(input, init)
+method steps are:
+
+
Let p be [=a new promise=]. + +
Let requestObject be the result of invoking the initial value of {{Request}} as + constructor with input and init as arguments. If this throws an exception, + reject p with it and return p. + +
Let request be requestObject's request. + +
If requestObject's signal is aborted, + then: + +
Abort the fetch() call with p, request, null, and
+ requestObject's signal's abort reason.
+
+
Return p. +
none".
+
+ Let responseObject be null. + +
Let relevantRealm be this's relevant realm. + +
Let locallyAborted be false. + +
This lets us reject promises with predictable timing, when the request to abort + comes from the same thread as the call to fetch. + +
Let controller be null. + +
Add the following abort steps to requestObject's + signal: + +
Set locallyAborted to true. + +
Assert: controller is non-null. + +
Abort controller with requestObject's + signal's abort reason. + +
Abort the fetch() call with p, request,
+ responseObject, and requestObject's signal's
+ abort reason.
+
+
Set controller to the result of calling fetch given + request and processResponse given response being + these steps: + +
If locallyAborted is true, then abort these steps. + +
If response's aborted flag is set, then: + +
Let deserializedError be the result of + deserialize a serialized abort reason given controller's + serialized abort reason and relevantRealm. + +
Abort the fetch() call with p, request,
+ responseObject, and deserializedError.
+
+
Abort these steps. +
If response is a network error, then reject p + with a {{TypeError}} and abort these steps. + +
Set responseObject to the result of creating a {{Response}}
+ object, given response, "immutable", and relevantRealm.
+
+
Resolve p with responseObject. +
Return p. +
To abort a fetch() call
+with a promise, request, responseObject, and an error:
+
+
Reject promise with error. + +
This is a no-op if promise has already fulfilled. + +
If request's body is non-null and is + readable, then cancel request's + body with error. + +
If responseObject is null, then return. + + +
Let response be responseObject's response. + +
If response's body is non-null and is + readable, then error response's + body with error. +
A {{FetchLaterResult}} has an associated activated getter steps, +which is an algorithm returning a boolean. + +
The activated getter steps are to return
+the result of running this's activated getter steps.
+
The fetchLater(input, init)
+method steps are:
+
+
Let requestObject be the result of invoking the initial value of {{Request}} as + constructor with input and init as arguments. + +
If requestObject's signal is aborted, + then throw signal's abort reason. + +
Let request be requestObject's request. + +
Let activateAfter be null. + +
If init is given and init["{{DeferredRequestInit/activateAfter}}"] + exists, then set activateAfter to + init["{{DeferredRequestInit/activateAfter}}"]. + +
If activateAfter is less than 0, then throw a {{RangeError}}. + +
If this's relevant global object's associated document is not + fully active, then throw a {{TypeError}}. + +
If request's URL's scheme is not an + HTTP(S) scheme, then throw a {{TypeError}}. + +
If request's URL is not a potentially trustworthy URL, + then throw a {{TypeError}}. + +
If request's body is not null, and request's + body length is null, then throw a {{TypeError}}. + +
Requests whose body is a {{ReadableStream}} object cannot be + deferred. + +
If the available deferred-fetch quota given request's + client and request's URL's + origin is less than request's total request length, then throw a + "{{QuotaExceededError}}" {{DOMException}}. + +
Let activated be false. + +
Let deferredRecord be the result of calling queue a deferred fetch given + request, activateAfter, and the following step: set activated to + true. + +
Add the following abort steps to requestObject's
+ signal: Set deferredRecord's
+ invoke state to "aborted".
+
+
Return a new {{FetchLaterResult}} whose + activated getter steps are to return activated. +
The following call would queue a request to be fetched when the document is terminated: + +
+fetchLater("https://report.example.com", {
+ method: "POST",
+ body: JSON.stringify(myReport),
+ headers: { "Content-Type": "application/json" }
+})
+The following call would also queue this request after 5 seconds, and the returned value would + allow callers to observe if it was indeed activated. Note that the request is guaranteed to be + invoked, even in cases where the user agent throttles timers. + +
+const result = fetchLater("https://report.example.com", {
+ method: "POST",
+ body: JSON.stringify(myReport),
+ headers: { "Content-Type": "application/json" },
+ activateAfter: 5000
+});
+
+function check_if_fetched() {
+ return result.activated;
+}
+The {{FetchLaterResult}} object can be used together with an {{AbortSignal}}. For example: + +
+let accumulated_events = [];
+let previous_result = null;
+const abort_signal = new AbortSignal();
+function accumulate_event(event) {
+ if (previous_result) {
+ if (previous_result.activated) {
+ // The request is already activated, we can start from scratch.
