Yet-another-fetch polyfill that adds AbortController support.

The fetch() function is a Promise-based mechanism for programmatically making web requests in the browser. This project provides a polyfill that implements a subset of the standard Fetch specification, enough to make fetch a viable replacement for most uses of XMLHttpRequest in traditional web applications.

The default CommonJS import path does not assign or polyfill window.fetch. Use import 'yetch/polyfill' (see Usage).

Table of Contents

• Read this first
• Installation
• Usage
  • HTML
  • JSON
  • Response metadata
  • Post form
  • Post JSON
  • File upload
  • Caveats
    • Handling HTTP error statuses
    • Sending cookies
    • Receiving cookies
    • Obtaining the Response URL
    • Aborting requests
• Browser Support

Read this first

• If you believe you found a bug with how fetch behaves in Chrome or Firefox, please don't open an issue in this repository. This project is a polyfill, and since Chrome and Firefox both implement the window.fetch function natively, no code from this project actually takes any effect in these browsers. See Browser support for detailed information.

• If you have trouble making a request to another domain (a different subdomain or port number also constitutes another domain), please familiarize yourself with all the intricacies and limitations of CORS requests. Because CORS requires participation of the server by implementing specific HTTP response headers, it is often nontrivial to set up or debug. CORS is exclusively handled by the browser's internal mechanisms which this polyfill cannot influence.

• This project doesn't work under Node.js environments. It's meant for web browsers only. You should ensure that your application doesn't try to package and run this on the server.

• If you have an idea for a new feature of fetch, submit your feature requests to the specification's repository. We only add features and APIs that are part of the Fetch specification.

Installation

You will need to have Promise polyfilled first (if neccesary), before you load yetch. We recommend taylorhakes/promise-polyfill.

npm install yetch --save
# or
yarn add yetch

Usage

yetch also polyfills AbortController, AbortSignal, and a few other related classes, but it does not polyfill Promise

If you'd like yetch to polyfill the global window.fetch, you should import the yetch/polyfill file; it doesn't export anything, it just polyfills the environment if needed.

// ES6+
import 'yetch/polyfill';
// CJS
require('yetch/polyfill');

Otherwise, if you'd like to just use yetch without actually polyfilling the global variables, you can import it directly:

import { fetch, AbortController } from 'yetch';

const controller = new AbortController();

fetch('/avatars', { signal: controller.signal })
  .catch(function(ex) {
    if (ex.name === 'AbortError') {
      console.log('request aborted')
    }
  });

// some time later...
controller.abort();

For a more comprehensive API reference that this polyfill supports, refer to https://Netflix.github.io/yetch/.

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('/users', {
  method: 'POST',
  body: new FormData(form)
})

Post JSON

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

File upload

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

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

fetch('/avatars', {
  method: 'POST',
  body: data
})

Caveats

• The Promise returned from fetch() won't reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, it will resolve normally, and it will only reject on network failure or if anything prevented the request from completing.

• For maximum browser compatibility when it comes to sending & receiving cookies, always supply the credentials: 'same-origin' option instead of relying on the default. See Sending cookies.

Handling HTTP error statuses

To have fetch Promise reject on HTTP error statuses, i.e. on any non-2xx status, define a custom response handler:

function checkStatus(response) {
  if (response.status >= 200 && response.status < 300) {
    return response
  } else {
    var error = new Error(response.statusText)
    error.response = response
    throw error
  }
}

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

fetch('/users')
  .then(checkStatus)
  .then(parseJSON)
  .then(function(data) {
    console.log('request succeeded with JSON response', data)
  }).catch(function(error) {
    console.log('request failed', error)
  })

Sending cookies

For CORS requests, use credentials: 'include' to allow sending credentials to other domains:

fetch('https://example.com:1234/users', {
  credentials: 'include'
})

To disable sending or receiving cookies for requests to any domain, including the current one, use the "omit" value:

fetch('/users', {
  credentials: 'omit'
})

The default value for credentials is "same-origin".

The default for credentials wasn't always the same, though. The following versions of browsers implemented an older version of the fetch specification where the default was "omit":

• Firefox 39-60
• Chrome 42-67
• Safari 10.1-11.1.2

If you target these browsers, it's advisable to always specify credentials: 'same-origin' explicitly with all fetch requests instead of relying on the default:

fetch('/users', {
  credentials: 'same-origin'
})

Receiving cookies

As with XMLHttpRequest, the Set-Cookie response header returned from the server is a forbidden header name and therefore can't be programmatically read with response.headers.get(). Instead, it's the browser's responsibility to handle new cookies being set (if applicable to the current URL). Unless they are HTTP-only, new cookies will be available through document.cookie.

Obtaining the Response URL

Due to limitations of XMLHttpRequest, the response.url value might not be reliable after HTTP redirects on older browsers.

The solution is to configure the server to set the response HTTP header X-Request-URL to the current URL after any redirect that might have happened. It should be safe to set it unconditionally.

# Ruby on Rails controller example
response.headers['X-Request-URL'] = request.url

This server workaround is necessary if you need reliable response.url in Firefox < 32, Chrome < 37, Safari, or IE.

Aborting requests

This polyfill supports the abortable fetch API. However, aborting a fetch requires use of two additional DOM APIs: AbortController and AbortSignal. Typically, browsers that do not support fetch will also not support AbortController or AbortSignal. Consequently, you will need to include an additional polyfill for these APIs to abort fetches.

Once you have an AbortController and AbortSignal polyfill in place, you can abort a fetch like so:

const controller = new AbortController()

fetch('/avatars', {
  signal: controller.signal
}).catch(function(ex) {
  if (ex.name === 'AbortError') {
    console.log('request aborted')
  }
})

// some time later...
controller.abort();

Browser Support

• Chrome
• Firefox
• Safari 6.1+
• Internet Explorer 10+

Note: modern browsers such as Chrome, Firefox, Microsoft Edge, and Safari contain native implementations of window.fetch and the latest versions even support AbortController. However, as a relatively new feature some of your users may have a version of these browsers that has support for window.fetch but does not have support for AbortController. In those cases the polyfilled version of fetch will be used instead of the native one. If you believe you've encountered an error with how window.fetch is implemented in any of these browsers, you should file an issue with that browser vendor instead of this project.

Credit

This project started as a fork of GitHub's whatwg-fetch, adding support for automatically polyfilling window.fetch so that it supports aborting requests with an AbortController. In additional yetch is a CJS module by default and does not replace window.fetch with a polyfill unless you import 'yetch/polyfill'.

As a fork, a majority of the work was done by GitHub and the community in whatwg-fetch.