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April 14, 2022

Fetch

JavaScript can send network requests to the server and load new information whenever it’s needed.

For example, we can use a network request to:

  • Submit an order,
  • Load user information,
  • Receive latest updates from the server,
  • …etc.

…And all of that without reloading the page!

There’s an umbrella term “AJAX” (abbreviated Asynchronous JavaScript And XML) for network requests from JavaScript. We don’t have to use XML though: the term comes from old times, that’s why that word is there. You may have heard that term already.

There are multiple ways to send a network request and get information from the server.

The fetch() method is modern and versatile, so we’ll start with it. It’s not supported by old browsers (can be polyfilled), but very well supported among the modern ones.

The basic syntax is:

let promise = fetch(url, [options])
  • url – the URL to access.
  • options – optional parameters: method, headers etc.

Without options, this is a simple GET request, downloading the contents of the url.

The browser starts the request right away and returns a promise that the calling code should use to get the result.

Getting a response is usually a two-stage process.

First, the promise, returned by fetch, resolves with an object of the built-in Response class as soon as the server responds with headers.

At this stage we can check HTTP status, to see whether it is successful or not, check headers, but don’t have the body yet.

The promise rejects if the fetch was unable to make HTTP-request, e.g. network problems, or there’s no such site. Abnormal HTTP-statuses, such as 404 or 500 do not cause an error.

We can see HTTP-status in response properties:

  • status – HTTP status code, e.g. 200.
  • ok – boolean, true if the HTTP status code is 200-299.

For example:

let response = await fetch(url);

if (response.ok) { // if HTTP-status is 200-299
  // get the response body (the method explained below)
  let json = await response.json();
} else {
  alert("HTTP-Error: " + response.status);
}

Second, to get the response body, we need to use an additional method call.

Response provides multiple promise-based methods to access the body in various formats:

  • response.text() – read the response and return as text,
  • response.json() – parse the response as JSON,
  • response.formData() – return the response as FormData object (explained in the next chapter),
  • response.blob() – return the response as Blob (binary data with type),
  • response.arrayBuffer() – return the response as ArrayBuffer (low-level representation of binary data),
  • additionally, response.body is a ReadableStream object, it allows you to read the body chunk-by-chunk, we’ll see an example later.

For instance, let’s get a JSON-object with latest commits from GitHub:

let url = 'https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits';
let response = await fetch(url);

let commits = await response.json(); // read response body and parse as JSON

alert(commits[0].author.login);

Or, the same without await, using pure promises syntax:

fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits')
  .then(response => response.json())
  .then(commits => alert(commits[0].author.login));

To get the response text, await response.text() instead of .json():

let response = await fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits');

let text = await response.text(); // read response body as text

alert(text.slice(0, 80) + '...');

As a show-case for reading in binary format, let’s fetch and show a logo image of “fetch” specification (see chapter Blob for details about operations on Blob):

let response = await fetch('/article/fetch/logo-fetch.svg');

let blob = await response.blob(); // download as Blob object

// create <img> for it
let img = document.createElement('img');
img.style = 'position:fixed;top:10px;left:10px;width:100px';
document.body.append(img);

// show it
img.src = URL.createObjectURL(blob);

setTimeout(() => { // hide after three seconds
  img.remove();
  URL.revokeObjectURL(img.src);
}, 3000);
Important:

We can choose only one body-reading method.

If we’ve already got the response with response.text(), then response.json() won’t work, as the body content has already been processed.

let text = await response.text(); // response body consumed
let parsed = await response.json(); // fails (already consumed)

Response headers

The response headers are available in a Map-like headers object in response.headers.

It’s not exactly a Map, but it has similar methods to get individual headers by name or iterate over them:

let response = await fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits');

// get one header
alert(response.headers.get('Content-Type')); // application/json; charset=utf-8

// iterate over all headers
for (let [key, value] of response.headers) {
  alert(`${key} = ${value}`);
}

Request headers

To set a request header in fetch, we can use the headers option. It has an object with outgoing headers, like this:

let response = fetch(protectedUrl, {
  headers: {
    Authentication: 'secret'
  }
});

…But there’s a list of forbidden HTTP headers that we can’t set:

  • Accept-Charset, Accept-Encoding
  • Access-Control-Request-Headers
  • Access-Control-Request-Method
  • Connection
  • Content-Length
  • Cookie, Cookie2
  • Date
  • DNT
  • Expect
  • Host
  • Keep-Alive
  • Origin
  • Referer
  • TE
  • Trailer
  • Transfer-Encoding
  • Upgrade
  • Via
  • Proxy-*
  • Sec-*

These headers ensure proper and safe HTTP, so they are controlled exclusively by the browser.