+ accumulated_events = [];
+ } else {
+ // Abort this request, and start a new one with all the events.
+ signal.abort();
+ }
+ }
+
+ accumulated_events.push(event);
+ result = fetchLater("https://report.example.com", {
+ method: "POST",
+ body: JSON.stringify(accumulated_events),
+ headers: { "Content-Type": "application/json" },
+ activateAfter: 5000,
+ abort_signal
+ });
+}
+Any of the following calls to fetchLater() would throw:
+
+
+// Only potentially trustworthy URLs are supported.
+fetchLater("http://untrusted.example.com");
+
+// The length of the deferred request has to be known when.
+fetchLater("https://origin.example.com", {body: someDynamicStream});
+
+// Deferred fetching only works on active windows.
+const detachedWindow = iframe.contentWindow;
+iframe.remove();
+detachedWindow.fetchLater("https://origin.example.com");
+See deferred fetch quota examples for examples + portraying how the deferred-fetch quota works. +
The user agent may terminate an ongoing fetch if that termination +is not observable through script. + +
"Observable through script" means observable through
+fetch()'s arguments and return value. Other ways, such as communicating
+with the server through a side-channel are not included.
+
+
The server being able to observe garbage collection has precedent, e.g., with +{{WebSocket}} and {{XMLHttpRequest}} objects. + +
The user agent can terminate the fetch because the termination cannot be observed. +
+fetch("https://www.example.com/")
+The user agent cannot terminate the fetch because the termination can be observed through + the promise. +
+window.promise = fetch("https://www.example.com/")
+The user agent can terminate the fetch because the associated body is not observable. +
+window.promise = fetch("https://www.example.com/").then(res => res.headers)
+The user agent can terminate the fetch because the termination cannot be observed. +
+fetch("https://www.example.com/").then(res => res.body.getReader().closed)
+The user agent cannot terminate the fetch because one can observe the termination by registering + a handler for the promise object. +
+window.promise = fetch("https://www.example.com/")
+ .then(res => res.body.getReader().closed)
+The user agent cannot terminate the fetch as termination would be observable via the registered + handler. +
+fetch("https://www.example.com/")
+ .then(res => {
+ res.body.getReader().closed.then(() => console.log("stream closed!"))
+ })
+(The above examples of non-observability assume that built-in properties and functions, such as + {{ReadableStream/getReader()|body.getReader()}}, have not been overwritten.) +
data: URLsFor an informative description of data: URLs, see RFC 2397. This section replaces
+that RFC's normative processing requirements to be compatible with deployed content. [[RFC2397]]
+
+
A data: URL struct is a struct that consists of a
+MIME type (a MIME type) and a
+body (a byte sequence).
+
+
The data: URL processor takes a URL
+dataURL and then runs these steps:
+
+
Let input be the result of running the URL serializer on + dataURL with exclude fragment set to true. + +
Remove the leading "data:" from input.
+
+
Let position point at the start of input. + +
Let mimeType be the result of collecting a sequence of code points that + are not equal to U+002C (,), given position. + +
Strip leading and trailing ASCII whitespace from mimeType. + +
This will only remove U+0020 SPACE code points, if any. + +
If position is past the end of input, then return failure. + +
Advance position by 1. + +
Let encodedBody be the remainder of input. + +
Let body be the percent-decoding of encodedBody. + +
If mimeType ends with U+003B (;), followed by zero or more U+0020 SPACE, followed by
+ an ASCII case-insensitive match for "base64", then:
+
+
+
Let stringBody be the isomorphic decode of body. + +
Set body to the forgiving-base64 decode of stringBody. + +
If body is failure, then return failure. + +
Remove the last 6 code points from mimeType. + +
Remove trailing U+0020 SPACE code points from mimeType, if any. + +
Remove the last U+003B (;) from mimeType. +
If mimeType starts with ";", then prepend
+ "text/plain" to mimeType.
