Async / await assignment to object keys: is it concurrent?

Question

I know that doing this:

const resultA = await a()
const resultB = await b()
// code here

Is effectively

a().then( resultA => {
   b().then( resultB => {
      // code here
   })
})

Basically, a() runs then b() runs. I nested them to show that both resultA and resultB are in our scope; yet both function didn't run at once.

But what about this:

const obj = {
  result1: await a(),
  result2: await b()
}

do a() and b() run concurrently?

For reference:

const asyncFunc = async (func) => await func.call()
const results = [funcA,funcB].map( asyncFunc )

I know here funcA and funcB do run concurrently.

Bonus:

How would you represent the object assignment

const obj = {
  result1: await a(),
  result2: await b()
}

using then / callbacks?

---

UPDATE:

@Bergi is correct in this answer, this occurs sequentially. To share a nice solution for having this work concurrently for an object without having to piece together the object from an array, one can also use Bluebird as follows

const obj2 = Bluebird.props(obj)

http://bluebirdjs.com/docs/api/promise.props.html

Urbanism

Your "for reference" example has a syntax error (and is misleading because of that). If you use await, you have to make the arrow function async, and then it becomes obvious that they might run concurrently because they're separate function evaluation. – Lonilonier

Thanks for catching that, corrected. – Urbanism

Note that the JS functions may trigger asynchronous processes that run concurrently, but the JS functions themselves aren't running concurrently in the sense of multi-threaded simultaneous execution. – Shannan

Answer 1

No, every await will stop the execution until the promise has fulfilled, even mid-expression. It doesn't matter whether they happen to be part of the same statement or not.

If you want to run them in parallel, and wait only once for their result, you have to use await Promise.all(…). In your case you'd write

const [result1, result2] = await Promise.all([a(), b()]);
const obj = {result1, result2};

---

How would you represent the object assignment using then / callbacks?

With temporary variables for each awaited value. Every await translates into one then call:

a().then(tmp1 => {
  return b().then(tmp2 => {
    const obj = {
      result1: tmp1,
      result2: tmp2
    };
    return …
  });
})

If we wanted to be pedantic, we'd have to pick apart the object creation:

const tmp0 = {};
a().then(tmp1 => {
  tmp0.result1 = tmp1;
  return b().then(tmp2 => {
    tmp0.result2 = tmp2;
    const obj = tmp0;
    return …
  });
})

Lonilonier

You're right! I just finished writing test code to try the above code using setTimeout and it does indeed run sequentially. – Urbanism

Answer 2

do a() and b() run concurrently?

No, they run sequentially.

The equivalent would be something like

a()
.then(result1 => b())
  .then(result2 => ({result1, result2}))

Faddist