Async / Await Warm Up

If you’re used to using Promises, async / await is an elegant way to make your javascript more legible. It evaluates as synchronous code but returns a promise that executes on the next microtask.

Let’s jump right in and poke around. Open up the babel repl and code along with me. Take a simple function and add async to the beginning:

async function sayHello() {
 return 'hello';
}
console.log(sayHello()); // => Promise {}

You’ll see that calling this function returns a Promise rather than “hello”. This is because anything returned from an async function is automatically wrapped in a promise. In order to log “hello” we could do this:

sayHello()
  .then(str => console.log(str)) // => 'hello'

OK so what about await?

First, write a function that returns a promise:

function mapLater(arr, fn, time) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(arr.map(fn));
    }, time);
  });
}

Pretty straight forward, we map over an array after given amount of time. Now let’s use that in an async function.

async function addAndMultiply(arr) {
  const added = await mapLater(arr, i => i + 2, 2000);
  const multiplied = await mapLater(added, i => i * 2, 2000);
  
  console.log(multiplied);
}
addAndMultiply([1, 3, 6, 20]); 
console.log('hello')
// => 'hello'
// 4 secs later => [6, 10, 16, 44]

You’ll see that “hello” is logged before we are returned our array. Our async function is non-blocking.

It’s important to note that await must always be inside of an async closure. Calling it in the global scope or in a function without the the async keyword will throw an error.

Handling errors

Imagine something goes wrong with our original mapLater function:

...
setTimeout(() => {
  reject('nope');
}, time);

Currently, we have no way of surfacing the error in our async addAndMultiply function. When we run it, the function will fail silently. To handle errors, one solution is to use try / catch:

async function addAndMultiply(arr) {
  try {
    const added = await mapLater(arr, i => i + 2, 2000);
    const multiplied = await mapLater(added, i => i * 2, 2000);
    console.log(multiplied); 
  } catch(err) {
    console.error(err); // 2 secs later => 'nope'
  }
}

Since errors bubble up by default, another practical solution would be to handle errors at the async entry point:

intricatelyNestedAsyncFunc().catch(err => console.error(err));

Real World

Use it today! So many great libraries provide APIs that return promises. For example, if you’re using fetch to retrieve data, you can start doing things like:

async function getProfileData(id) {
  try {
    const users = await getUser(id);
    ...  
  } catch {
    ...
  }
}

To use async / await now you’ll need to use Babel’s transform-async-to-generator.