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dr-fetch

This is one more package for fetching. The difference with other packages is that this one does it right.

This package:

  • Uses the modern, standardized fetch function.
  • Does not throw on non-OK HTTP responses.
  • Allows to fully type all possible HTTP responses depending on the HTTP status code.

Does a Non-OK Status Code Warrant an Error?

No. Non-OK status codes may communicate things like validation errors, which usually requires that the application informs the user about which piece of data is wrong. Why should this logic be inside a catch block? The fact is, try..catch would be there as a replacement of branching (using if or switch). This is a very smelly code smell.

The second reason is that in most runtimes, unwinding the call stack is costly. Why should we pay a price in performance just to include the code smell of using try..catch as a branching statement? There is no reason to do such thing.

Quickstart

  1. Install the package.
  2. Create your custom fetch function, usually including logic to inject an authorization header/token.
  3. Create a fetcher object.
  4. Optionally add body parsers.
  5. Use the fetcher for every HTTP request needed.

Installation

npm i dr-fetch

Create Custom Fetch Function

This is optional and only needed if you need to do something before or after fetching. By far the most common task to do is to add the authorization header and the accept header to every call.

// myFetch.ts
import { obtainToken } from './magical-auth-stuff.js';
import { setHeaders } from 'dr-fetch';

export function myFetch(url: Parameters<typeof fetch>[0], init?: Parameters<typeof fetch>[1]) {
    const token = obtainToken();
    // Make sure there's an object where headers can be added:
    init ??= {};
    // With setHeaders(), you can add headers to 'init' with a map, an array of tuples, a Headers 
    // object or a POJO object.
    setHeaders(init, { 'Accept': 'application/json', 'Authorization': `Bearer ${token}`});
    // Finally, do fetch.
    return fetch(url, init);
}

Think of this custom function as the place where you do interceptions (if you are familiar with this term from axios).

Create Fetcher Object

// fetcher.ts
import { DrFetch } from "dr-fetch";
import { myFetch } from "./myFetch.js";

export default new DrFetch(myFetch);
// If you don't need a custom fetch function, just do:
export default new DrFetch();

Adding a Custom Body Parser

This step is also optional.

One can say that the DrFetch class comes with 2 basic body parsers:

  1. JSON parser when the value of the coontent-type response header is application/json or similar (application/problem+json, for instance).
  2. Text parser when the value of the content-type response header is text/<something>, such as text/plain or text/csv.

If your API sends a content type not covered by any of the above two cases, use DrFetch.withParser() to add a custom parser for the content type you are expecting. The class allows for fluent syntax, so you can chain calls:

// fetcher.ts
...

export default new DrFetch(myFetch)
    .withParser('custom/contentType', async (response) => {
        // Do what you must with the provided response object.  In the end, you must return the parsed body.
        return finalBody;
    });
    ;

NOTE: The content type can also be matched passing a regular expression instead of a string.

Now the fetcher object is ready for use.

Using the Fetcher Object

This is the fun part where we can enumerate the various shapes of the body depending on the HTTP status code:

import type { MyData } from "./my-datatypes.js";
import fetcher from './fetcher.js';

const response = await fetcher
    .for<200, MyData[]>()
    .for<401, { loginUrl: string; }>()
    .fetch('/api/mydata/?active=true')
    ;

The object stored in the response variable will contain the following properties:

  • ok: Same as Response.ok.
  • status: Same as Response.status.
  • statusText: Same as Response.statusText.
  • body: The HTTP response body, already parsed and typed according to the specification: MyData[] if the status code was 200, or { loginUrl: string; } if the status code was 401.

Your editor's Intellisense should be able to properly and accurately tell you all this:

if (response.status === 200) { // In this example, doing response.ok in the IF narrows the type just as well.
    // Say, display the data somehow/somewhere.  In Svelte, we would set a store, perhaps?
    myDataStore.set(response.body);
}
else {
    // TypeScript/Intellisense will tell you that the only other option is for the status code to be 401:
    window.location.href = response.body.loginUrl;
}

Smarter Uses

It is smart to create just one fetcher, configure it, then use it for every fetch call. Because generally speaking, different URL's will carry a different body type, the fetcher object should be kept free of for<>() calls. However, what if your API is standardized so all status 400 bodies look the same? Then configure that type:

// root-fetcher.ts
import { DrFetch } from "dr-fetch";
import { myFetch } from "./my-fetch.js";
import type { BadRequestBody } from "my-datatypes.js";

export default new DrFetch(myFetch)
    .withParser(...) // Optional parsers
    .withParser(...)
    .for<400, BadRequestBody>()
    ;

You can now consume this root fetcher object and it will be pre-typed for the 400 status code.

Specializing the Root Parser

Ok, nice, but what if we needed a custom parser for just one particular URL? It makes no sense to add it to the root fetcher, and maybe it is even harmful to do so. In that case, clone the fetcher.

Cloning a fetcher produces a new fetcher with the same data-fetching function, the same body parsers and the same body typings, unless we specify we want something different, like not cloning the body types, or specifying a new data-fetching function.

import rootFecher from "./root-fetcher.js";

function specialFetch(url: Parameters<typeof fetch>[0], init?: Parameters<typeof fetch>[1]) {
    ...
}

const localFetcher = rootFetcher.clone(true); // Same data-fetching function, body parsers and body typing.
const localFetcher = rootFetcher.clone(false); // Same data-fetching function and body parsers.  No body typing.
const localFetcher = rootFetcher.clone(true, { fetchFn: specialFetch }); // Different data-fetching function.
const localFetcher = rootFetcher.clone(true, { includeParsers: false }); // No custom body parsers.
const localFetcher = rootFetcher.clone(true, { fetchFn: false }); // Identical parsers and body typing, stock fetch().

