JavaScript ES6 - Possible race condition between promise resolution and event?

Question

Is it possible to have user code executed between a promise resolution and a promise await return?

function a () {
  return new Promise(resolve => {
    setTimeout(() => {
      // Between here...
      resolve()
    }, 1000))
  }
}

async function b () {
  await a()
  // ...and here ?
}

Does the specification enforces that Promise callbacks are called immediatly? I wonder if an event could be handled by the virtual machine between the 2 points, possibly causing side-effects.

Answer 1

No, it doesn't enforce immediate calling. The spec runs through a number of steps upon resolving a promise, one of which is:

12. Perform EnqueueJob ("PromiseJobs", PromiseResolveThenableJob, «‍promise, resolution, thenAction»)

Note that depending on the state of the promise, different routes can be taken. However, they all end at EnqueueJob, which says:

8. Add pending at the back of the Job Queue named by queueName.

So any further executions are deferred to the end of the job queue.

Answer 2

Promise callbacks are not called immediately. Your function b() transforms to

function b () {
  return a().then(() => {
    // Do something
  });
}

where the function which is given to then is executed at some point of time by the eventloop after the promise from a was resolved. It will never be called synchronously, because the specification guarantees that Promise continuations always run with an empty call stack (which means in a fresh iteration of the eventloop). Therefore some other code (e.g. from another promise resolval) can run between the time where the promise was resolved and when the continuation runs.