Excerpt

The Fetch API provides a JavaScript interface for accessing and manipulating parts of the HTTP pipeline, such as requests and responses. It also provides a global fetch() method that provides an easy, logical way to fetch resources asynchronously across the network.

The Fetch API provides a JavaScript interface for accessing and manipulating parts of the HTTP pipeline, such as requests and responses. It also provides a global fetch() method that provides an easy, logical way to fetch resources asynchronously across the network.

This kind of functionality was previously achieved using XMLHttpRequest. Fetch provides a better alternative that can be easily used by other technologies such as Service Workers. Fetch also provides a single logical place to define other HTTP-related concepts such as CORS and extensions to HTTP.

The fetch specification differs from jQuery.ajax() in the following significant ways:

• The Promise returned from fetch() won't reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, as soon as the server responds with headers, the Promise will resolve normally (with the ok property of the response set to false if the response isn't in the range 200 -299), and it will only reject on network failure or if anything prevented the request from completing.
• fetch() won't send cross-origin cookies unless you set the credentials init option. (Since April 2018. The spec changed the default credentials policy to same-origin. Firefox changed since 61.0b13.)

A basic fetch request is really simple to set up. Have a look at the following code:

fetch('http://example.com/movies.json')
  .then(response => response.json())
  .then(data => console.log(data));

Here we are fetching a JSON file across the network and printing it to the console. The simplest use of fetch() takes one argument — the path to the resource you want to fetch — and does not directly return the JSON response body but instead returns a promise that resolves with a Response object.

The Response object, in turn, does not directly contain the actual JSON response body but is instead a representation of the entire HTTP response. So, to extract the JSON body content from the Response object, we use the json() method, which returns a second promise that resolves with the result of parsing the response body text as JSON.

Note: See the Body section for similar methods to extract other types of body content.

Fetch requests are controlled by the connect-src directive of Content Security Policy rather than the directive of the resources it's retrieving.

Supplying request options

The fetch() method can optionally accept a second parameter, an init object that allows you to control a number of different settings:

See fetch() for the full options available, and more details.

js
// Example POST method implementation:
async function postData(url = '', data = {}) {
  // Default options are marked with *
  const response = await fetch(url, {
    method: 'POST', // *GET, POST, PUT, DELETE, etc.
    mode: 'cors', // no-cors, *cors, same-origin
    cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
    credentials: 'same-origin', // include, *same-origin, omit
    headers: {
      'Content-Type': 'application/json'
      // 'Content-Type': 'application/x-www-form-urlencoded',
    },
    redirect: 'follow', // manual, *follow, error
    referrerPolicy: 'no-referrer', // no-referrer, *no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin, unsafe-url
    body: JSON.stringify(data) // body data type must match "Content-Type" header
  });
  return response.json(); // parses JSON response into native JavaScript objects
}

postData('https://example.com/answer', { answer: 42 })
  .then(data => {
    console.log(data); // JSON data parsed by `data.json()` call
  });

Note that mode: "no-cors" only allows a limited set of headers in the request:

• Accept
• Accept-Language
• Content-Language
• Content-Type with a value of application/x-www-form-urlencoded, multipart/form-data, or text/plain

Sending a request with credentials included

To cause browsers to send a request with credentials included on both same-origin and cross-origin calls, add credentials: 'include' to the init object you pass to the fetch() method.

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

Note: Access-Control-Allow-Origin is prohibited from using a wildcard for requests with credentials: 'include'. In such cases, the exact origin must be provided; even if you are using a CORS unblocker extension, the requests will still fail.

Note: Browsers should not send credentials in preflight requests irrespective of this setting. For more information see: CORS > Requests with credentials.

If you only want to send credentials if the request URL is on the same origin as the calling script, add credentials: 'same-origin'.

// The calling script is on the origin 'https://example.com'

fetch('https://example.com', {
  credentials: 'same-origin'
});

To instead ensure browsers don't include credentials in the request, use credentials: 'omit'.

fetch('https://example.com', {
  credentials: 'omit'
})

Uploading JSON data

Use fetch() to POST JSON-encoded data.

const data = { username: 'example' };

fetch('https://example.com/profile', {
  method: 'POST', // or 'PUT'
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(data),
})
.then(response => response.json())
.then(data => {
  console.log('Success:', data);
})
.catch((error) => {
  console.error('Error:', error);
});

Uploading a file

Files can be uploaded using an HTML <input type="file" /> input element, FormData() and fetch().

const formData = new FormData();
const fileField = document.querySelector('input[type="file"]');

formData.append('username', 'abc123');
formData.append('avatar', fileField.files[0]);

fetch('https://example.com/profile/avatar', {
  method: 'PUT',
  body: formData
})
.then(response => response.json())
.then(result => {
  console.log('Success:', result);
})
.catch(error => {
  console.error('Error:', error);
});

Uploading multiple files

Files can be uploaded using an HTML <input type="file" multiple /> input element, FormData() and fetch().

const formData = new FormData();
const photos = document.querySelector('input[type="file"][multiple]');

formData.append('title', 'My Vegas Vacation');
for (let i = 0; i < photos.files.length; i++) {
  formData.append(`photos_${i}`, photos.files[i]);
}

fetch('https://example.com/posts', {
  method: 'POST',
  body: formData,
})
.then(response => response.json())
.then(result => {
  console.log('Success:', result);
})
.catch(error => {
  console.error('Error:', error);
});

