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  • Do await statements unblock synchronously?

Do await statements unblock synchronously?

# /#!/JoePea (9 years ago)

Is code that follows an await statement supposed to get executed in the same tick in which the statement's awaited promise is resolved? F.e.:

let resolve = null
const somePromise = new Promise(r => resolve = r)

~async function() {
  await somePromise
  doSomething()
}()

// ... time passes
resolve()

Should doSomething() fire in that same tick as when resolve() is called? I already know that if this is true, there's at least one exception: await Promise.resolve(), in that the await statement must still defer to a future tick even though the given Promise is already resolved. I suppose I'm asking for cases where the await statement's promise is unresolved.

# Andrea Giammarchi (9 years ago)

I suppose I'm asking for cases where the await statement's promise is

unresolved.

isn't that a "forever pending"? then AFAIK it should "forever await" ... right?

# Jordan Harband (9 years ago)

As I understand it, await always fires in the next tick, if it's observable.

The opportunity to optimize that to "same tick" exists if an engine can prove it's not observable.

# Mark S. Miller (9 years ago)

Not necessarily "the next tick", but some future tick. Definitely not in this tick or the tick in which the promise is resolved. Definitely in its own tick.

And yes, engines can always do whatever unobservable optimizations they want.

# Mark Miller (9 years ago)

On Mon, Apr 11, 2016 at 9:31 PM, Mark S. Miller <erights at google.com> wrote:

Not necessarily "the next tick", but some future tick. Definitely not in this tick or the tick in which the promise is resolved.

Meant: "or the tick in which the promise is settled."

# /#!/JoePea (9 years ago)

So just to clarify, the code following an await statement should never run in the tick in which the await statement is executed, should never run in the same tick in which the promise it is awaiting gets resolved, and so should always run in a 3rd tick separate from those other two?

# Mark Miller (9 years ago)

Essentially yes. Minor issues inline

On Mon, Apr 11, 2016 at 10:32 PM, /#!/JoePea <joe at trusktr.io> wrote:

So just to clarify, the code following an await statement should never run in the tick in which the await statement is executed, should never run in the same tick in which the promise it is awaiting gets resolved,

Settled.

If unresolved promise p becomes resolved to unresolved promise q, then p is resolved but not settled.

If q is then fulfilled or rejected, then q is settled and p is settled in the same way.

and so should always run in a 3rd tick separate from those other two?

It should always run in a tick after the ticks in which those other two events happen. However, to be fully pedantic, those other two events may happen in one tick, so the post-await computation, happening after both, would happen in a second tick.

Btw, I assume your "tick" is equivalent to our "turn" or "job". Tick is not bad but neither is it clearly better. We should not introduce a third term since we already have more than we need.

# /#!/JoePea (9 years ago)

Thanks!

"tick" is equivalent to our "turn" or "job".

I thought about that, but figured tick was fine since Node has process.nextTick. I'll stick to "turn".