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

do a() and b() run concurrently?

No, they run sequentially.

The equivalent would be something like

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

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 …
  });
})