User Script-enhanced version of the HTML5 fetch() API

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 is based on GitHub's fetch polyfill.

Compatibility

This is a quick conversion, and is currently not well tested.

• Only tested in TamperMonkey
• Only basic GET with no custom headers/cookies/security/anything has been tested
• request.credentials: 'include' isn't implemented - it might be effectively forced true, not sure

Installation

Save off the script somewhere, then include it in your user script with @require.
Make sure you have requested GM_xmlhttpRequest permissions.

// @grant GM_xmlhttpRequest
// @require https://www.example.com/some/js/GM_fetch.js

Usage

The fetch function supports any HTTP method. We'll focus on GET and POST example requests.

HTML

fetch('/users.html')
  .then(function(response) {
    return response.text()
  }).then(function(body) {
    document.body.innerHTML = body
  })

JSON

fetch('/users.json')
  .then(function(response) {
    return response.json()
  }).then(function(json) {
    console.log('parsed json', json)
  }).catch(function(ex) {
    console.log('parsing failed', ex)
  })

Response metadata

fetch('/users.json').then(function(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

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

var 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.status >= 200 && response.status < 300) {
    return response
  }
  throw new Error(response.statusText)
}

function json(response) {
  return response.json()
}

fetch('/users')
  .then(status)
  .then(json)
  .then(function(json) {
    console.log('request succeeded with json response', json)
  }).catch(function(error) {
    console.log('request failed', error)
  })

Response URL caveat

The Response object has a URL attribute for the final responded resource. Usually this is the same as the Request url, but in the case of a redirect, its all transparent. Newer versions of XHR include a responseURL attribute that returns this value. But not every browser supports this. The compromise requires setting a special server side header to tell the browser what URL it just requested (yeah, I know browsers).

response.headers['X-Request-URL'] = request.url

If you want response.url to be reliable, you'll want to set this header. The day that you ditch this polyfill and use native fetch only, you can remove the header hack.