Define a property as string or function that returns a string in Typescript

Question

I want to create an interface where a property can be either a string or a Function that has to return a string. I currently have the following:

interface IExample {
  prop: string|Function;
}

But that's not explicit enough for me because the Function is allowed to return anything. I want to tell the compiler that the return value has to be a string.

How is this possible in TypeScript? Or is it possible at all?

Answer 1

type propType = () => string;

interface IExample {
   field : string | propType;
}

class MyClass1 implements IExample {
    field : string;
}

class MyClass2 implements IExample {
    field() {
        return "";
    }
}

Update 1

type PropertyFunction<T> = () => T;

interface IExample {
   field : string | PropertyFunction<string>;
}