[feature] How to get the Promise result value?

[kuoruan]

ctx, _ := v8go.NewContext(nil)

val, err := ctx.RunScript(`new Promise((resolve) => setTimeout(() => resolve(1), 1000)).then(console.log)`, "value.js")
if err != nil {
  panic(err)
}

fmt.Printf("result: %s", val.String())

result: [object Promise]

I want to get the Promise result, for the example code is 1

[kuoruan]

I think this will be possible after merging #68.

[rogchap]

Hey @kuoruan;
This is a bit of a tricky one right now; #68 could help if you create your own callback function to manage the resolved value; but that would be just a workaround.
What we really need is #61 but this has a large scope because the V8 Object class includes arrays, dates, regex, promises as well as regular objects (and more).
Once I have #68 working, I'll move to addressing the Object class (I already have a branch where I made a start)

[fizx]

I'd like this too, but for simplicity, I don't need complex/useful value types. The following code would be totally fine:

if val.IsPromise() {
	val.PromiseCallback(func(thePromiseResult *v8go.Value, e error) { println(thePromiseResult.String()) })
}

I think I can effectively get there with FunctionTemplates and some ugly code, but also happy to contribute.

[rogchap]

Hey @fizx I'm working on the Object/Promise code right now.
I've not fleshed out the complete API, but it will be something like:

val.AsPromise().Result()

[kuoruan]

Hi @rogchap , I see you added the support for Object, do you have a plan to add Promise?

I'm trying to get Promise result from a promise result with your code:

RtnValue PromiseResult(ValuePtr ptr) {
  LOCAL_OBJECT(ptr);
  RtnValue rtn = {nullptr, nullptr};

  Local<Promise> promise = obj.As<Promise>();

  std::cout << promise->State() << std::endl;

  while (promise->State() == Promise::kPending) {
    iso->RunMicrotasks();
  }

  if (promise->State() != Promise::kFulfilled) {
    rtn.error = ExceptionError(try_catch, iso, local_ctx);
  } else {
    MaybeLocal<Value> result = promise->Result();

    m_value* new_val = new m_value;
    new_val->iso = iso;
    new_val->ctx.Reset(iso, local_ctx);
    new_val->ptr.Reset(iso, Persistent<Value>(iso, result.ToLocalChecked()));

    rtn.value = static_cast<ValuePtr>(new_val);
  }

  return rtn;
}

If I run code like Promise.resolve(1), I can get the result.

But code like new Promise(resolve => setTimeout(function() { resolve(1) }, 1000)) the state is always Promise::kRejected

Any thing I'm wrong?

[rogchap]

Hey @kuoruan

Yes, I am working on full support for Promise API as I can imagine people wanting to be able to do thinks like the fetch API etc; #74 Was just a prerequisite.

You seem to be on the right track to get the Promise Result, I know that iso.RunMicrotasks() API no longer exists so you might want to try iso.PerformMicrotaskCheckpoint() instead.

[rogchap]

@kuoruan I've noticed another error: setTimeout is not part of ECMA-262 and therefore not implemented in V8 😱
You'll need to implement that yourself with a Function Template and setting the global object via the Object Template and creating a new Context. This is why you are getting kRejected

[kuoruan]

Hi @rogchap
Seems v8 already has setTimeout function. https://stackoverflow.com/a/60355125/5266411

I tried this:

$ brew install v8
$ d8 -v
V8 version 8.8.278.15

create a new file like:

console.log("a");
setTimeout(() => {
    console.log("c");
}, 5000);
console.log("b");

d8 example.js

no error found.

[kuoruan]

OK, there is only setTimeout but no clearTimeout in d8.

https://github.com/v8/v8/blob/b38bf5b0b1f149f7af3fd90a2ce12344e7191d03/src/d8/d8.cc#L2443-L2444

[rogchap]

d8 is different from v8; d8 implements setTimeout for debugging v8. d8 is an example of a CLI tool that uses v8, just like any program that uses v8go would also.