+
+
Let mimeTypeRecord be the result of parsing + mimeType. + +
If mimeTypeRecord is failure, then set mimeTypeRecord to
+ text/plain;charset=US-ASCII.
+
+
Return a new data: URL struct whose
+ MIME type is mimeTypeRecord and
+ body is body.
+
This section and its subsections are informative only. + +
For the purposes of fetching, there is an API layer (HTML's img, CSS's
+background-image), early fetch layer, service worker layer, and network & cache
+layer. `Accept` and `Accept-Language` are set in the early fetch layer
+(typically by the user agent). Most other headers controlled by the user agent, such as
+`Accept-Encoding`, `Host`, and `Referer`, are set in the
+network & cache layer. Developers can set headers either at the API layer or in the service
+worker layer (typically through a {{Request}} object). Developers have almost no control over
+forbidden request-headers, but can control `Accept` and have the means to
+constrain and omit `Referer` for instance.
+
+
+
Redirects (a response whose status or +internal response's (if any) status is a +redirect status) are not exposed to APIs. Exposing redirects might leak information not +otherwise available through a cross-site scripting attack. + +
A fetch to https://example.org/auth that includes a
+Cookie marked HttpOnly could result in a redirect to
+https://other-origin.invalid/4af955781ea1c84a3b11. This new URL contains a
+secret. If we expose redirects that secret would be available through a cross-site
+scripting attack.
+
+
+
For resources where data is protected through IP authentication or a firewall +(unfortunately relatively common still), using the CORS protocol is +unsafe. (This is the reason why the CORS protocol had to be +invented.) + +
However, otherwise using the following header is +safe: + +
+Access-Control-Allow-Origin: *
+Even if a resource exposes additional information based on cookie or HTTP
+authentication, using the above header will not reveal
+it. It will share the resource with APIs such as
+{{XMLHttpRequest}}, much like it is already shared with
+curl and wget.
+
+
Thus in other words, if a resource cannot be accessed from a random device connected to
+the web using curl and wget the aforementioned
+header is not to be included. If it can be accessed
+however, it is perfectly fine to do so.
+
+
+
If CORS protocol requirements are more complicated than setting
+`Access-Control-Allow-Origin` to * or a static
+origin, `Vary` is to be used.
+[[!HTML]] [[!HTTP]] [[!HTTP-CACHING]]
+
+
+Vary: Origin
+In particular, consider what happens if `Vary` is not used and a server is
+configured to send `Access-Control-Allow-Origin` for a certain
+resource only in response to a CORS request. When a user agent receives a response to a
+non-CORS request for that resource (for example, as the result of a navigation
+request), the response will lack `Access-Control-Allow-Origin`
+and the user agent will cache that response. Then, if the user agent subsequently encounters a
+CORS request for the resource, it will use that cached response from the previous
+non-CORS request, without `Access-Control-Allow-Origin`.
+
+
But if `Vary: Origin` is used in the same scenario described above, it will cause
+the user agent to fetch a response that includes
+`Access-Control-Allow-Origin`, rather than using the cached response
+from the previous non-CORS request that lacks
+`Access-Control-Allow-Origin`.
+
+
However, if `Access-Control-Allow-Origin` is set to
+* or a static origin for a particular resource, then configure the server
+to always send `Access-Control-Allow-Origin` in responses for the
+resource — for non-CORS requests as well as CORS
+requests — and do not use `Vary`.
