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# window.fetch polyfill
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The `fetch()` function is a Promise-based mechanism for programmatically making
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web requests in the browser. This project is a polyfill that implements a subset
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of the standard [Fetch specification][], enough to make `fetch` a viable
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replacement for most uses of XMLHttpRequest in traditional web applications.
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This project adheres to the [Open Code of Conduct][]. By participating, you are
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expected to uphold this code.
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## Table of Contents
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* [Read this first](#read-this-first)
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* [Installation](#installation)
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* [Usage](#usage)
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* [HTML](#html)
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* [JSON](#json)
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* [Response metadata](#response-metadata)
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* [Post form](#post-form)
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* [Post JSON](#post-json)
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* [File upload](#file-upload)
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* [Caveats](#caveats)
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* [Handling HTTP error statuses](#handling-http-error-statuses)
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* [Sending cookies](#sending-cookies)
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* [Receiving cookies](#receiving-cookies)
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* [Obtaining the Response URL](#obtaining-the-response-url)
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* [Browser Support](#browser-support)
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## Read this first
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* If you believe you found a bug with how `fetch` behaves in Chrome or Firefox,
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please **avoid opening an issue in this repository**. This project is a
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_polyfill_, and since Chrome and Firefox both implement the `window.fetch`
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function natively, no code from this project actually takes any effect in
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these browsers. See [Browser support](#browser-support) for detailed
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information.
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* If you have trouble **making a request to another domain** (a different
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subdomain or port number also constitutes as another domain), please
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familiarize yourself with all the intricacies and limitations of [CORS][]
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requests. Because CORS requires participation of the server by implementing
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specific HTTP response headers, it is often nontrivial to set up or debug.
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CORS is exclusively handled by the browser's internal mechanisms which this
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polyfill cannot influence.
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* If you have trouble **maintaining the user's session** or [CSRF][] protection
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through `fetch` requests, please ensure that you've read and understood the
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[Sending cookies](#sending-cookies) section.
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* If this polyfill **doesn't work under Node.js environments**, that is expected,
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because this project is meant for web browsers only. You should ensure that your
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application doesn't try to package and run this on the server.
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* If you have an idea for a new feature of `fetch`, please understand that we
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are only ever going to add features and APIs that are a part of the
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[Fetch specification][]. You should **submit your feature requests** to the
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[repository of the specification](https://github.com/whatwg/fetch/issues)
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itself, rather than this repository.
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## Installation
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* `npm install whatwg-fetch --save`; or
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* `bower install fetch`.
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You will also need a Promise polyfill for [older browsers](http://caniuse.com/#feat=promises).
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We recommend [taylorhakes/promise-polyfill](https://github.com/taylorhakes/promise-polyfill)
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for its small size and Promises/A+ compatibility.
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For use with webpack, add this package in the `entry` configuration option
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before your application entry point:
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```javascript
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entry: ['whatwg-fetch', ...]
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```
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For Babel and ES2015+, make sure to import the file:
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```javascript
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import 'whatwg-fetch'
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```
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## Usage
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For a more comprehensive API reference that this polyfill supports, refer to
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https://github.github.io/fetch/.
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### HTML
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```javascript
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fetch('/users.html')
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.then(function(response) {
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return response.text()
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}).then(function(body) {
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document.body.innerHTML = body
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})
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```
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### JSON
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```javascript
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fetch('/users.json')
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.then(function(response) {
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return response.json()
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}).then(function(json) {
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console.log('parsed json', json)
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}).catch(function(ex) {
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console.log('parsing failed', ex)
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})
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```
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### Response metadata
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```javascript
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fetch('/users.json').then(function(response) {
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console.log(response.headers.get('Content-Type'))
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console.log(response.headers.get('Date'))
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console.log(response.status)
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console.log(response.statusText)
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})
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```
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### Post form
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```javascript
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var form = document.querySelector('form')
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fetch('/users', {
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method: 'POST',
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body: new FormData(form)
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})
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```
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### Post JSON
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```javascript
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fetch('/users', {
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method: 'POST',
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headers: {
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'Content-Type': 'application/json'
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},
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body: JSON.stringify({
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name: 'Hubot',
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login: 'hubot',
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})
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})
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```
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### File upload
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```javascript
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var input = document.querySelector('input[type="file"]')
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var data = new FormData()
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data.append('file', input.files[0])
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data.append('user', 'hubot')
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fetch('/avatars', {
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method: 'POST',
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body: data
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})
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```
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### Caveats
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The `fetch` specification differs from `jQuery.ajax()` in mainly two ways that
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bear keeping in mind:
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* The Promise returned from `fetch()` **won't reject on HTTP error status**
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even if the response is an HTTP 404 or 500. Instead, it will resolve normally,
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and it will only reject on network failure or if anything prevented the
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request from completing.
