Getting Started with Async/Await
Inspired by the Zeit team’s post on the subject, my team at PayPal recently migrated our main server-side codebase to use Async/Await. I’m excited to share with you some of things we learned along the way.
Let’s start with some terminology:
- Async function
- Await keyword
People usually say async/await which is lovely and nice really, but you should know that they aren’t the same thing. There are async functions and there is the await keyword. They are certainly tied together in some ways, but async functions in particular can be used without await. How is that?
Async Functions Return a Promise
When you create a function with the async keyword that function will always return a Promise. When you return inside your async function it wraps your value in a Promise.
Even if your code throws inside an async function it won’t automatically bubble up, instead it will return a rejected promise
Async Functions are the only Place where you can use Await
In addition to converting your returns into a Promise, an async function is also special in that it’s the only place where you can use the await keyword.*
What is the await keyword? Await lets you pause the execution of an async function until it receives the results of a promise. This lets you write async code that reads in the order that it’s executed.
Await allows you to write asynchronous code with no callbacks at all. This makes your code much more readable. And await works with any promise, not just promises created by async functions.
Error Handling in Async Functions
Because an async function is a Promise, when you throw inside of an async function it’s swallowed up and returned as a rejected Promise.
If you are using await to call the Promise you can wrap it in try/catch or you’ll need to add a catch handler to the returned Promise.
Promise errors usually bubble up to their parent, so you usually only need that try/catch on your top-level Promise.
Putting it all Together
Taking advantage of the error handling properties of promises and the concise syntax of async functions can yield some powerfully simple results.
In this simplified example you can see how one might take advantage of the inherent error handling capabilities async functions to simplify error handling in an Express app
On my team at PayPal we usually handle errors with next(err). However with async/await we can simply throw errors anywhere in the code and the router will forward them to the next function provided by Express. This is a huge simplification.
Moving from callbacks to promises and async/await has condensed error handling in our app and will improve comprehension for our more complicated code paths. It took me a couple of hours to migrate most of our routes from plain callbacks to this new approach. Really the only thing you need to get started is a solid knowledge of Promises and an understanding of how to setup babel.
* Top-level await is not allowed in the current spec (Though there has been some discussion about possibly allowing this in the future).