How to add a method to a prototype of a generic class in typescript

Question

I'm trying to add a method to a prototype of PromiseLike<T> With String it is not a problem:

declare global {
    interface String {
        handle(): void;
    }
}

String.prototype.handle = function() {
}

Compiles OK

But if I try to do the same with PromiseLike<T>, I get a compile error 'PromiseLike' only refers to a type, but is being used as a value here.:

declare global {
    interface PromiseLike<T> {
        handle(): PromiseLike<T>;
    }
}

PromiseLike.prototype.handle = function<T>(this: T):T {
    return this;
}

Obviously the problem here is that PromiseLike is generic. How can I do this properly in typescript?

Answer 1

Interfaces do not exist at runtime, they are erased during compilation, so setting the value of a function on an interface is not possible. What you are probably looking for is adding the function to Promise. You can do this similarly:

declare global {
  interface Promise<T> {
      handle(): Promise<T>;
  }
}

Promise.prototype.handle = function<T>(this: Promise<T>): Promise<T> {
  return this;
}