GM_xmlhttpRequest powered implementation of window.fetch

The global fetch function is an easier way to make web requests and handle responses than using an XMLHttpRequest. This implementation uses GM_xmlhttpRequest as the underlying implementation, allowing user scripts to make cross-domain requests using the fetch API.

GM_fetch.ts differs from upstream GM_fetch by being rewritten in TypeScript and featuring a more consistent API. GM_fetch is based on GitHub's fetch polyfill.

Compatibility

• GM_fetch.ts uses the legacy GM_xmlhttpRequest API as opposed to GreaseMonkey 4.0's GM.xmlHttpRequest for compatibility with a wider variety of userscript engines. As such, while it is compatible with TamperMonkey, ViolentMonkey, Scriptish, and legacy GreaseMonkey, it is unlikely to work on GreaseMonkey 4.0.
• The default UTF-8 TextDecoder encoding is used for decoding response ArrayBuffer instances, with the exception of Response.text().
• Response.text() is untested.
• Body.formData() is unsupported for response bodies.
• request.credentials: 'include' isn't explicitly implemented.

Installation

For regular userscripts, without any type of module bundler, you can add an @require clause to your script's header pointing to dist/GM_fetch.js. This is a UMD file that will adapt to either RequireJS, AMD, or Webpack, or export a GM_fetch variable to the current scope. The Headers, Request, and Response classes are only publicly visible if a module loader is used.

// @grant    GM_xmlhttpRequest
// @require  https://raw.githubusercontent.com/uwx/GM_fetch.ts/master/dist/GM_fetch.js

Note: This always links to the most up-to-date version, which may introduce breaking changes. Pinning to a commit, or self-hosting, is recommended.

Alternatively, for TypeScript projects, you can copy all the files in src to your project directly, and import ./GM_fetch.ts.

Usage

The fetch function supports the GET, HEAD and POST HTTP methods, as with GM_xmlhttpRequest. Here are some examples.

HTML

fetch('/users.html')
  .then(res => res.text())
  .then(body => document.body.innerHTML = body);

JSON

fetch('/users.json')
  .then(res => res.json())
  .then(json => console.log('parsed json', json)
  .catch(err => console.error('parsing failed', err);

Response metadata

fetch('/users.json').then(response => {
  console.log(response.headers.get('Content-Type'));
  console.log(response.headers.get('Date'));
  console.log(response.status);
  console.log(response.statusText);
});

Post form

var form = document.querySelector('form')

fetch('/query', {
  method: 'post',
  body: new FormData(form)
});

Post JSON

fetch('/users', {
  method: 'post',
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    name: 'Hubot',
    login: 'hubot',
  })
});

File upload

const input = document.querySelector('input[type="file"]')

const form = new FormData()
form.append('file', input.files[0])
form.append('user', 'hubot')

fetch('/avatars', {
  method: 'post',
  body: form
});

Success and error handlers

This causes fetch to behave like jQuery's $.ajax by rejecting the Promise on HTTP failure status codes like 404, 500, etc. The response Promise is resolved only on successful, 200 level, status codes.

function status(response) {
  if (response.ok) {
    return response;
  }
  throw new Error(response.statusText);
}

fetch('/users')
  .then(status)
  .then(res => res.json())
  .then(json => console.log('request succeeded with json response', json))
  .catch(err => console.log('request failed', err));