POST requests

To make a POST request, or a request with another method, we need to use fetch options:

  • method – HTTP-method, e.g. POST,
  • body – the request body, one of:
    • a string (e.g. JSON-encoded),
    • FormData object, to submit the data as multipart/form-data,
    • Blob/BufferSource to send binary data,
    • URLSearchParams, to submit the data in x-www-form-urlencoded encoding, rarely used.

The JSON format is used most of the time.

For example, this code submits user object as JSON:

let user = {
  name: 'John',
  surname: 'Smith'
};

let response = await fetch('/article/fetch/post/user', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json;charset=utf-8'
  },
  body: JSON.stringify(user)
});

let result = await response.json();
alert(result.message);

Please note, if the request body is a string, then Content-Type header is set to text/plain;charset=UTF-8 by default.

But, as we’re going to send JSON, we use headers option to send application/json instead, the correct Content-Type for JSON-encoded data.

Sending an image

We can also submit binary data with fetch using Blob or BufferSource objects.

In this example, there’s a <canvas> where we can draw by moving a mouse over it. A click on the “submit” button sends the image to the server:

<body style="margin:0">
  <canvas id="canvasElem" width="100" height="80" style="border:1px solid"></canvas>

  <input type="button" value="Submit" onclick="submit()">

  <script>
    canvasElem.onmousemove = function(e) {
      let ctx = canvasElem.getContext('2d');
      ctx.lineTo(e.clientX, e.clientY);
      ctx.stroke();
    };

    async function submit() {
      let blob = await new Promise(resolve => canvasElem.toBlob(resolve, 'image/png'));
      let response = await fetch('/article/fetch/post/image', {
        method: 'POST',
        body: blob
      });

      // the server responds with confirmation and the image size
      let result = await response.json();
      alert(result.message);
    }

  </script>
</body>

Please note, here we don’t set Content-Type header manually, because a Blob object has a built-in type (here image/png, as generated by toBlob). For Blob objects that type becomes the value of Content-Type.

The submit() function can be rewritten without async/await like this:

function submit() {
  canvasElem.toBlob(function(blob) {
    fetch('/article/fetch/post/image', {
      method: 'POST',
      body: blob
    })
      .then(response => response.json())
      .then(result => alert(JSON.stringify(result, null, 2)))
  }, 'image/png');
}

Summary

A typical fetch request consists of two await calls:

let response = await fetch(url, options); // resolves with response headers
let result = await response.json(); // read body as json

Or, without await:

fetch(url, options)
  .then(response => response.json())
  .then(result => /* process result */)

Response properties:

  • response.status – HTTP code of the response,
  • response.ok – true if the status is 200-299.
  • response.headers – Map-like object with HTTP headers.

Methods to get response body:

  • response.text() – return the response as text,
  • response.json() – parse the response as JSON object,
  • response.formData() – return the response as FormData object (multipart/form-data encoding, see the next chapter),
  • response.blob() – return the response as Blob (binary data with type),
  • response.arrayBuffer() – return the response as ArrayBuffer (low-level binary data),

Fetch options so far:

  • method – HTTP-method,
  • headers – an object with request headers (not any header is allowed),
  • body – the data to send (request body) as string, FormData, BufferSource, Blob or UrlSearchParams object.

In the next chapters we’ll see more options and use cases of fetch.

Tasks

Fetch users from GitHub

Create an async function getUsers(names), that gets an array of GitHub logins, fetches the users from GitHub and returns an array of GitHub users.

The GitHub url with user information for the given USERNAME is: https://api.github.com/users/USERNAME.

There’s a test example in the sandbox.

Important details:

  1. There should be one fetch request per user.
  2. Requests shouldn’t wait for each other. So that the data arrives as soon as possible.
  3. If any request fails, or if there’s no such user, the function should return null in the resulting array.

Open a sandbox with tests.

To fetch a user we need: fetch('https://api.github.com/users/USERNAME').

If the response has status 200, call .json() to read the JS object.

Otherwise, if a fetch fails, or the response has non-200 status, we just return null in the resulting array.

So here’s the code:

async function getUsers(names) {
  let jobs = [];

  for(let name of names) {
    let job = fetch(`https://api.github.com/users/${name}`).then(
      successResponse => {
        if (successResponse.status != 200) {
          return null;
        } else {
          return successResponse.json();
        }
      },
      failResponse => {
        return null;
      }
    );
    jobs.push(job);
  }

  let results = await Promise.all(jobs);

  return results;
}

Please note: .then call is attached directly to fetch, so that when we have the response, it doesn’t wait for other fetches, but starts to read .json() immediately.

If we used await Promise.all(names.map(name => fetch(...))), and call .json() on the results, then it would wait for all fetches to respond. By adding .json() directly to each fetch, we ensure that individual fetches start reading data as JSON without waiting for each other.

That’s an example of how low-level Promise API can still be useful even if we mainly use async/await.

Open the solution with tests in a sandbox.

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