Explaining Promises, Async, and Await in JavaScript

March 15, 2024

1,835 words

Post contents

When using JavaScript, you'll likely to've run into a function like fetch that doesn't pass you a value immediately.

const res = fetch("example.com/something");   // ^? Promise<"pending">

If you try to rely on the value returned from fetch without anything else; you'll likely run into something you won't expect:

const json = res.json();// VM303:1 Uncaught TypeError: res.json is not a function

This is because res is a Promise, which does not have a json method on it. It isn't unless you do:

fetch("example.com/something")    .then(res => res.json());


const res = await fetch("example.com/something");const json = res.json();

That you'll see the JSON returned from this endpoint.

Why is this? What is .then or await doing? Are they interchangeable?

Let's dive in and explain more.

Sync and Async Values

Let's take a step back for a moment and talk about sync and async functions.

Look at the following code that's written to add up as many numbers as you pass:

function sumNumbers(numbers) {  let total = 0;  for (let i = 0; i < numbers.length; i++) {    total += numbers[i];  }  return total;}const sum = sumNumbers([1, 2, 3]);   // ^? 6

Here, sum is assigned the number of 6 from sumNumbers. This type of code is known as synchronous code; meaning that it runs each line of code sequentially until the execution of the code is finished.

For example, you don't have to use any kind of special syntax to tell the for loop that you want to wait for the next value. It runs each items in the for loop one-by-one:

function logEachItem(items) {  for (let i = 0; i < items.length; i++) {    console.log(items[i]);  }}/** * This will always log `1`, then `2`, then `3` * Because the `for` loop is sequential,  * it runs the same order every time */logEachItem([1, 2, 3]);

This is often the default for many functions in JavaScript.

But every now and then this type of code becomes a problem when you remember that JavaScript is single-threaded, and that expensive synchronous code can block the main thread.

When the main thread is blocked, it means a few things for your users:

  • The user can't select text
  • The user can't type into a text area or text input
  • Any other interactivity is blocked

We can see this behavior when the main thread is blocked on the Unicorn Utterances homepage:

So how can you accidentally block the main thread?

Let's say we take the previous logEachItem function and apply it over a massive array with a length of 300,000 items:

function logEachItem(items) {  for (let i = 0; i < items.length; i++) {    console.log(items[i]);  }}const items = Array.from({length: 300000}, (_, i) => i);logEachItem(items)

While adding these numbers might not block the main thread, any I/O operation is fairly expensive comparatively; which leads to this usage blocking the main thread.

This isn't inherently unique to I/O operations, however; you can block the main thread with any sufficiently expensive synchronous code.

Async Operations

Now assume we have the following implementation of sleep, which forces our codebase to wait a certain number of seconds:

function sleep(seconds) {  setTimeout(() => {		// How do we know when this timer is done?  }, seconds * 1000);}

Were this setTimeout function synchronous, it would prevent the user from interacting with your homepage for however long you asked to wait.

But instead, if we run sleep in our code, you'll notice that other code is able to execute before the setTimeout finished:

sleep(1000)console.log("I am running before the setTimeout is done");

So then how can we tell our code that sleep is done and it should execute the next line of code?

There're two ways to do this:

  • Callbacks
  • Promises


If we piggy-back off of the idea that you can pass a function to another function, we can make it so that you pass a function that's called when the setTimeout is done executing:

function sleep(callback, seconds) {  setTimeout(() => {    callback();  }, seconds * 1000);}sleep(() => {  console.log("The sleep is finished");}, 1)

This is how older JavaScript APIs functioned due to its intuitive nature, but comes with some flaws; Namely when you want to compose multiple sleeps one-by-one:

sleep(() => {    sleep(() => {        sleep(() => {            sleep(() => {                sleep(() => {                    sleep(() => {                        sleep(() => {                            sleep(() => {                                sleep(() => {                                    sleep(() => {                                        console.log("10 seconds have passed")                                    }, 1)                                }, 1)                            }, 1)                        }, 1)                    }, 1)                }, 1)            }, 1)        }, 1)    }, 1)}, 1)

This is the biggest problem with callbacks; it leads to multiple chained items leading to a nested structure colloquially called a "Christmas Tree".

To solve this, we can reach for promises to handle the async nature of sleep.

Async Promises

Luckily for us, JavaScript introduced a primitive to handle async code in 2015 called a "Promise".

