Bookmarking a good article about async/await

Original source

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. Other values are wrapped in a resolved promise automatically.

For instance, this function 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:

 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.

If we try to use await in non-async function, there would be a syntax error:

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

We will get this error if we do not put async before a function. As said, await only works inside an async function.

await won’t work in the top-level code

People who are just starting to use await tend to forget the fact that we can’t use await in top-level code. For example, this will not work:

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

We can wrap it into an anonymous async function, like this:

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

await accepts “thenables” Like promise.then, await allows to use thenable objects (those with a callable then method). The idea is that a 3rd-party object may not be a promise, but promise-compatible: if it supports .then, that’s enough to use with await.

Here’s a demo Thenable class, the await below accepts its instances:

 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();

If await gets a non-promise object with .then, it calls that method providing native functions resolve, reject as arguments. Then await waits until one of them is called (in the example above it happens in the line (*)) and then proceeds with the result.

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. In that case there will be a delay before await throws 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.

async/await and promise.then/catch

When we use async/await, we rarely need .then, because await handles the waiting for us. And we can use a regular try..catch instead of .catch. That’s usually (not always) more convenient.

But at the top level of the code, when we’re outside of any async function, we’re syntactically unable to use await, so it’s a normal practice to add .then/catch to handle the final result or falling-through errors.

Like in the line (*) of the example above.

async/await works well with Promise.all

When we need to wait for multiple promises, we can wrap them in Promise.all and then await:

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

In case of an error, it propagates as usual: from the failed promise to Promise.all, and then becomes an exception that we can catch using try..catch around the call.

Summary

The async keyword before a function has two effects:

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

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

  • If it’s an error, the exception is generated, same as if throw error were called at that very place.
  • Otherwise, it returns the result.

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.