The fetch()
function is a Promise-based mechanism for programatically 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.
This project adheres to the Open Code of Conduct. By participating, you are expected to uphold this code.
-
If you believe you found a bug with how
fetch
behaves in Chrome or Firefox, please avoid opening an issue in this repository. This project is a polyfill, and since Chrome and Firefox both implement thewindow.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 as 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.
-
If you have trouble maintaining the user's session or CSRF protection through
fetch
requests, please ensure that you've read and understood the Sending cookies section. -
If this polyfill doesn't work under Node.js environment, that is expected, because this project is 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
, please understand that we are only ever going to add features and APIs that are a part of the Fetch specification. You should submit your feature requests to the repository of the specification itself, rather than this repository.
-
npm install whatwg-fetch --save
; or -
bower install fetch
.
You will also need a Promise polyfill for older browsers. We recommend taylorhakes/promise-polyfill for its small size and Promises/A+ compatibility.
For use with webpack, add this package in the entry
configuration option
before your application entry point:
entry: ['whatwg-fetch', ...]
For Babel and ES2015+, make sure to import the file:
import '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 fetch
specification differs from jQuery.ajax()
in mainly two ways that
bear keeping in mind:
-
The Promise returned from
fetch()
won't reject on HTTP error status even if the response is a 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. -
By default,
fetch
won't send or receive any cookies from the server, resulting in unauthenticated requests if the site relies on maintaining a user session. See Sending cookies for how to opt into cookie handling.
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)
})
To automatically send cookies for the current domain, the credentials
option
must be provided:
fetch('/users', {
credentials: 'same-origin'
})
The "same-origin" value makes fetch
behave similarly to XMLHttpRequest with
regards to cookies. Otherwise, cookies won't get sent, resulting in these
requests not preserving the authentication session.
For CORS requests, use the "include" value to allow sending credentials to other domains:
fetch('https://example.com:1234/users', {
credentials: 'include'
})
Like with XMLHttpRequest, the Set-Cookie
response header returned from the
server is a forbidden header name and therefore can't be programatically
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
.
Bear in mind that the default behavior of fetch
is to ignore the Set-Cookie
header completely. To opt into accepting cookies from the server, you must use
the credentials
option.
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.
- Chrome
- Firefox
- Safari 6.1+
- Internet Explorer 10+
Note: modern browsers such as Chrome, Firefox, and Microsoft Edge contain native
implementations of window.fetch
, therefore the code from this polyfill doesn't
have any affect 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 on this project.