# 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.

This project adheres to the [Open Code of Conduct][]. By participating, you are
expected to uphold this code.

## Table of Contents

* [Read this first](#read-this-first)
* [Installation](#installation)
* [Usage](#usage)
  * [HTML](#html)
  * [JSON](#json)
  * [Response metadata](#response-metadata)
  * [Post form](#post-form)
  * [Post JSON](#post-json)
  * [File upload](#file-upload)
  * [Caveats](#caveats)
    * [Handling HTTP error statuses](#handling-http-error-statuses)
    * [Sending cookies](#sending-cookies)
    * [Receiving cookies](#receiving-cookies)
    * [Obtaining the Response URL](#obtaining-the-response-url)
* [Browser Support](#browser-support)

## Read this first

* 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 the `window.fetch`
  function natively, no code from this project actually takes any effect in
  these browsers. See [Browser support](#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](#sending-cookies) section.

* If this polyfill **doesn't work under Node.js environments**, 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](https://github.com/whatwg/fetch/issues)
  itself, rather than this repository.

## Installation

* `npm install whatwg-fetch --save`; or

* `bower install fetch`.

You will also need a Promise polyfill for [older browsers](http://caniuse.com/#feat=promises).
We recommend [taylorhakes/promise-polyfill](https://github.com/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:

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

For Babel and ES2015+, make sure to import the file:

```javascript
import 'whatwg-fetch'
```

## Usage

For a more comprehensive API reference that this polyfill supports, refer to
https://github.github.io/fetch/.

### HTML

```javascript
fetch('/users.html')
  .then(function(response) {
    return response.text()
  }).then(function(body) {
    document.body.innerHTML = body
  })
```

### JSON

```javascript
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

```javascript
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

```javascript
var form = document.querySelector('form')

fetch('/users', {
  method: 'POST',
  body: new FormData(form)
})
```

### Post JSON

```javascript
fetch('/users', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    name: 'Hubot',
    login: 'hubot',
  })
})
```

### File upload

```javascript
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 `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 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.

* 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](#sending-cookies) for how to opt into cookie
  handling.

#### 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:

```javascript
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

To automatically send cookies for the current domain, the `credentials` option
must be provided:

```javascript
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:

```javascript
fetch('https://example.com:1234/users', {
  credentials: 'include'
})
```

#### 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`.

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.

#### 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
# 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.

## Browser Support

- 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 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.


  [fetch specification]: https://fetch.spec.whatwg.org
  [open code of conduct]: http://todogroup.org/opencodeofconduct/#fetch/opensource@github.com
  [cors]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
    "Cross-origin resource sharing"
  [csrf]: https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet
    "Cross-site request forgery"
  [forbidden header name]: https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name