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* By default, `fetch` **won't send or receive any cookies** from the server,
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resulting in unauthenticated requests if the site relies on maintaining a user
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session. See [Sending cookies](#sending-cookies) for how to opt into cookie
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handling.
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#### Handling HTTP error statuses
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To have `fetch` Promise reject on HTTP error statuses, i.e. on any non-2xx
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status, define a custom response handler:
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```javascript
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function checkStatus(response) {
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if (response.status >= 200 && response.status < 300) {
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return response
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} else {
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var error = new Error(response.statusText)
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error.response = response
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throw error
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}
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}
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function parseJSON(response) {
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return response.json()
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}
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fetch('/users')
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.then(checkStatus)
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.then(parseJSON)
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.then(function(data) {
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console.log('request succeeded with JSON response', data)
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}).catch(function(error) {
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console.log('request failed', error)
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})
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```
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#### Sending cookies
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To automatically send cookies for the current domain, the `credentials` option
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must be provided:
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```javascript
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fetch('/users', {
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credentials: 'same-origin'
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})
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```
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The "same-origin" value makes `fetch` behave similarly to XMLHttpRequest with
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regards to cookies. Otherwise, cookies won't get sent, resulting in these
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requests not preserving the authentication session.
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For [CORS][] requests, use the "include" value to allow sending credentials to
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other domains:
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```javascript
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fetch('https://example.com:1234/users', {
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credentials: 'include'
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})
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```
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#### Receiving cookies
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As with XMLHttpRequest, the `Set-Cookie` response header returned from the
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server is a [forbidden header name][] and therefore can't be programmatically
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read with `response.headers.get()`. Instead, it's the browser's responsibility
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to handle new cookies being set (if applicable to the current URL). Unless they
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are HTTP-only, new cookies will be available through `document.cookie`.
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Bear in mind that the default behavior of `fetch` is to ignore the `Set-Cookie`
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header completely. To opt into accepting cookies from the server, you must use
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the `credentials` option.
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#### Obtaining the Response URL
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Due to limitations of XMLHttpRequest, the `response.url` value might not be
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reliable after HTTP redirects on older browsers.
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The solution is to configure the server to set the response HTTP header
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`X-Request-URL` to the current URL after any redirect that might have happened.
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It should be safe to set it unconditionally.
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``` ruby
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# Ruby on Rails controller example
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response.headers['X-Request-URL'] = request.url
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```
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This server workaround is necessary if you need reliable `response.url` in
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Firefox < 32, Chrome < 37, Safari, or IE.
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## Browser Support
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- Chrome
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- Firefox
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- Safari 6.1+
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- Internet Explorer 10+
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Note: modern browsers such as Chrome, Firefox, and Microsoft Edge contain native
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implementations of `window.fetch`, therefore the code from this polyfill doesn't
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have any effect on those browsers. If you believe you've encountered an error
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with how `window.fetch` is implemented in any of these browsers, you should file
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an issue with that browser vendor instead of this project.
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[fetch specification]: https://fetch.spec.whatwg.org
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[open code of conduct]: http://todogroup.org/opencodeofconduct/#fetch/opensource@github.com
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[cors]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
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"Cross-origin resource sharing"
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[csrf]: https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet
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"Cross-site request forgery"
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[forbidden header name]: https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name
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