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Co-authored-by: Titus <tituswormer@gmail.com>

lib/constants.js

@@ -1,70 +1,100 @@
 /** @constant */
 export const DEFAULT_HEADERS = {
-  // Ensures that the page can't be displayed in a frame,
-  // regardless of where the request came from.
-  // It's a security feature to prevent "clickjacking" attacks,
-  // where an attacker might use an iframe to overlay a legitimate page over a deceptive page.
+  /**
+   * Ensures that the page can’t be displayed in a frame,
+   * regardless of where the request came from.
+   * It’s a security feature to prevent “clickjacking” attacks,
+   * where an attacker might use an iframe to overlay a legitimate page over a
+   * deceptive page.
+   */
   'X-Frame-Options': 'deny',
-  // The browser should enable the XSS filter and if a XSS attack is detected,
-  // rather than sanitizing the page, the browser will prevent rendering of the page entirely.
+  /**
+   * The browser should enable the XSS filter and if a XSS attack is detected,
+   * rather than sanitizing the page, the browser will prevent rendering of the
+   * page entirely.
+   */
   'X-XSS-Protection': '1; mode=block',
-  // Trust what the server says and not to perform MIME type sniffing.
-  // This can prevent certain security risks where the browser might interpret
-  // files as a different MIME type, which can be exploited in attacks.
+  /**
+   * Trust what the server says and not to perform MIME type sniffing.
+   * This can prevent certain security risks where the browser might interpret
+   * files as a different MIME type, which can be exploited in attacks.
+   */
   'X-Content-Type-Options': 'nosniff',
-  // By default, do not load content from any source (default-src 'none').
-  // Images can be loaded from data: URLs (like Base64 encoded images).
-  // Styles can be loaded inline (style-src 'unsafe-inline'),
-  // which usually means from within the HTML itself rather than from external files.
+  /**
+   * By default, do not load content from any source (`default-src 'none'`),
+   * images can be loaded from `data:` URLs (like Base64 encoded images),
+   * and styles can be loaded inline (`style-src 'unsafe-inline'`),
+   * which usually means from within the HTML itself rather than from external
+   * files.
+   */
   'Content-Security-Policy':
     "default-src 'none'; img-src data:; style-src 'unsafe-inline'",
-  // This header is often called HSTS (HTTP Strict Transport Security).
-  // It's a security feature that ensures a website can only be accessed over HTTPS instead of HTTP.
-  // The max-age parameter specifies how long (in seconds) the browser should remember to enforce this policy.
-  // The includeSubDomains directive extends this rule to all subdomains of the domain sending the header.
-  // This ensures that even if a user tries to access the site or its subdomains via HTTP,
-  // their browser will automatically upgrade the request to HTTPS.
+  /**
+   * This header is often called HSTS (HTTP Strict Transport Security).
+   * It’s a security feature that ensures a website can only be accessed over
+   * HTTPS instead of HTTP.
+   * The `max-age` parameter specifies how long (in seconds) the browser should
+   * remember to enforce this policy.
+   * The `includeSubDomains` directive extends this rule to all subdomains of
+   * the domain sending the header.
+   * This ensures that even if a user tries to access the site or its
+   * subdomains via HTTP,
+   * their browser will automatically upgrade the request to HTTPS.
+   */
   'Strict-Transport-Security': 'max-age=31536000; includeSubDomains'
 }
 
 /**
- * http request headers that are acceptable to pass from
- * the client to the remote server. Only those present and true, are forwarded.
- * @constant
+ * HTTP request headers that are acceptable to pass from the client to the
+ * remote server.
+ * Only those present and true are forwarded.
+ * 
+ * @type {Readonly<Map<string, boolean>>}
  */
 export const REQUEST_HEADERS = new Map([
   ['Accept', true],
   ['Accept-Charset', true],
-  ['Accept-Encoding', false], // images (aside from xml/svg), don't typically benefit from compression
+  // No `Accept-Encoding` because images (other than from XML or SVG), don’t
+  // typically benefit from compression:
+  ['Accept-Encoding', false],
   ['Accept-Language', true],
   ['Cache-Control', true],
   ['If-None-Match', true],
   ['If-Modified-Since', true],
-  ['X-Forwarded-For', false], // x-forwarded-for header is not blindly passed without additional custom processing
-  ['Range', true] // required to support safari byte range requests for video
+  // `Range` is used by Safari for byte range requests on video:
+  ['Range', true],
+  // No `X-Forwarded-For` because ???
+  ['X-Forwarded-For', false]
 ])
 
 /**
- * http response headers that are acceptable to pass from
- * the remote server to the client. Only those present and true, are forwarded.
- * @constant
+ * HTTP response headers that are acceptable to pass from the remote server to
+ * the client.
+ * Only those present and true are forwarded.
+ *
+ * @type {Readonly<Map<string, boolean>>}
  */
 export const RESPONSE_HEADERS = new Map([
-  ['Accept-Ranges', true], // required to support Safari byte range requests for video
-  ['Content-Length', true],
-  ['Content-Range', true],
-
+  // `Accept-Ranges` is used by Safari for byte range requests on video:
+  ['Accept-Ranges', true],
   ['Cache-Control', true],
+  ['Content-Length', true],
   ['Content-Encoding', true],
+  ['Content-Range', true],
   ['Content-Type', true],
   ['ETag', true],
   ['Expires', true],
   ['Last-Modified', true],
-  ['Server', false], // override in response with either nothing, or ServerNameVer
+  // `Server` is disallowed because ???
+  ['Server', false],
   ['Transfer-Encoding', true]
 ])
 
-/** @constant */
+/**
+ * MIME types that we allow.
+ * 
+ * @type {ReadonlyArray<string>}
+ */
 export const IMAGE_MIME_TYPES = [
   'image/bmp',
   'image/cgm',