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.
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. 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.
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.
For a more comprehensive API reference that this polyfill supports, refer to https://github.github.io/fetch/.
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', ...]
fetch('/users.html') .then(function(response) { return response.text() }).then(function(body) { document.body.innerHTML = body })
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) })
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) })
var form = document.querySelector('form') fetch('/users', { method: 'POST', body: new FormData(form) })
fetch('/users', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ name: 'Hubot', login: 'hubot', }) })
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 })
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.
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) })
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”:
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' })
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
.
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.
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 'abortcontroller-polyfill/dist/abortcontroller-polyfill-only' 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()
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.