If you have faced a code like below, then this article will help you in multiple ways 😁.
fetchPizzas()
.then((pizzas) => {
return sortByToppings(pizzas)
.then((pizzas) => {
return checkDeliveryOptions(pizzas)
.then((pizzasWithDelivery) => {
return checkBirthdayGift(pizzasWithDelivery)
.then((pizza) => {
return sendToCustomer(pizza);
});
});
});
});
A little bit of background
There are many a times where we have a bunch of tasks to be executed sequentially. The examples are from File handling to calling databases multiple times based on the result of the previous call. Or calling multiple APIs in a sequence where one call is dependent on another.
Prior to introduction of async/await
, many used callbacks alongside setTimeOut
to simulated the behaviour they wanted (aka callback hell). Later on people started to use promises which made the code much more readable but they would end up in the same place when the number of calls where high (aka promise hell).
Async functions
A function in JavaScript is async
when it operates asynchronously via the event loop, using an implicit promise to return its result. Furthermore, the type of its result should be an AsyncFunction
object.
This function is nothing but a combination of promises and generators. I will not going into details of generators, but they usually contains one or many yield
keywords.
Now lets see the async
function in action. Assume we have a function which returns a string:
function hi() {
return 'Hi from JavaScript';
}
hi(); // 'Hi from JavaScript'
If we put async
in front of the function, then it no longer returns string, it will be a promise which is wrapped around the string value automatically.
async function hi() {
return 'Hi from JavaScript';
}
hi(); // Promise {<resolved>: "Hi from JavaScript"}
Now in order to get the value from the promise we act like before:
hi().then(console.log); // 'Hi from JavaScript'
You might be wondering how this can help to solve the promise hell. Just bare with me and we'll get there step by step with examples so it'd be clear when we're finished.
Await
The await
makes JavaScript engine to wait until a promise is resolved/rejected and returns it's result. This keyword can only be used inside an async
function.
const doSomething = async () => {
console.log(await hi())
};
// 'Hi from JavaScript'
You might think since await
forces the JavaScript engine to wait, it will have some cost on CPU. But that's not the case because the engine can perform other scripts while waiting for the promise to get resolves/rejected. Plus this is way more elegant that using promises
and .then
.
Warning: If you try to invoke an async
function using await
inside a normal function, you will get a syntax error.
function doSomething() {
await hi(); // Uncaught SyntaxError: await is only valid in async function
}
A small catch
Most people who start working with async/await
forget that they can't invoke an async
function on top level code. This is due to the fact that we can't have await
inside a normal function and the top level functions are normal by default.
let response = await hi(); // syntax error in top-level code
console.log(response);
What you can do however, is to wrap your code in an async
IIFE
(immediately invoked function execution) and call it right there:
(async () => {
let response = await hi();
console.log(response); // 'Hi from JavaScript'
...
})();
Update: As Nick Tyler mentioned in the comments, there is a stage 3 proposal to support await
in top level code. So stay tuned and watch this space:
tc39 / proposal-top-level-await
top-level `await` proposal for ECMAScript (stage 3)
ECMAScript proposal: Top-level await
Champion: Myles Borins
Status: Stage 3
Synopsis
Top-level await
enables modules to act as big async functions: With top-level await
, ECMAScript Modules (ESM) can await
resources, causing other modules who import
them to wait before they start evaluating their body.
Motivation
Limitations on IIAFEs
With await
only available within async
functions, a module can include an await
in the code that executes at startup by factoring that code into an async
function:
// awaiting.mjs
import { process } from "./some-module.mjs";
let output;
async function main() {
const dynamic = await import(computedModuleSpecifier);
const data = await fetch(url);
output = process(dynamic.default, data);
}
main();
export { output };
This pattern can also be immediately invoked. You could call this an Immediately Invoked Async Function…
Error handling
As I said before, most async
functions can be written as a normal function with promises. However, async
functions are less error-prone
when it comes to error handling. If an awaited call fails, the exception is automatically caught and the Error
object will be propagated to the caller using the implicit return promise.
