Thanks for the pointer Ben. I encountered another issue while following
your suggestion:
For evaluating the script "new Promise(() => {})", I'd expect
result->IsPromise() is true. However, in some cases I'm seeing
result->IsPromise() is false if some code are executed before.
I don't have a small example as I couldn't pinpoint the issue. But I'll see
this issue if I execute the tensorflow.js source code (from
https://cdn.jsdelivr.net/npm/@tensorflow/tfjs@2.4.0/dist/tf.js) before the
promise, in which case I'll see result->IsPromise() is false. However, when
I print the status from v8::String::Utf8Value(isolate,
result->ToDetailString(context).ToLocalChecked()), it still outputs
"#<Promise>". So I'm confused.
It seems that the execution of tensorflow.js is interfering with something,
but I'm not sure how to debug. FYI, I *think* the execution of
tensorflow.js succeeded, because I saw no compile/run error, and after that
I could still compile/run some tensorflow functions, or just compile/run
some other toy functions. It's just that a promise is no longer evaluated
to be a promise.
Any ideas why this happens?
On Wednesday, September 16, 2020 at 3:35:10 AM UTC-4, Ben Noordhuis wrote:
>
> On Wed, Sep 16, 2020 at 8:56 AM Yao Xiao <yao...@chromium.org
> <javascript:>> wrote:
> >
> > Hi v8-users,
> >
> > I'm new to v8.
> >
> > For a "script" that has a call to a sync function that returns an
> integer, I figured I could do the following to get the result in C++:
> >
> > v8::Local<v8::Value> result = script->Run(context).ToLocalChecked();
> > int result_int = result->Int32Value(context).FromJust();
> >
> > I'm wondering if there's a way to get (i.e. make C++ aware of) the
> return value an async function? I tried to use await but it seems that the
> script won't compile.
>
> You can attach your own .then and .catch callbacks if the script's
> return value is a promise (which is what the return value of an async
> function really is):
>
> if (result->IsPromise()) {
> Local<Promise> promise = result.As<Promise>();
> if (promise->State() == Promise::kPending) {
> Local<Function> resolve = Function::New(context,
> Resolve).ToLocalChecked();
> Local<Function> reject = Function::New(context,
> Reject).ToLocalChecked();
> promise = promise.Then(resolve, reject).ToLocalChecked();
> } else {
> result = promise->Result(); // note: distinguish between
> kFulfilled and kRejected
> // ...
> }
>
> You'll need to pump the event loop and/or the microtask queue in order
> for the promise to resolve.
>
> Apropos the await keyword, that isn't accepted at the top-level scope
> (outside an async function) unless you're executing the script as an
> ES module and the --harmony_top_level_await flag is set.
>
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