Processing a text file line by line

The chunks that are read from a response are not broken neatly at line boundaries and are Uint8Arrays, not strings. If you want to fetch a text file and process it line by line, it is up to you to handle these complications. The following example shows one way to do this by creating a line iterator (for simplicity, it assumes the text is UTF-8, and doesn't handle fetch errors).

async function* makeTextFileLineIterator(fileURL) {
  const utf8Decoder = new TextDecoder('utf-8');
  const response = await fetch(fileURL);
  const reader = response.body.getReader();
  let { value: chunk, done: readerDone } = await reader.read();
  chunk = chunk ? utf8Decoder.decode(chunk) : '';

  const re = /\n|\r|\r\n/gm;
  let startIndex = 0;
  let result;

  for (;;) {
    let result = re.exec(chunk);
    if (!result) {
      if (readerDone) {
        break;
      }
      let remainder = chunk.substr(startIndex);
      ({ value: chunk, done: readerDone } = await reader.read());
      chunk = remainder + (chunk ? utf8Decoder.decode(chunk) : '');
      startIndex = re.lastIndex = 0;
      continue;
    }
    yield chunk.substring(startIndex, result.index);
    startIndex = re.lastIndex;
  }
  if (startIndex < chunk.length) {
    // last line didn't end in a newline char
    yield chunk.substr(startIndex);
  }
}

async function run() {
  for await (let line of makeTextFileLineIterator(urlOfFile)) {
    processLine(line);
  }
}

run();

Checking that the fetch was successful

A fetch() promise will reject with a TypeError when a network error is encountered or CORS is misconfigured on the server-side, although this usually means permission issues or similar — a 404 does not constitute a network error, for example. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true. The code would look something like this:

fetch('flowers.jpg')
  .then(response => {
    if (!response.ok) {
      throw new Error('Network response was not OK');
    }
    return response.blob();
  })
  .then(myBlob => {
    myImage.src = URL.createObjectURL(myBlob);
  })
  .catch(error => {
    console.error('There has been a problem with your fetch operation:', error);
  });

Supplying your own request object

Instead of passing a path to the resource you want to request into the fetch() call, you can create a request object using the Request() constructor, and pass that in as a fetch() method argument:

const myHeaders = new Headers();

const myRequest = new Request('flowers.jpg', {
  method: 'GET',
  headers: myHeaders,
  mode: 'cors',
  cache: 'default',
});

fetch(myRequest)
  .then(response => response.blob())
  .then(myBlob => {
    myImage.src = URL.createObjectURL(myBlob);
  });

Request() accepts exactly the same parameters as the fetch() method. You can even pass in an existing request object to create a copy of it:

const anotherRequest = new Request(myRequest, myInit);

This is pretty useful, as request and response bodies are one use only. Making a copy like this allows you to make use of the request/response again while varying the init options if desired. The copy must be made before the body is read, and reading the body in the copy will also mark it as read in the original request.

Note: There is also a clone() method that creates a copy. Both methods of creating a copy will fail if the body of the original request or response has already been read, but reading the body of a cloned response or request will not cause it to be marked as read in the original.

The Headers interface allows you to create your own headers object via the Headers() constructor. A headers object is a simple multi-map of names to values:

const content = 'Hello World';
const myHeaders = new Headers();
myHeaders.append('Content-Type', 'text/plain');
myHeaders.append('Content-Length', content.length.toString());
myHeaders.append('X-Custom-Header', 'ProcessThisImmediately');

The same can be achieved by passing an array of arrays or an object literal to the constructor:

const myHeaders = new Headers({
  'Content-Type': 'text/plain',
  'Content-Length': content.length.toString(),
  'X-Custom-Header': 'ProcessThisImmediately'
});

The contents can be queried and retrieved:

console.log(myHeaders.has('Content-Type')); // true
console.log(myHeaders.has('Set-Cookie')); // false
myHeaders.set('Content-Type', 'text/html');
myHeaders.append('X-Custom-Header', 'AnotherValue');

console.log(myHeaders.get('Content-Length')); // 11
console.log(myHeaders.get('X-Custom-Header')); // ['ProcessThisImmediately', 'AnotherValue']

myHeaders.delete('X-Custom-Header');
console.log(myHeaders.get('X-Custom-Header')); // null

Some of these operations are only useful in ServiceWorkers, but they provide a much nicer API for manipulating headers.

All of the Headers methods throw a TypeError if a header name is used that is not a valid HTTP Header name. The mutation operations will throw a TypeError if there is an immutable guard (see below). Otherwise, they fail silently. For example:

const myResponse = Response.error();
try {
  myResponse.headers.set('Origin', 'http://mybank.com');
} catch (e) {
  console.log('Cannot pretend to be a bank!');
}

A good use case for headers is checking whether the content type is correct before you process it further. For example:

fetch(myRequest)
  .then(response => {
     const contentType = response.headers.get('content-type');
     if (!contentType || !contentType.includes('application/json')) {
       throw new TypeError("Oops, we haven't got JSON!");
     }
     return response.json();
  })
  .then(data => {
      /* process your data further */
  })
  .catch(error => console.error(error));

Guard

Since headers can be sent in requests and received in responses, and have various limitations about what information can and should be mutable, headers' objects have a guard property. This is not exposed to the Web, but it affects which mutation operations are allowed on the headers object.