JavaScript's Async + Await in 5 Minutes

Dec 15, 2019

~5min

Bye Bye Promise inception and callback fury! 👋🎉

It’s likely that you’ve encountered Promises in your JavaScript (If you haven’t check out this guide quick 👍). They allow you to hook into the completion of asynchronous calls. They make it simple to chain asynchronous operations or even group them together. There is one tiny downside. When consuming Promises, the syntax isn’t always the prettiest.

Introducing async + await 🎉

For those in camp TL;DR async + await are syntactic sugar for consuming your Promises 🍭 They aid in understanding the flow of your code. There are no new concepts, it’s Promises with nicer shoes 👟 Scroll down for a gist ⌨️

Baking a cake with code 🍰 permalink

We are going to bake a cake 🍰 yum! To bake the cake, we first need to get the ingredients. I’m sorry, it’s a plain sponge 😅

Butter
Flour
Sugar
Eggs 🥚

In our code, getting each ingredient requires an asynchronous operation.

For example, here is the method getButter:


            javascript
            
          const getButter = () => new Promise((resolve, reject) => {
  setTimeout(() => resolve('Butter'), 3000)
})

These operations will become part of a getIngredients method. When we bake the cake, we will need to invoke getIngredients before mixing, etc.

With Promises permalink

Let’s assume we need to chain each asynchronous operation. getIngredients is a journey around a supermarket picking up one ingredient at a time 🛒

In most cases, we only need to chain operations if they are dependent on each other. For example, if the second operation needs the return value from the first operation and so on.

In our example, it may be that we can only add one item to our shopping basket at a time. That means we need to progress through the ingredients one by one. Remember the code here is hypothetical and to show the use of Promises 😉

How might getIngredients look with Promises? I’ve certainly seen nested Promises like this before 👀


            javascript
            
          const getIngredients = () => new Promise((resolve, reject) => {
  getButter().then((butter) => {
    updateBasket(butter)
    getFlour().then((flour) => {
      updateBasket(flour)
      getSugar().then((sugar) => {
        updateBasket(sugar)
        getEggs().then((eggs) => {
          updateBasket(eggs)
          resolve(basket)
        })
      })
    })
  })
})

This works but doesn’t look great 👎 It would look better with a Promise chain.


            javascript
            
          const getIngredients = () => getButter()
  .then(updateBasket)
  .then(getFlour)
  .then(updateBasket)
  .then(getSugar)
  .then(updateBasket)
  .then(getEggs)
  .then(updateBasket)

If we were doing our grocery shopping online, we could use Promise.all 🤓


            javascript
            
          const getIngredients = () => Promise.all([
  getButter(),
  getFlour(),
  getSugar(),
  getEggs(),
])

These look much tidier but we still need to use a callback to get those ingredients.


            javascript
            
          getIngredients().then(ingredients => doSomethingWithIngredients(ingredients))

Tidying it up with async + await permalink

Let’s sprinkle on that syntactic sugar 🍭 To use the await keyword, we must first declare a method as asynchronous with the async keyword. It’s important to note that an async method will always return a Promise. That means there is no need to return a Promise 🎉

Let’s declare getIngredients as async


            javascript
            
          const getIngredients = async () => {}

Now, how might those Promises look with sugar? The await keyword allows us to wait for a Promise and define a variable with the return value of that Promise. It's a little verbose for this example, but let’s apply that sugar to getIngredients.


            javascript
            
          const getIngredients = async () => {
  const butter = await getButter()
  const flour = await getFlour()
  const sugar = await getSugar()
  const eggs = await getEggs()
  return [
    butter,
    flour,
    sugar,
    eggs,
  ]
}

The code isn't smaller, but it's more verbose and concise 👍 No more callbacks. It's when we consume a Promise that the syntactic sugar comes into play.


            javascript
            
          const bakeACake = async () => {
  const ingredients = await getIngredients()
  // do something with the ingredients, no more ".then" 🙌
}

Wow! 😎 How much cleaner is that?

The use of async and await makes our code procedural and comprehensive. It looks cleaner and does exactly the same thing. It’s important to remember here that we aren’t replacing Promises, we're still using them under the hood. Now we're using them with a new cleaner syntax.

