Typescript: Type inference when using decorator

Question

I wonder why when I use decorators or annotations in Typescript on a class. The compiler can't infer the new type of the class. If I don't use decorators and use the old way to do this in ES5 (ie. call manually the decorator) it obviously works.

For instance, here a sample that shows the issue:

function decorate(Target: typeof Base): IExtendedBaseConstructor {
  return class extends Target implements IExtendedBase {
    public extendedtMethod(): number {
      return 3;
    }
  };
}

interface IBase {
  baseMethod(): number;
}

interface IExtendedBase extends Base {
  extendedtMethod(): number;
}

interface IExtendedBaseConstructor {
  new(): IExtendedBase;
}

@decorate
class Base implements IBase {
  public baseMethod(): number {
    return 5;
  }
}

const test = new Base();
test.baseMethod(); // OK
test.extendedtMethod(); // NOT OK, typescript think, Base is still Base but we decorated it.

With the older way, it works:

class Base implements IBase {
  public baseMethod(): number {
    return 5;
  }
}

const ExtendedBase = decorate(Base);

const test = new ExtendedBase();
test.baseMethod(); // OK
test.extendedtMethod(); // OK

Thanks in advance.

Answer 1

Right now this doesn't work. There is a pending issue on github to allow class decorators to change the type of the class.

I would recommend doing the "old way" you mentioned until this is implemented.