IMPORTANT: The first parameter to the clone function cannot be a variable. It is just used as a TypeScript trick to reset the body typing. The value itself means nothing in runtime because types are not a runtime thing.

Shortcut Functions

Since v0.3.0

DrFetch objects now provide the shortcut functions get, head, post, patch, put and delete. Except for get and head, all these accept a body parameter. When this body is a POJO or an array, the body is stringified and the Content-Type header is given the value application/json. If a body of any other type is given (that the fetch() function accepts, such as FormData), no headers are explicitly specified and therefore it is up to what fetch() (or the custom data-fetching function you provide) does in these cases.

import type { Todo } from './myTypes.js';

const newTodo = { text: 'I am new.  Insert me!' };
const response = await fetcher
    .for<200, { success: true; entity: Todo; }>()
    .for<400, { errors: string[]; }>()
    .post('/api/todos', newTodo);

const newTodos = [{ text: 'I am new.  Insert me!' }, { text: 'Me too!' }];
const response = await fetcher
    .for<200, { success: true; entities: Todo[]; }>()
    .for<400, { errors: string[]; }>()
    .post('/api/todos', newTodos);

As stated, your custom fetch can be used to further customize the request because these shortcut functions will, in the end, call it.

setHeaders and makeIterableHeaders

Since v0.4.0

These are two helper functions that assist you in writing custom data-fetching functions.

If you haven't realized, the init paramter in fetch() can have the headers specified in 3 different formats:

  • As a Headers object (an instance of the Headers class)
  • As a POJO object, where the property key is the header name, and the property value is the header value
  • As an array of tuples of type [string, string], where the first element is the header name, and the second one is its value

To further complicate this, the POJO object also accepts an array of strings as property values for headers that accept multiple values.

So writing a formal custom fetch without setHeaders() looks like this:

export function myFetch(URL: Parameters<typeof fetch>[0], init?: Parameters<typeof fetch>[1]) {
    const acceptHdrKey = 'Accept';
    const acceptHdrValue = 'application/json';
    init ??= {};
    init.headers ??= new Headers();
    if (Array.isArray(init.headers)) {
        // Tuples, so push a tuple per desired header:
        init.headers.push([acceptHdrKey, acceptHdrValue]);
    }
    else if (init.headers instanceof Headers) {
        init.headers.set(acceptHdrKey, acceptHdrValue);
    }
    else {
        // POJO object, so add headers as properties of an object:
        init.headers[acceptHdrKey] = acceptHdrValue;
    }
    return fetch(url, init);
}

This would also get more complex if you account for multi-value headers. Now the same thing, using setHeaders():

export function myFetch(URL: Parameters<typeof fetch>[0], init?: Parameters<typeof fetch>[1]) {
    init ??= {};
    setHeaders(init, [['Accept', 'application/json']]);
    // OR:
    setHeaders(init, new Map([['Accept', ['application/json', 'application/xml']]]));
    // OR:
    setHeaders(init, { 'Accept': ['application/json', 'application/xml'] });
    // OR:
    setHeaders(init, new Headers([['Accept', 'application/json']]));
    return fetch(url, init);
}

The difference is indeed pretty shocking. Also note that adding arrays of values doesn't increase the complexity of the code.

makeIterableHeaders

This function is the magic trick that powers the setHeaders function, and is very handy for troubleshooting or unit testing because it can take a collection of HTTP header specifications in the form of a map, a Headers object, a POJO object or an array of tuples and return an iterator object that iterates through the definitions in the same way: A list of tuples.

const myHeaders1 = new Headers();
myHeaders1.set('Accept', 'application/json');
myHeaders1.set('Authorization', 'Bearer x');

const myHeaders2 = new Map();
myHeaders2.set('Accept', 'application/json');
myHeaders2.set('Authorization', 'Bearer x');

const myHeaders3 = {
    'Accept': 'application/json',
    'Authorization': 'Bearer x'
};

const myHeaders4 = [
    ['Accept', 'application/json'],
    ['Authorization', 'Bearer x'],
];

// The output of these is identical.
console.log([...makeIterableHeaders(myHeaders1)]);
console.log([...makeIterableHeaders(myHeaders2)]);
console.log([...makeIterableHeaders(myHeaders3)]);
console.log([...makeIterableHeaders(myHeaders4)]);

This function is a generator function, so what returns is an iterator object. The two most helpful ways of using it are in for..of statements and spreading:

for (let [key, value] of makeIterableHeaders(myHeaders)) { ... }

// In unit-testing, perhaps:
expect([...makeIterableHeaders(myHeaders)].length).to.equal(2);

Usage Without TypeScript (JavaScript Projects)

Why are you a weird fellow/gal? Anyway, prejudice aside, body typing will mean nothing to you, so forget about for() and anything else regarding types. Do your custom data-fetching function, add your custom body parsers and fetch away using .fetch(), .get(), head(), .post(), .put(), .patch() or .delete().