How to set the prototype of DynamicObject correctly?

[zllovesuki]

On godoc:

The Object's prototype is initially set to Object.prototype, but can be changed using regular mechanisms (Object.SetPrototype() in Go or Object.setPrototypeOf() in JS).

The Object cannot have own Symbol properties, however its prototype can. If you need an iterator support for example, you could create a regular object, set Symbol.iterator on that object and then use it as a prototype. See TestDynamicObjectCustomProto for more details.

However, I'm having trouble of doing so from Go side.

Suppose that I have a class:

class FetchEvent {
    get [Symbol.toStringTag]() {
        return 'FetchEvent'
    }
}

Calling .toString() on an instance of FetchEvent returns [object FetchEvent] correctly. but I'm trying to set a DynamicObject's prototype to FetchEvent, so I don't get TypeError: Could not convert &{{0xc00011f380 0xc000735b90} 0xc000b84000} to primitive when I'm trying to call .toString() on the DynamicObject.

What's the correct way of doing so? I have tried:

class FetchEvent {
    get [Symbol.toStringTag]() {
        return 'FetchEvent'
    }
}

const instance = new FetchEvent()

	evt.nativeEvt = vm.NewDynamicObject(evt)
	class := vm.Get("FetchEvent")
	instance := vm.Get("instance")
	evt.nativeEvt.SetPrototype(class.ToObject(vm))
	fmt.Printf("%+v\n", evt.nativeEvt)
	evt.nativeEvt.SetPrototype(instance.ToObject(vm))
	fmt.Printf("%+v\n", evt.nativeEvt)
	evt.nativeEvt.SetPrototype(class.ToObject(vm).Prototype())
	fmt.Printf("%+v\n", evt.nativeEvt)
	evt.nativeEvt.SetPrototype(instance.ToObject(vm).Prototype())
	fmt.Printf("%+v\n", evt.nativeEvt)

But all of them still fails with %!v(PANIC=String method: TypeError: Could not convert &{{0xc0003b0f30 0xc0005c5bc0} 0xc000972000} to primitive)

Maybe I'm just bad at JavaScript 😅

[zllovesuki]

I figured it out. Even if this is dynamic, it is still trying to access .toString or .valueOf from the DynamicObject itself, you can 302 those .Get() to the underlying instance itself:

// ...
	evt.nativeEvt = evt.vm.NewDynamicObject(evt)
	evt.nativeEvt.SetPrototype(evt.nativeEvtInstance)
// ...

func (evt *fetchEvent) Get(key string) goja.Value {
	switch key {
	case "respondWith":
		return evt.vm.ToValue(evt.nativeRespondWith)
	case "waitUntil":
		return evt.vm.ToValue(evt.nativeWaitUntil)
	case "request":
		return evt.requestProxy.nativeReq
	default:
		return evt.nativeEvtInstance.Get(key)
	}
}

Now it is no longer exploding 😆

2023-03-11T03:46:35.274-0800    INFO    heresy/runtime_logger.go:23     [object FetchEvent]     {"script": "event.js"}

[dop251]

Yes, I was just writing to suggest that your DynamicObject implementation that either returns a primitive or a function that returns an object when asked for the toString property.

The following works as expected for me:

func TestDynamicObjectProtoSymbol1(t *testing.T) {
	vm := New()
	_, err := vm.RunString(`
class FetchEvent {
    get [Symbol.toStringTag]() {
        return 'FetchEvent'
    }
}

const instance = new FetchEvent()
	`)
	if err != nil {
		t.Fatal(err)
	}
	m := make(map[string]Value)
	//m["toString"] = vm.ToValue("")
	dynObj := &testDynObject{
		r: vm,
		m: m,
	}
	o := vm.NewDynamicObject(dynObj)
	class := vm.Get("FetchEvent")
	instance := vm.Get("instance")
	o.SetPrototype(class.ToObject(vm))
	t.Logf("%+v\n", o)
	o.SetPrototype(instance.ToObject(vm))
	t.Logf("%+v\n", o)
	o.SetPrototype(class.ToObject(vm).Prototype())
	t.Logf("%+v\n", o)
	o.SetPrototype(instance.ToObject(vm).Prototype())
	t.Logf("%+v\n", o)
}

Produces:

    object_dynamic_test.go:487: %!v(PANIC=String method: TypeError: Function.prototype.toString requires that 'this' be a Function)
    object_dynamic_test.go:489: [object FetchEvent]
    object_dynamic_test.go:491: %!v(PANIC=String method: TypeError: Function.prototype.toString requires that 'this' be a Function)
    object_dynamic_test.go:493: [object FetchEvent]

(BTW, the second scenario is the right one)

However, if I uncomment the //m["toString"] = vm.ToValue("") line, I get the following:

    object_dynamic_test.go:487: %!v(PANIC=String method: TypeError: Could not convert &{{0xc000206870 0xc000206180} 0xc0001f8df0} to primitive)
    object_dynamic_test.go:489: %!v(PANIC=String method: TypeError: Could not convert &{{0xc000206870 0xc0002067e0} 0xc0001f8df0} to primitive)
    object_dynamic_test.go:491: %!v(PANIC=String method: TypeError: Could not convert &{{0xc000206870 0xc00010f8c0} 0xc0001f8df0} to primitive)
    object_dynamic_test.go:493: %!v(PANIC=String method: TypeError: Could not convert &{{0xc000206870 0xc000206120} 0xc0001f8df0} to primitive)

[zllovesuki]

so o.SetPrototype(instance.ToObject(vm).Prototype()) is the one I should be using, correct? thank you 🙏

[dop251]

Yes, or it can be class.ToObject(vm).Get("prototype").ToObject(vm), this way you don't need the instance. Also, Symbol.toStringTag can be a simple property, no need to make it a getter.