Async and Await

Drawback of using promises

You have seen how promises are superior to callbacks, but there’s one drawback to using promises. The structure of synchronous imperative code is drastically different from the structure of the asynchronous code that uses promises. Unlike the sequential code that flows naturally from one statement or expression to the next, we have to get our head wrapped around the then() and catch() sequences.

Solution async-await

The async and await feature was introduced to keep the code structure identical between synchronous and asynchronous code. This does not affect the way we write asynchronous functions, but it largely changes the way we use them.

Rules of using async-await

There are two rules for using this feature:

• async: To use an asynchronous function as if it were a synchronous function, optionally mark the promise-returning asynchronous function with the async keyword.
• await: To call the asynchronous function as if it were a synchronous function, place the await keyword right in front of a call. The await keyword may be used only within functions marked async.

Synchronous vs asynchronous

Let’s explore this feature by first comparing synchronous to asynchronous code structure. Then, we’ll look at how async and await help.

Let’s create two functions—one synchronous and one asynchronous. They both take a number and return the double of the number.