How to retrieve a class defined in a module?

[Ben Ernst]

I am migrating my V8 code from using "plain scripts" to using "ES6 Modules". There are a few concepts that I am struggling with. 

First, how might I retrieve a class definition defined in a module? Previously I would have used the "global" object on the "context" (i.e. context->Global()), and found the class definition there. I can't find a good example on how to do that in a module. I am experimenting with Module::GetModuleNamespace(), but I'm not sure that I'm on the right track.

Second, am I allowed to take a persistent handle to a module? I am finding that after restoring a module from a persistent handle, I encounter the failure below.

#

# Fatal error in C:\621300de\v8\src/api/api-inl.h, line 128

# Debug check failed: allow_empty_handle || that != nullptr.

#

#

#

#FailureMessage Object: 0000006114FBC2E0

==== C stack trace ===============================

        v8::base::debug::StackTrace::StackTrace [0x0,000,7FF,60D,F82,80B+27]

        v8::platform::DefaultPlatform::PostJob [0x0,000,7FF,60D,F7F,1E1+401]

        V8_Fatal [0x0,000,7FF,60C,251,B87+167]

        v8::base::PrintCheckOperand<unsigned __int64> [0x0,000,7FF,60C,251,683+627]

        v8::Module::GetStatus [0x0,000,7FF,60C,1E0,3FD+141]

        v8::Module::GetModuleNamespace [0x0,000,7FF,60C,1D1,243+19]

[Camillo Bruni]

Hi,

The ModuleNamespace object is the proper choice, you can have a look at the existing tests.

For future reference, you will usually find (a bit hidden and obscure, I agree) existing tests in V8's test-api.cc.

Could you provide some more example code for the error you get with persistent handles?

cheers,

Camillo

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Camillo Bruni |

Software Engineer, V8 |

Google Germany GmbH |

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[Ben Ernst]

Thank you for the advice Camillo, the test you referred me to is very helpful. I will prepare a good code sample and come back to this conversation.

Regards,

Ben

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-users/CAOeS1i9mEbcpxGQr1uNk%3DfAMPDi%2Bz%2Bzkun0eiopfphqErJJLAQ%40mail.gmail.com.

[Ben Ernst]

Thanks for the advice, that got me back on track. The failure mentioned in my original post only occurs when there's nothing at all exported from the module. I speculate the namespace object gets garbage collected in that situation. When I tested with actual valid modules, everything works fine.

Thanks again,

Ben

[Ben Ernst]

There's one thing I'm struggling with here. Given the simple class definition below, using the V8 C++ API I can call the ordinary function "lamingtonsIsUndefined", but how would I call the accessor "get numberOfLamingtons()"?

Many thanks in advance,

Ben

export class DemonstrationClass {

constructor(lamingtons) {

this.lamingtons = lamingtons

}

get numberOfLamingtons() {

return this.lamingtons

}

lamingtonsIsUndefined(){

return this.lamingtons === undefined

}

}

[Alex Kodat]

You wouldn't call it any more than you "call" it in JavaScript. You just get the value via the Object Get function: 
Local<Value> pastries = object->Get(context, String::NewFromUtf8( isolate, "lamingtons").ToLocalChecked());

[Ben Ernst]

Thanks for that thought Alex! Drafting a reply to you led me to the answer, which is obvious in hindsight.

Yes, I could just get "lamingtons" in this case, but that wouldn't help me if it was a non-trivial accessor. In javascript I would be able to call "instance.numberOfLamingtons". In this case, I can simply call "Get" with value "numberOfLamingtons". Yes, obvious in hindsight. The code ends up looking something like the below. Thanks again, thoroughly appreciate the help.

Ben

    // Call an accessor function on the instance.

    v8::Local<v8::Value> lamingtons;

    CHECK(instance->Get(context, ToV8Value(isolate, "numberOfLamingtons")).ToLocal(&lamingtons));

    std::cout << ConvertTo<double>(*isolate, lamingtons->ToNumber(context).ToLocalChecked()) << std::endl;