What's the order of execution when awaiting a non-Promise?

Question

My understanding is that the promise in await promise expression gets pushed to Microtask Queue, so it would settle after the call stack is empty. The below code works as I expected:

async function func() {
  console.log(await Promise.resolve('bagel'));
}

func();
console.log('synchronous log');

// Result:
// synchronous log
// bagel

But if I await something not a Promise like this, the order of the logs changed:

async function func() {
  await console.log('bagel');
}

func();
console.log('synchronous log');

// Result:
// bagel
// synchronous log

What happens behind-the-scenes when I call await? What is the order of executing when awaiting a non-Promise? And specifically await console.log?

`await console.log(…)` makes no sense, that's the same as `console.log(…); await undefined;`. You probably meant `console.log(await 'bagel')`? — Bergi, Apr 02 '23 at 08:48
"*My understanding is that the promise gets pushed to Microtask Queue*" - no, the task queue does not contain promises. The *task* (`.then()` handler) that resumes the execution of the `async` function is pushed to the microtask queue when the promise is resolved. — Bergi, Apr 02 '23 at 08:50

Dimava · Accepted Answer · 2023-04-02T08:47:15.817

In your case you have

async function func() {
  let un = console.log('bagel');
  await un;
}

func();
console.log('synchronous log');

So the order is obvious

In case of

async function func() {
  await 0;
  console.log('bagel');
}

func();
console.log('synchronous log');

synchronous one will also be the first, as await pauses function execution

If you want your "syncronous" await to be syncronous you have to not await it if it's not a promise

async function func(wait) {
  if (wait?.then) {
    await wait
  }
}

What's the order of execution when awaiting a non-Promise?

1 Answers1