You can create a promise like so:

const promise = new Promise((resolve, reject) => {	resolve();  // or  reject();})// Alternatively:const fulfilledPromise = Promise.resolve()const rejectedPromise = Promise.reject();

A promise can have one of three potential states:

  • Pending - The promise has not yet "settled" to being "fulfilled" or "rejected"
  • Fulfilled - The promise has successfully resolved a value
  • Rejected - The promise has thrown an error which rejected the promise

A promise-based sleep function might look something like this:

function sleep(seconds) {  return new Promise(resolve => {    setTimeout(() => {      resolve();    }, seconds * 1000);      })}sleep(1)  .then(resolvedValue => {    console.log("The sleep is finished");	})	.catch(rejectedError => {  	console.error("The sleep had an error")	})

Here, we're using .then and .catch to handle the success and failure states from the promise. We can even gather the value resolved inside of the .then and the rejected value inside of .catch.

.then is also able to chain together to handle a promise returned from a previous .then state:

sleep(1)	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))	.then(() => sleep(1))  .then(() => {    console.log("The sleep is finished");	})

Wow! There's no more Christmas Tree effect!

That's right! Now we can chain these promises and avoid the headaches that callbacks can cause.

Async Functions and the Await Keyword

Now that we understand promises, let's explore two nice addons to promises that were made in 2017:

  • Async functions
  • The await keyword

Using these, your ability to use promises is made even easier.

Let's start with the async function:

Async Functions

The most simple usage of an async function is by wrapping any normal function in async:

async function returnNumber() {	return 123;}const promise = returnNumber();   // ^? Promise<"pending">

Here, the behavior of returnNumber hasn't changed with one minor exception:

Any value returned from an async function is automatically marked as a promise.

This means that you can now add .then to our returnNumber like so:

async function returnNumber() {	return 123;}returnNumber()  .then(num => console.log(num));

While the ability to return a promise quickly and easily is nice, it isn't the main superpower of async functions.

await keyword

The main utility of the async function is its ability to use an await on an existing promise. Let's go back to our sleep function and demonstrate its use within an async function:

function sleep(seconds) {  return new Promise(resolve => {    setTimeout(() => {      resolve();    }, seconds * 1000);      })}async function main() {  await sleep(1);  console.log("One second has passed");  await sleep(1);  console.log("Two seconds have passed");}

This is functionally equivalent to:

function main() {  return sleep(1)  	.then(() => {      console.log("One second has passed");      return sleep(1);    	  })  	.then(() => {	    console.log("Two seconds have passed");  	  })}

But with a much more readable syntax.

Error Handling with await

Manually handling promises allows us to use the .catch handle to catch rejected promises, but how do we do this with await?

Well, we can use the classic try/catch to handle rejected errors in await usage:

async function main() {  try {		await Promise.reject("There was an error");  } catch (e) {    console.error(e);  }}

Without this, the rejected promise will bubble up past main and into whatever was used to call main; which can cause a ton of problems on its own.


Remember to handle errors in async functions like you would elsewhere. Resilient code is good code!

await usage mistakes

It's important to note that you're only able to use await inside of a function marked as async:

// ✅ Goodasync function fn() {	await sleep(1);}// ✅ Goodconst fn = async () => {	await sleep(1);}// ❌ Badconst fn = () => {	await sleep(1);}// ❌ Badfunction fn() {	await sleep(1);}

If you try to use await inside of a non-async function, you'll get the error:

Uncaught SyntaxError: await is only valid in async functions and the top level bodies of modules

To fix this, wrap your functions that use await in an async keyword.

Mix and match async and new Promise

As you may have caught onto; you're able to implicitly mix and match async functions, the await keyword, new Promise construction, and .then.

To showcase this, let's create a code sample that does just that:

function fetchItems() {  return fetch("https://example.com/items")  	.then(res => res.ok ? Promise.resolve(res) : Promise.reject(res))  	.then((res) => res.json())}async function loadList() {  // Default to an empty array if there is an error;  const list = await fetchItems()  	.catch(err => {      console.warn(err);      // This resolves as a fulfilled promise now      return [];    });  // Wait a second since `example.com` is too fast  // And our users are suspicious without a loading spinner 	await new Promise(resolve => {    setTimeout(() => {      resolve();    }, 1000)  });    return list;}let loading = true;loadList.then((list) => {  loading = false;  console.log("The list is", list)}).catch((err) => {    loading = false;})


That's all for today! There's so much more to learn about promises that we'll explore in the future. This will include:

  • Methods of concurrent promise running:
    • Promise.all
    • Promise.allSettled
    • Promise.any
    • Promise.race
  • Async function generators:
    • for await (const item of list)
    • async function*

And more!

In the meantime if this article caught your attention, consider joining our Discord to ask more questions and hang out with like-minded engineers ready to help with any engineering problems you might have.

Subscribe to our newsletter!

Subscribe to our newsletter to get updates on new content we create, events we have coming up, and more! We'll make sure not to spam you and provide good insights to the content we have.