Async/await

There’s a special syntax to work with promises in a more comfortable fashion, called “async/await”. It’s surprisingly easy to understand and use.

Async functions

Let’s start with the async keyword. It can be placed before a function, like this:

async function f() {
  return 1;
}

The word “async” before a function means one simple thing: a function always returns a promise. Even If a function actually returns a non-promise value, prepending the function definition with the “async” keyword directs Javascript to automatically wrap that value in a resolved promise.

For instance, the code above returns a resolved promise with the result of 1, let’s test it:

async function f() {
  return 1;
}

f().then(alert); // 1

…We could explicitly return a promise, that would be the same as:

async function f() {
  return Promise.resolve(1);
}

f().then(alert); // 1

So, async ensures that the function returns a promise, and wraps non-promises in it. Simple enough, right? But not only that. There’s another keyword, await, that works only inside async functions, and it’s pretty cool.

Await

The syntax:

// works only inside async functions
let value = await promise;

The keyword await makes JavaScript wait until that promise settles and returns its result.

Here’s an example with a promise that resolves in 1 second:

async function f() {

  let promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve("done!"), 1000)
  });

  let result = await promise; // wait till the promise resolves (*)

  alert(result); // "done!"
}

f();

The function execution “pauses” at the line (*) and resumes when the promise settles, with result becoming its result. So the code above shows “done!” in one second.

Let’s emphasize: await literally makes JavaScript wait until the promise settles, and then go on with the result. That doesn’t cost any CPU resources, because the engine can do other jobs meanwhile: execute other scripts, handle events etc.

It’s just a more elegant syntax of getting the promise result than promise.then, easier to read and write.

function f() {
  let promise = Promise.resolve(1);
  let result = await promise; // Syntax error
}

Let’s take the showAvatar() example from the chapter Promises chaining and rewrite it using async/await:

1. We’ll need to replace .then calls with await.
2. Also we should make the function async for them to work.

async function showAvatar() {

  // read our JSON
  let response = await fetch('/article/promise-chaining/user.json');
  let user = await response.json();

  // read github user
  let githubResponse = await fetch(`https://api.github.com/users/${user.name}`);
  let githubUser = await githubResponse.json();

  // show the avatar
  let img = document.createElement('img');
  img.src = githubUser.avatar_url;
  img.className = "promise-avatar-example";
  document.body.append(img);

  // wait 3 seconds
  await new Promise((resolve, reject) => setTimeout(resolve, 3000));

  img.remove();

  return githubUser;
}

showAvatar();

Pretty clean and easy to read, right? Much better than before.

// syntax error in top-level code
let response = await fetch('/article/promise-chaining/user.json');
let user = await response.json();

(async () => {
  let response = await fetch('/article/promise-chaining/user.json');
  let user = await response.json();
  ...
})();

class Thenable {
  constructor(num) {
    this.num = num;
  }
  then(resolve, reject) {
    alert(resolve);
    // resolve with this.num*2 after 1000ms
    setTimeout(() => resolve(this.num * 2), 1000); // (*)
  }
};

async function f() {
  // waits for 1 second, then result becomes 2
  let result = await new Thenable(1);
  alert(result);
}

f();

class Waiter {
  async wait() {
    return await Promise.resolve(1);
  }
}

new Waiter()
  .wait()
  .then(alert); // 1

Error handling

If a promise resolves normally, then await promise returns the result. But in case of a rejection, it throws the error, just as if there were a throw statement at that line.

This code:

async function f() {
  await Promise.reject(new Error("Whoops!"));
}

…Is the same as this:

async function f() {
  throw new Error("Whoops!");
}

In real situations, the promise may take some time before it rejects. So await will wait, and then throw an error.

We can catch that error using try..catch, the same way as a regular throw:

async function f() {

  try {
    let response = await fetch('http://no-such-url');
  } catch(err) {
    alert(err); // TypeError: failed to fetch
  }
}

f();

In case of an error, the control jumps to the catch block. We can also wrap multiple lines:

async function f() {

  try {
    let response = await fetch('/no-user-here');
    let user = await response.json();
  } catch(err) {
    // catches errors both in fetch and response.json
    alert(err);
  }
}

f();

If we don’t have try..catch, then the promise generated by the call of the async function f() becomes rejected. We can append .catch to handle it:

async function f() {
  let response = await fetch('http://no-such-url');
}

// f() becomes a rejected promise
f().catch(alert); // TypeError: failed to fetch // (*)

If we forget to add .catch there, then we get an unhandled promise error (viewable in the console). We can catch such errors using a global event handler as described in the chapter Error handling with promises.

// wait for the array of results
let results = await Promise.all([
  fetch(url1),
  fetch(url2),
  ...
]);

Microtask queue

As we’ve seen in the chapter Microtasks and event loop, promise handlers are executed asynchronously. Every .then/catch/finally handler first gets into the “microtask queue” and executed after the current code is complete.

Async/await is based on promises, so it uses the same microtask queue internally, and has the similar priority over macrotasks.

For instance, we have:

• setTimeout(handler, 0), that should run handler with zero delay.
• let x = await f(), function f() is async, but returns immediately.

Which one runs first if await is below setTimeout in the code?

async function f() {
  return 1;
}

(async () => {
    setTimeout(() => alert('timeout'), 0);

    await f();

    alert('await');
})();

There’s no ambiguity here: await always finishes first, because (as a microtask) it has a higher priority than setTimeout handling.

Summary

The async keyword before a function has two effects:

1. Makes it always return a promise.
2. Allows to use await in it.

The await keyword before a promise makes JavaScript wait until that promise settles, and then:

1. If it’s an error, the exception is generated, same as if throw error were called at that very place.
2. Otherwise, it returns the result, so we can assign it to a value.

Together they provide a great framework to write asynchronous code that is easy both to read and write.

With async/await we rarely need to write promise.then/catch, but we still shouldn’t forget that they are based on promises, because sometimes (e.g. in the outermost scope) we have to use these methods. Also Promise.all is a nice thing to wait for many tasks simultaneously.