6 points you need to know about async/await in JavaScript

If you have faced a code like below, then this article will help you in multiple ways ūüėĀ.

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 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:

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.

Now in order to get the value from the promise we act like before:

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.

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.

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.

What you can do however, is to wrap your code in an async IIFE(immediately invoked function execution) and call it right there:

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:

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.

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.

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.

Promise.all compatibility

You can use async/await alongside Promise.all to wait for multiple promises:

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.

async class methods

You can have an¬†async¬†class method. Just prepend it with¬†async¬†and you’re good to go.

Summary

Just to quickly go through what we discussed so far:

  1. async keyword makes a method asynchronous, which in turn always returns a promise and allows await to be used.
  2. 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.
  3. Together, they provide a great opportunity for us to write clean, more testable, asynchronous code.
  4. With¬†async/await¬†you wouldn’t need¬†.then/.catch, but just note that they are still based on promises.
  5. You can use Promise.all to wait for multiple async functions calls.
  6. 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.

Credit @Dev.To

TranIt.Co Comments - Leave your comment or ask a question
Tags: