Async/await
There’s a special syntax to work with promises in a more comfortable fashion, called “async/await”. It’s surprisingly easy to understand and use.
Async functions
Let’s start with the async keyword. It can be placed before a function, like this:
async function f() {
return 1;
}
The word “async” before a function means one simple thing: a function always returns a promise.
Other values are wrapped in a resolved promise automatically.
For instance, this function returns a resolved promise with the result of 1; let’s test it:
…We could explicitly return a promise, which would be the same:
async function f() {
return Promise.resolve(1);
}
f().then(alert); // 1
So, async ensures that the function returns a promise, and wraps non-promises in it. Simple enough, right?
But not only that. There’s another keyword, await, that works only inside async functions, and it’s pretty cool.
Await
The syntax:
// works only inside async functions let value = await promise;
The keyword await makes JavaScript wait until that promise settles and returns its result.
Here’s an example with a promise that resolves in 1 second:
async function f() {
let promise = new Promise((resolve, reject) => {
setTimeout(() => resolve("done!"), 1000)
});
let result = await promise; // wait until the promise resolves (*)
alert(result); // "done!"
}
f();
The function execution “pauses” at the line (*) and resumes when the promise settles, with result becoming its result.
So the code above shows “done!” in one second.
Let’s emphasize: await literally makes JavaScript wait until the promise settles, and then go on with the result.
That doesn’t cost any CPU resources, because the engine can do other jobs in the meantime: execute other scripts, handle events, etc.
It’s just a more elegant syntax of getting the promise result than promise.then, easier to read and write.
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