fetch module

This project adheres to the [Open Code of Conduct][code-of-conduct]. By participating, you are expected to uphold this code. [code-of-conduct]: http://todogroup.org/opencodeofconduct/#fetch/[email protected]

This module exposes the fetch function, which is an easier way to make web requests and handle responses than using an XMLHttpRequest. This module is written as closely as possible to the standard Fetch specification at https://fetch.spec.whatwg.org.

Installation

You'll need a Promise polyfill for older browsers.

$ bower install es6-promise

For a node.js implementation, try node-fetch.

For use with webpack, refer to Using WebPack with shims and polyfills.

Differences with github/fetch

The major difference is the addition of the following methods and properties to the Request object:

• abort: cancels a request
• send: sends a request
• then, catch: both update the internal promise object, making requests look like promises
• getXHR: returns the used XHR object
• isAborted: set to true when the request is aborted
• promise: promise of the request
• xhr: XMLHttpRequest object used for the request

As a result, this module is not a polyfill. Instead, it exports the following objects to enable their import in environments that already have an implementation of fetch:

• Body
• Request
• Response
• Headers
• fetch

Usage

The fetch function supports any HTTP method. We'll focus on GET and POST example requests.

HTML

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

JSON

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

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

var form = document.querySelector('form')

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

Post JSON

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

File upload

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 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 any cookies to the server, resulting in unauthenticated requests if the site relies on maintaining a user session.

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:

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)
  })

Aborting a request

To abort (cancel) a fetch, simply call abort() on the request object returned by fetch:

var request = fetch('/big-data')
  .then(checkStatus)
  .then(parseJSON)
  .catch(function(error) {
    if (request.isAborted) {
      console.log('request was aborted')
    }
  })
// Later…
request.abort()

Sending cookies

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

fetch('/users', {
  credentials: 'same-origin'
})

This option makes fetch behave similar to XMLHttpRequest with regards to cookies. Otherwise, cookies won't get sent, resulting in these requests not preserving the authentication session.

Use the include value to send cookies in a cross-origin resource sharing (CORS) request.

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

Receiving cookies

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.

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 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

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