Implement an object with multiple constructors

[LijieZhang1998]

Hi,

I'm implementing Intl for i18n support. Intl is actually a namespace that has multiple constructors. Can I use goja to implement it? I tried the following code. I expected to print out true, but got false. If I remove new from new Intl.DateTimeFormat("haha"), then it's working. I want to have new before Intl.DateTimeFormat("haha") because that complies with specification.

func DateTimeFormat(date string) string {
	return date
}

type DateTimeFormatConstructor func(date string) string

type Intl struct {
	DateTimeFormat DateTimeFormatConstructor
}

func main(){
	vm := goja.New()
	var intl = Intl{
		DateTimeFormat: DateTimeFormat,
	}
	vm.Set("Intl", &intl)
	_, err := vm.RunString(`
			 function intlTest() {
				const result = new Intl.DateTimeFormat("haha");
				return result;
			};
	 `)
	if err != nil {
		panic(err)
	}

	var fn func() string
	err = vm.ExportTo(vm.Get("intlTest"), &fn)
	if err != nil {
		panic(err)
	}
	fmt.Println(fn()=="haha")
}

[dop251]

Hi.

To be more standard compliant, you need something like this:

	vm := New()
	intl := vm.NewObject()
	dateTimeFormat := vm.ToValue(func(call ConstructorCall) *Object {
		// initialise the instance (call.This) according to the arguments supplied to the constructor (call.Arguments)
		return nil
	})
	dateTimeProto := dateTimeFormat.(*Object).Get("prototype").(*Object)
	dateTimeProto.DefineDataProperty("format", vm.ToValue(func(call FunctionCall) Value {
		instance, _ := call.This.(*Object)
		// do the formatting using the instance parameters and the supplied arguments
		_ = instance
		result := vm.ToValue("passed")
		return result
	}), FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	intl.DefineDataProperty("DateTimeFormat", dateTimeFormat, FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	vm.GlobalObject().DefineDataProperty("Intl", intl, FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	res, err := vm.RunString(`
		const formatter = new Intl.DateTimeFormat();
		formatter.format();
	`)

[dop251]

You can also use a Go type as an instance, like this:

	type formatter struct {

	}
	vm := New()
	intl := vm.NewObject()
	dateTimeFormat := vm.ToValue(func(call ConstructorCall) *Object {
		instance := &formatter{}
		// initialise the instance according to the arguments supplied to the constructor (call.Arguments)
                // instance.whatever = ....
		instanceValue := vm.ToValue(instance).(*Object)
		instanceValue.SetPrototype(call.This.Prototype())
		return instanceValue
	})
	dateTimeProto := dateTimeFormat.(*Object).Get("prototype").(*Object)
	dateTimeProto.DefineDataProperty("format", vm.ToValue(func(call FunctionCall) Value {
		instance, ok := call.This.Export().(*formatter)
		if !ok {
			panic(vm.NewTypeError("this should be a formatter"))
		}
                return vm.ToValue(formatter.format(....))
	}), FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)

I'm going to add this to the documentation too.

[LijieZhang1998]

Awesome 🎉 This is very helpful. I have one more question about how I can get instance parameters inside the definition of "format" in the first solution. call.Arguments returns the arguments for format functionCall. How can I get instance parameters for dateTimeFormat? Sorry, i'm a newbie for goja. The second solution is working for me.

	vm := New()
	intl := vm.NewObject()
	dateTimeFormat := vm.ToValue(func(call ConstructorCall) *Object {
		// initialise the instance (call.This) according to the arguments supplied to the constructor (call.Arguments)
		return nil
	})
	dateTimeProto := dateTimeFormat.(*Object).Get("prototype").(*Object)
	dateTimeProto.DefineDataProperty("format", vm.ToValue(func(call FunctionCall) Value {
		instance, _ := call.This.(*Object)
		// do the formatting using the instance parameters and the supplied arguments
       	        _ = instance
		result := vm.ToValue("passed")
		return result
	}), FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	intl.DefineDataProperty("DateTimeFormat", dateTimeFormat, FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	vm.GlobalObject().DefineDataProperty("Intl", intl, FLAG_TRUE, FLAG_TRUE, FLAG_FALSE)
	res, err := vm.RunString(`
		const formatter = new Intl.DateTimeFormat();
		formatter.format();
	`)

[dop251]

You'd have to set them as properties of the instance (maybe Symbol properties to minimise the exposure). That's why the second solution is better.

[LijieZhang1998]

Sounds good to me 👍 Thank you very much.

You'd have to set them as properties of the instance (maybe Symbol properties to minimise the exposure). That's why the second solution is better.

[dop251]

No worries!