+
+
+
As part of establishing a connection, the {{WebSocket}} object initiates a special kind of
+fetch (using a request whose mode is
+"websocket") which allows it to share in many fetch policy decisions, such
+HTTP Strict Transport Security (HSTS). Ultimately this results in fetch calling into
+WebSockets to obtain a dedicated connection. [[WEBSOCKETS]]
+[[HSTS]]
+
+
Fetch used to define +obtain a WebSocket connection and +establish a WebSocket connection directly, but +both are now defined in WebSockets. [[WEBSOCKETS]] + + + +
In its essence fetching is an exchange of a request for a +response. In reality it is rather complex mechanism for standards to adopt and use +correctly. This section aims to give some advice. + +
Always ask domain experts for review. + +
This is a work in progress. + + +
The first step in fetching is to create a request, and populate its +items. + +
Start by setting the request's URL and method,
+as defined by HTTP. If your `POST` or `PUT` request needs a
+body, you set request's body to a byte sequence, or to
+a new body whose stream is a {{ReadableStream}} you created. [[HTTP]]
+
+
Choose your request's destination using the guidance in the +destination table. Destinations affect +Content Security Policy and have other implications such as the [:Sec-Fetch-Dest:] +header, so they are much more than informative metadata. If a new feature requires a +destination that's not in the destination table, +please +file an issue +to discuss. [[CSP]] + +
Set your request's client to the +environment settings object you're operating in. Web-exposed APIs are generally defined with +Web IDL, for which every object that implements an interface has a +relevant settings object you can use. For example, a request associated with an +element would set the request's client to the element's +node document's relevant settings object. All features that are directly web-exposed +by JavaScript, HTML, CSS, or other {{Document}} subresources should have a +client. + +
If your fetching is not directly web-exposed, e.g., it is sent in the background +without relying on a current {{Window}} or {{Worker}}, leave request's +client as null and set the request's origin, +policy container, service-workers mode, and +referrer to appropriate values instead, e.g., by copying them from the +environment settings object ahead of time. In these more advanced cases, make sure the +details of how your fetch handles Content Security Policy and +referrer policy are fleshed out. Also make sure you handle concurrency, as callbacks +(see [[#fetch-elsewhere-fetch]]) would be posted on a parallel queue. [[REFERRER]] [[CSP]] + +
Think through the way you intend to handle cross-origin resources. Some features may only work in
+the same origin, in which case set your request's mode to
+"same-origin". Otherwise, new web-exposed features should almost always set their
+mode to "cors". If your feature is not web-exposed, or you think
+there is another reason for it to fetch cross-origin resources without CORS, please
+file an issue
+to discuss.
+
+
For cross-origin requests, also determines if credentials are to be included with
+the requests, in which case set your request's credentials mode to
+"include".
+
+
Figure out if your fetch needs to be reported to Resource Timing, and with which +initiator type. By passing an initiator type to the +request, reporting to Resource Timing will be done automatically once the +fetch is done and the response is fully downloaded. [[RESOURCE-TIMING]] + +
If your request requires additional HTTP headers, set its header list to
+a header list that contains those headers, e.g., « (`My-Header-Name`,
+`My-Header-Value`) ». Sending custom headers may have implications, such as requiring a
+CORS-preflight fetch, so handle with care.
+
+
If you want to override the default caching mechanism, e.g., disable caching for this
+request, set the request's cache mode to a value other than
+"default".
+
+
Determine whether you want your request to support redirects. If you don't, set its
+redirect mode to "error".
+
+
Browse through the rest of the parameters for request to see if something else is +relevant to you. The rest of the parameters are used less frequently, often for special purposes, +and they are documented in detail in the [[#requests]] section of this standard. + + +
Aside from a request the fetch operation takes several optional +arguments. For those arguments that take an algorithm: the algorithm will be called from a task (or +in a parallel queue if useParallelQueue is true). + +
Once the request is set up, to determine which algorithms to pass to +fetch, determine how you would like to process the response, and in +particular at what stage you would like to receive a callback: + +
This is how most callers handle a response, for example + scripts and style resources. + The response's body is read in its entirety into a + byte sequence, and then processed by the caller. + +
To process a response upon completion, pass an algorithm as the + processResponseConsumeBody argument of fetch. The given + algorithm is passed a response and an argument representing the fully read + body (of the response's + internal response). The second argument's values have the following + meaning: + +
Fully reading the contents of the response's + internal response's body succeeded. + +
A byte sequence containing the full contents will be passed also for a
+ request whose mode is "no-cors". Callers have to
+ be careful when handling such content, as it should not be accessible to the requesting
+ origin. For example, the caller may use contents of a "no-cors"
+ response to display image contents directly to the user, but those image contents
+ should not be directly exposed to scripts in the embedding document.
+
Let request be a request whose URL is
+ https://stuff.example.com/ and client is this's
+ relevant settings object.
