async/await vs combining generators and promises?

Question

The purpose of async/await functions is to simplify the behavior of using promises synchronously and to perform some behavior on a group of Promises. Just as Promises are similar to structured callbacks, async/await is similar to combining generators and promises.

I understand basic concepts of async/await, generators and promises. However, I don't fully understand what it means by saying async/await is similar to combining generators and promises.

So async/await simplify what generators and promises can do together? Any example?

Read this article to understand what is difference between promises, generators and async/await. — alexmac
Generators were used together with an intricate helper function as a hack to do what async/await does now. I would recommend to avoid this comparison however, it should be seen as nothing but a historic artefact. I think MDN needs a good edit… — Bergi

JJWesterkamp JJWesterkamp · Accepted Answer · 2017-09-04T20:05:21

Yes, you're right. An example with a generator and promises first:

function *gen () {
    const promiseValue = yield new Promise((resolve) => resolve(42));
    console.log(promiseValue);
}

// .. and at some other context using '*gen'

const iterator = gen();
const { value: promise } = iterator.next(); // Promise (42)

promise.then((resolvedValue) => iterator.next(resolvedValue)); // logs 42

This generator yields a Promise to the outside world, one whose value we pass back into the generator by passing it as an argument to the iterator.next call, once that promise resolves.

This pattern at least intersects with what is said to be a task. The disadvantage of this is that we DO have to call next manually on the iterator, every time a yielded promise resolves. That's where async await comes in:

async function task() {
    const promiseValue = await new Promise((resolve) => resolve(42));
    console.log(promiseValue);
}

And that's all there is to it. An async function will pause until a promise expression preceded by the await keyword resolves, and the expression will evaluate to an automatic unwrap of the promise - i.e. its final value.

Then the await expression's result (42, in our case) will be assigned to promiseValue, like yield works as well. This all happens before the function continues its execution to the next statement.

This is essentially equal behaviour as the above snippet with the generator.

Although in my experience this example seems to be the most common use-case for both generators and async functions, one where async/await is definitely the cleaner approach, generators do have some really strong powers that async/await does not have in the same way (but that is another topic).

async/await vs combining generators and promises?

1 Answers