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);
});
});
});
});
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 dependenat 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).
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 AsyncFuncton
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.
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
.
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
}
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'
...
})();
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);
}
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 methodsYou 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()
.delivery()
.then(console.log); // Pizza
Just to quickly go through what we discussed so far:
async
keyword makes a method asynchronous, which in turn always returns a promise and allows await
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.async/await
you wouldn’t need .then/.catch
, but just note that they are still based on promises.Promise.all
to wait for multiple async
functions calls.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.