+
+
Fetch request, with + processResponseConsumeBody set to the following steps given + a response response and null, failure, or a byte sequence + contents: + +
If contents is null or failure, then present an error to the user. + +
Otherwise, parse contents considering the metadata from response, + and perform your own operations on it. +
In some cases, for example when playing video or progressively loading images, callers might + want to stream the response, and process it one chunk at a time. The response is + handed over to the fetch caller once the headers are processed, and the caller + continues from there. + +
To process a response chunk-by-chunk, pass an algorithm to the + processResponse argument of fetch. The given + algorithm is passed a response when the response's headers have been + received and is responsible for reading the response's + body's stream in order to download the rest + of the response. For convenience, you may also pass an algorithm to the + processResponseEndOfBody argument, which is called once you have finished + fully reading the response and its body. Note that unlike + processResponseConsumeBody, passing the + processResponse or processResponseEndOfBody arguments + does not guarantee that the response will be fully read, and callers are responsible to + read it themselves. + +
The processResponse argument is also useful for handling the + response's header list and status without + handling the body at all. This is used, for example, when handling responses + that do not have an ok status. + +
Let request be a request whose URL is
+ https://stream.example.com/ and client is this's
+ relevant settings object.
+
+
Fetch request, with + processResponse set to the following steps given a + response response: + +
If response is a network error, then present an error to the user. + +
Otherwise, if response's status is not an + ok status, present some fallback value to the user. + +
Otherwise, get a reader for response's + body's stream, and process in an appropriate way for the + MIME type identified by extracting a MIME type from response's headers list. +
In some cases, there is no need for a response at all, e.g., in the case of + {{Navigator/sendBeacon()|navigator.sendBeacon()}}. Processing a response and passing callbacks to + fetch is optional, so omitting the callback would fetch without + expecting a response. In such cases, the response's body's + stream will be discarded, and the caller does not have to worry about downloading + the contents unnecessarily. + +
Fetch a request whose
+ URL is https://fire-and-forget.example.com/,
+ method is `POST`, and client is this's
+ relevant settings object.
+
Apart from the callbacks to handle responses, fetch accepts additional callbacks +for advanced cases. processEarlyHintsResponse is intended specifically for +responses whose status is 103, and is currently handled only by +navigations. processRequestBodyChunkLength and +processRequestEndOfBody notify the caller of request body uploading +progress. + +
Note that the fetch operation starts in the same thread from which it was called, +and then breaks off to run its internal operations in parallel. The aforementioned callbacks +are posted to a given event loop which is, by default, the +client's global object. To process +responses in parallel and handle interactions with the main thread by yourself, +fetch with useParallelQueue set to true. + + +
To manipulate a fetch operation that has already started, use the +fetch controller returned by calling fetch. For example, you may +abort the fetch controller due the user or page logic, or +terminate it due to browser-internal circumstances. + +
In addition to terminating and aborting, callers may report timing
+if this was not done automatically by passing the initiator type, or
