whatwg-fetch
Advanced tools
The whatwg-fetch npm package is a polyfill for the Fetch API, a modern interface for making network requests in browsers and Node.js. It allows developers to make HTTP requests to retrieve or send data to remote servers in an easy and efficient way. The Fetch API provides a more powerful and flexible feature set compared to older technologies like XMLHttpRequest.
Making GET requests
This code sample demonstrates how to make a GET request to retrieve data from a specified URL and then process the response as JSON.
fetch('https://api.example.com/data')
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error('Error:', error));Making POST requests
This code sample shows how to make a POST request to send JSON data to a server and then handle the JSON response.
fetch('https://api.example.com/data', {
method: 'POST',
headers: {
'Content-Type': 'application/json'
},
body: JSON.stringify({ key: 'value' })
})
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error('Error:', error));Handling HTTP errors
This code sample illustrates how to handle HTTP errors by checking the response status before processing the response.
fetch('https://api.example.com/data')
.then(response => {
if (!response.ok) {
throw new Error('Network response was not ok');
}
return response.json();
})
.then(data => console.log(data))
.catch(error => console.error('Error:', error));Axios is a popular HTTP client for the browser and Node.js. It provides a promise-based API and has a similar feature set to whatwg-fetch, including interceptors, automatic transforms for JSON data, and cancellation. Axios is often preferred for its more convenient error handling and wider browser support without polyfills.
node-fetch is a light-weight module that brings the Fetch API to Node.js. It is similar to whatwg-fetch but is specifically designed for Node.js environments. It provides a simple interface for making HTTP requests and is a drop-in replacement for native fetch in Node.js.
Ky is a tiny and elegant HTTP client based on the Fetch API. It extends the capabilities of fetch with additional features like retrying failed requests, timeout handling, and more convenient methods for parsing responses. Ky is designed to offer a more developer-friendly API compared to whatwg-fetch.
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.
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.
npm install whatwg-fetch --save
As an alternative to using npm, you can obtain fetch.umd.js from the
Releases section. The UMD distribution is compatible with AMD and CommonJS
module loaders, as well as loading directly into a page via <script> tag.
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.
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.
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'
})
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'
})
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.
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.
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.
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 '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()
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.
FAQs
A window.fetch polyfill.
The npm package whatwg-fetch receives a total of 10,161,252 weekly downloads. As such, whatwg-fetch popularity was classified as popular.
We found that whatwg-fetch demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 3 open source maintainers collaborating on the project.
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