window.fetch polyfill

The fetch() function is a Promise-based mechanism for programmatically making
web requests in the browser. This project is 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.

Table of Contents

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

Read this first

• If you believe you found a bug with how fetch behaves in your browser,
  please don't open an issue in this repository unless you are testing in
  an old version of a browser that doesn't support window.fetch natively.
  Make sure you read this entire readme, especially the Caveats
  section, as there's probably a known work-around for an issue you've found.
  This project is a polyfill, and since all modern browsers now implement the
  fetch function natively, no code from this project actually takes any
  effect there. 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

npm install whatwg-fetch --save

You will also need a Promise polyfill for older browsers.
We recommend taylorhakes/promise-polyfill
for its small size and Promises/A+ compatibility.

Usage

Importing

Importing will automatically polyfill window.fetch and related APIs:

 import 'whatwg-fetch'

window.fetch(...)

If for some reason you need to access the polyfill implementation, it is
available via exports:

 import {fetch as fetchPolyfill} from 'whatwg-fetch'

window.fetch(...)   // use native browser version
fetchPolyfill(...)  // use polyfill implementation

This approach can be used to, for example, use abort
functionality in browsers that implement a native but
outdated version of fetch that doesn't support aborting.

For use with webpack, add this package in the entry configuration option
before your application entry point:

 entry: ['whatwg-fetch', ...]

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.

• Not all Fetch standard options are supported in this polyfill. For instance,
  redirect and
  cache directives are ignored.

• keepalive is not supported because it would involve making a synchronous XHR, which is something this project is not willing to do. See issue #700 for more information.

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'
})

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'
})

Note: due to limitations of
XMLHttpRequest,
using credentials: 'omit' is not respected for same domains in browsers where
this polyfill is active. Cookies will always be sent to same domains in older
browsers.

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.

Redirect modes

The Fetch specification defines these values for the redirect
option: "follow"
(the default), "error", and "manual".

Due to limitations of XMLHttpRequest, only the "follow" mode is available in
browsers where this polyfill is active.

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:

 import 'yet-another-abortcontroller-polyfill'
import {fetch} from 'whatwg-fetch'

// use native browser implementation if it supports aborting
const abortableFetch = ('signal' in new Request('')) ? window.fetch : fetch

const controller = new AbortController()

abortableFetch('/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, therefore the code from this polyfill doesn't
have any effect on those browsers. 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.