And yes, this works with Promise.all too. So if we had done the shopping online, our code gets even smaller.


            javascript
            
          const getIngredients = async () => {
  const ingredients = await Promise.all([
    getButter(),
    getFlour(),
    getSugar(),
    getEggs(),
  ])
  return ingredients
}

We don't need that wrapper function anymore!


            javascript
            
          const getIngredients = async () =>
  await Promise.all([getButter(), getFlour(), getSugar(), getEggs()]);

Awaiting a non-Promise permalink

How about if the value you await on is not a Promise? In our example, the asynchronous functions are returning a String after a setTimeout.


            javascript
            
          const egg = await 🥚

There will be no error, the value becomes a resolved Promise 😅

What about rejections? permalink

Up until now, we’ve dealt with the happy path 😃 But how about in the case where a Promise rejects?

For example, what if there are no eggs in stock? Our asynchronous function for getEggs would reject with a potential error.

To accommodate for this, a simple try/catch statement will do the trick 👍


            javascript
            
          const getIngredients = async () => {
  try {
    const butter = await 'Butter'
    const flour = await getFlour()
    const sugar = await getSugar()
    const eggs = await getEggs()
    return [
      butter,
      flour,
      sugar,
      eggs,
    ]
  } catch(e) { return e }
}

We could wrap at this level or higher up where we invoke getIngredients 👍

Consuming our function and baking the cake 🍰 permalink

If you’ve got this far, we’ve created our function for getIngredients with the new async + await keywords. What might the rest of it look like?


            javascript
            
          const bakeACake = async () => {
  try {
    // get the ingredients
    const ingredients = await getIngredients()
    // mix them together
    const cakeMix = await mix(ingredients)
    // put in oven on 180C, gas mark 4for 20-25 minutes
    const hotCake = await cook(cakeMix)
    // allow to stand before serving
    const cake = await stand(hotCake)
    return cake
  } catch (e) { return e }
}

Much cleaner than what we might have done previously with Promises 🎉

That’s it! Baking a cake with async + await in 5 minutes 🍰 permalink

If you’ve got this far, thanks for reading 😃 I’ve put together a gist with some possible example code that can be seen below along with some further resources on async + await.

The important takeaways ⚠️;

async functions will always return a Promise
await will in most cases be used against a Promise or a group of Promises
Handle any potential errors with a try/catch statement 👍
We haven’t touched on this but you can await an await. Making a fetch request you might await the request and then await the json function.


            javascript
            
          const data = await (await fetch(`${dataUrl}`)).json()

As always, any questions or suggestions, please feel free to leave a response or tweet me 🐦! Be sure to follow me on the socials 😎


            javascript
            
          const PROB = 0.2
const grabIngredient = ingredient => () => {
	return new Promise((resolve, reject) => {
      setTimeout(() => {
        if (Math.random() > PROB) {
            resolve(ingredient)
        } else {
        	reject(`Sorry, we've got no ${ingredient}`)
        }
      }, Math.random() * 1000)
    })
}


// boilerplate functions for getting the different ingredients
const getButter = grabIngredient('Butter')
const getFlour = grabIngredient('Flour')
const getSugar = grabIngredient('Sugar')
const getEggs = grabIngredient('Eggs')

const getIngredientsFromTheSuperMarket = async () => {
  try {
    const butter = await getButter()
    const flour = await getFlour()
    const sugar = await getSugar()
    const eggs = await getEggs()
    return [
      butter,
      flour,
      sugar,
      eggs,
    ]
  } catch(e) { return e }
}

const getIngredientsOnline = async () => await Promise.all([
  getButter(),
  getFlour(),
  getSugar(),
  getEggs(),
])

// boilerplate async functions that return strings
const mix = async (ingredients) => `Mixing ${ingredients}`
const cook = async (cakeMix) => 'Hot Cake'
const stand = async (hotCake) => '🍰'

const bakeACake = async () => {
  try {
    const ingredients = await getIngredientsOnline()
    const cakeMix = await mix(ingredients)
    const hotCake = await cook(cakeMix)
    const cake = await stand(hotCake)
    console.info('BAKED', cake)
    return cake
  } catch (e) { console.info(e) }
}

bakeACake()

Further resources permalink

await — MDN
async function — MDN
Async + Await podcast — Syntax.fm