V8 adds support for top-level await

Re:V8 adds support for top-level await by mstade on Wednesday September 25, 2019 @06:17AM (#21069327)

The point isn't so much whether qux() is async or not, it's whether the caller wants it to execute before subsequent statements or not. In the current state of the world:

    const a = await qux()
    const b = qux()
    const c = await qux()
    const d = (await b) + 5

But if I had my little way:

    const a = qux()
    const b = async qux()
    const c = qux()
    const d = b + 5

The weird one is `d` of course, why would it implicitly await there? Because `+` tries to get the value of `b` in order to add `5`.

How the runtime would optimize this scenario I don't know, it could probably statically determine that qux() may be async and therefore determine it would have to implicitly await its return value even if it's just the number. Obviously this is a contrived scenario, but I'd rather pay a small runtime cost, than pay the cognitive overhead of remembering to await things everywhere. Like most people, I forget things...

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Re:V8 adds support for top-level await by laughinghan on Wednesday September 25, 2019 @03:42PM (#21074511)

I think you missed my point. I understood how you inverted explicit and implicit, you saw me explain that to someone else who asked about Promise.all().

What I'm asking you is, how does anyone, be it the programmer, compiler, or runtime, know whether to block on qux() or not, given that it sometimes synchronously returns 1?

Are you saying that in your proposal, it would become part of the calling convention that any time any function at all returns (and no 'async' keyword was used), the callee would check if a Promise was returned and if so, return a continuation promise?

That is no small runtime cost.

In an untyped language like JS, the cases where static analysis could determine whether a function returns a Promise or not are vanishingly few, probably about the same as the cases where a function can be inlined. Remember, anyone at any point can overwrite Array.prototype.push to be anything they want, including a function that returns a Promise. Any function that takes and calls a callback might be calling an async function. Any math or string operation could be calling .valueOf() or .toString(), which might be async.

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