Function instead of interface name when using Reflect

[pooyaho]

Hi
I've a problem with using reflect in field decorators. I've created a decorator with below signature:

export function Inject() {
    return function (clazz, name) {
        .......
        let typeName: string = Reflect.getOwnMetadata("design:type", clazz, name).name;
        .........
    }
}

class MyClass{
    @Inject()
    private $q:ng.IQService
}

And I use @Inject on fields. It works when field type is class but when field type is interface, it returns Function not the name of interface. How can I fix it?

[mhegazy]

TypeScript types, e.g. interfaces, type aliases, etc.., do not exist at runtime. they are design-time constructs that the compiler uses to check the consistency of the code, and are removed when generating the final JS.

The refectMetaData feature only preserves metadata about runtime values. classes do have a runtime value, i.e. the constructor function, whereas interfaces do not.

if the compiler knows there is a runtime value packaging the type (classes or enums), then it will use it; if not, it will try find the best "guess". so number will be Number, string will be String, [] becomes Array, async functions become Promise, and if your interface has a call or construct signatue on it, it will be Function, if none of these matches, it becomes Object.

[mhegazy]

you can find more relevant discussions in #3628

[pooyaho]

I think my problem is about reflection not type checking, And I know all thing that you wrote. I want to know the interface name with reflection and nothing more. I want to replace Function in the below generated code to an string that contains name of interface.

class MyClass{
     @Inject
     private service:ng.IHttpService;
}

it compiles to

........
__decorate([
          Inject(), 
          __metadata('design:type', Function)
  ], MyClass.prototype, "service", void 0);
........

[mhegazy]

First, the interface name by itself is not sufficient. the name is not unique. Second, and more importantly, TS has a structural type system, not a nominal one, so the interface name is of inconsequential in this matter, you could pass in an anonymous type that matches the interface, or another interface, etc..

This issue has been discussed in length before, please see issue #3060 (linked from #3628 among others)