+extract full timing info and handle it on the caller side (this is
+done only by navigations). The fetch controller is also used to
+process the next manual redirect for requests with
+redirect mode set to "manual".
+
+
+
Thanks to +Adam Barth, +Adam Lavin, +Alan Jeffrey, +Alexey Proskuryakov, +Andreas Kling, +Andrés Gutiérrez, +Andrew Sutherland, +Andrew Williams, +Ángel González, +Anssi Kostiainen, +Arkadiusz Michalski, +Arne Johannessen, +Artem Skoretskiy, +Arthur Barstow, +Arthur Sonzogni, +Asanka Herath, +Axel Rauschmayer, +Ben Kelly, +Benjamin Gruenbaum, +Benjamin Hawkes-Lewis, +Benjamin VanderSloot, +Bert Bos, +Björn Höhrmann, +Boris Zbarsky, +Brad Hill, +Brad Porter, +Bryan Smith, +Caitlin Potter, +Cameron McCormack, +Carlo Cannas, +白丞祐 (Cheng-You Bai), +Chirag S Kumar, +Chris Needham, +Chris Rebert, +Clement Pellerin, +Collin Jackson, +Daniel Robertson, +Daniel Veditz, +Dave Tapuska, +David Benjamin, +David Håsäther, +David Orchard, +Dean Jackson, +Devdatta Akhawe, +Domenic Denicola, +Dominic Farolino, +Dominique Hazaël-Massieux, +Doug Turner, +Douglas Creager, +Eero Häkkinen, +Ehsan Akhgari, +Emily Stark, +Eric Lawrence, +Eric Orth, +Feng Yu, +François Marier, +Frank Ellerman, +Frederick Hirsch, +Frederik Braun, +Gary Blackwood, +Gavin Carothers, +Glenn Maynard, +Graham Klyne, +Gregory Terzian, +Guohui Deng(邓国辉), +Hal Lockhart, +Hallvord R. M. Steen, +Harris Hancock, +Henri Sivonen, +Henry Story, +Hiroshige Hayashizaki, +Honza Bambas, +Ian Hickson, +Ilya Grigorik, +isonmad, +Jake Archibald, +James Graham, +Jamie Mansfield, +Janusz Majnert, +Jeena Lee, +Jeff Carpenter, +Jeff Hodges, +Jeffrey Yasskin, +Jensen Chappell, +Jeremy Roman, +Jesse M. Heines, +Jianjun Chen, +Jinho Bang, +Jochen Eisinger, +John Wilander, +Jonas Sicking, +Jonathan Kingston, +Jonathan Watt, +최종찬 (Jongchan Choi), +Jordan Stephens, +Jörn Zaefferer, +Joseph Pecoraro, +Josh Matthews, +jub0bs, +Julian Krispel-Samsel, +Julian Reschke, +송정기 (Jungkee Song), +Jussi Kalliokoski, +Jxck, +Kagami Sascha Rosylight, +Keita Suzuki, +Keith Yeung, +Kenji Baheux, +Lachlan Hunt, +Larry Masinter, +Liam Brummitt, +Linus Groh, +Louis Ryan, +Luca Casonato, +Lucas Gonze, +Łukasz Anforowicz, +呂康豪 (Kang-Hao Lu), +Maciej Stachowiak, +Malisa, +Manfred Stock, +Manish Goregaokar, +Marc Silbey, +Marcos Caceres, +Marijn Kruisselbrink, +Mark Nottingham, +Mark S. Miller, +Martin Dürst, +Martin O'Neal, +Martin Thomson, +Matt Andrews, +Matt Falkenhagen, +Matt Menke, +Matt Oshry, +Matt Seddon, +Matt Womer, +Mhano Harkness, +Michael Ficarra, +Michael Kohler, +Michael™ Smith, +Mike Pennisi, +Mike West, +Mohamed Zergaoui, +Mohammed Zubair Ahmed, +Moritz Kneilmann, +Ms2ger, +Nico Schlömer, +Nicolás Peña Moreno, +Nidhi Jaju, +Nikhil Marathe, +Nikki Bee, +Nikunj Mehta, +Noam Rosenthal, +Odin Hørthe Omdal, +Olli Pettay, +Ondřej Žára, +O. Opsec, +Patrick Meenan, +Perry Jiang, +Philip Jägenstedt, +R. Auburn, +Raphael Kubo da Costa, +Robert Linder, +Rondinelly, +Rory Hewitt, +Ross A. Baker, +Ryan Sleevi, +Sam Atkins, +Samy Kamkar, +Sébastien Cevey, +Sendil Kumar N, +Shao-xuan Kang, +Sharath Udupa, +Shivakumar Jagalur Matt, +Shivani Sharma, +Sigbjørn Finne, +Simon Pieters, +Simon Sapin, +Simon Wülker, +Srirama Chandra Sekhar Mogali, +Stephan Paul, +Steven Salat, +Sunava Dutta, +Surya Ismail, +Tab Atkins-Bittner, +Takashi Toyoshima, +吉野剛史 (Takeshi Yoshino), +Thomas Roessler, +Thomas Steiner, +Thomas Wisniewski, +Tiancheng "Timothy" Gu, +Tobie Langel, +Tom Schuster, +Tomás Aparicio, +triple-underscore, +保呂毅 (Tsuyoshi Horo), +Tyler Close, +Ujjwal Sharma, +Vignesh Shanmugam, +Vladimir Dzhuvinov, +Wayne Carr, +Xabier Rodríguez, +Yehuda Katz, +Yoav Weiss, +Youenn Fablet, +Yoichi Osato, +平野裕 (Yutaka Hirano), and +Zhenbin Xu +for being awesome. + +
This standard is written by Anne van Kesteren +(Apple, annevk@annevk.nl).