Prior to this, we had to reject the promise which was returned from the normal function and use a .catch
in the caller. I've seen many places where the developers used a try/catch and throw a new exception which meant the stack trace would be reset.
async function hi() {
throw new Error("Whoops!");
};
async function doSomething() {
try {
let response = await hi();
return response;
} catch(err) {
console.log(err);
}
}
doSomething();
Or you can avoid the try/catch
because the promise generated by the call to hi
becomes rejected. Then simply use .catch
to handle the error.
async function hi() {
throw new Error("Whoops!");
};
async function doSomething() {
let response = await hi();
return response;
}
doSomething().catch(err => {
console.log(err);
});
You can ignore the catch all together and handle all the exceptions using a global exception handler if you think that's more suitable to your situation. Something like this which uses the onrejectionhandled
property of WindowsEventHandlers
mixin.
window.onrejectionhandled = function(e) {
console.log(e.reason);
}
Promise.all compatibility
You can use async/await
alongside Promise.all
to wait for multiple promises:
const responses = await Promise.all([
fetch('yashints.dev/rss'),
hi(),
...
])
If an error occurs, it propagates as usual, from the failed promise to Promise.all
and then turns to an exception that you can catch using any of the above methods.
await
can take in a "thenable"
Similar to promise.then
, if you have any object which has a .then
method, await
will accepts it. This is to support scenarios where a 3rd-party object which is not a promise, but promise-compatible (it supports .then
), it would be enough to use it with await
.
class Greeting {
constructor(name) {
this.name = name;
}
then(resolve, reject) {
console.log(resolve);
setTimeout(() => resolve(`Hi ${this.name}`));
}
};
async function greet() {
const greeting = await Greeting('Yaser');
console.log(greeting); // Hi Yaser
};
greet();
async
class methods
You can have an async
class method. Just prepend it with async
and you're good to go.
class Order {
async deliver() {
return await Promise.resolve('Pizza');
}
}
new Order()
.deliver()
.then(console.log); // Pizza
Summary
Just to quickly go through what we discussed so far:
-
async
keyword makes a method asynchronous, which in turn always returns a promise and allowsawait
to be used. -
await
keyword before a promise makes JavaScript wait until that is resolved/rejected. If the promise is rejected, an exception is generated, otherwise the result is returned. - Together, they provide a great opportunity for us to write clean, more testable, asynchronous code.
- With
async/await
you wouldn't need.then/.catch
, but just note that they are still based on promises. - You can use
Promise.all
to wait for multipleasync
functions calls. - You can have an
async
method in a class.
I know there are many great articles around async/await
, but I tried to cover some items where I had to constantly remind myself of. Hope it will help you to have a centralised place for most of what you need to write clean asynchronous JavaScript.
Have fun exploring these points.
Top comments (12)
I’m not sure why the first block of code in the article uses promises like the old callback hell, but here it is the proper way to write that first block of code:
And, in my opinion, it looks better then the alternatives offered by the article, not mentioning the testability of it.
Agree, I think promises are better that
async/await
when it comes to function composition.That was a bad example, not to be used
Thanks for the post Yaser!
One thing I'd add is that although we don't have top-level await, it's most likely going to happen as it's at Stage 3.
tc39 / proposal-top-level-await
top-level `await` proposal for ECMAScript (stage 3)
ECMAScript proposal: Top-level
await
Champion: Myles Borins
Status: Stage 3
Synopsis
Top-level
await
enables modules to act as big async functions: With top-levelawait
, ECMAScript Modules (ESM) canawait
resources, causing other modules whoimport
them to wait before they start evaluating their body.Motivation
Limitations on IIAFEs
With
await
only available withinasync
functions, a module can include anawait
in the code that executes at startup by factoring that code into anasync
function:This pattern can also be immediately invoked. You could call this an Immediately Invoked Async Function Expression (IIAFE), as a play on IIFE idiom.
Thanks for sharing, didn't know about this 👌🏽
Updated the post to reflect this, thanks again Nick
Great article Yaser! btw:
AsyncFuncton
->AsyncFunction
Nice catch, I should double check my spell checker in VS Code
Good read, but I believe next to last code line should be ‘deliver()’. Also, looking for more on thenable, any good coverage somewhere? Thanks!!
Fixed, thanks, check this out promisesaplus.com/
Just what I needed when I needed. Thank you!!
You're welcome, glad it helped