What is F-Bounded Polymorphism in TypeScript

Question

I notice version 1.8 of TypeScript supports F-Bounded Polymorphism. In layman's terms, what is it and how is this helpful? I am assuming since this feature was included early it must be pretty important.

Answer 1

It basically means that you have your list of generics that the function references, and within that list of generics, one type can reference another type, to define a relationship between the two generic types.

function someFunction <T, U> (t: T, u: U): T {
  return t;
}

const dog = someFunction(new Dog(), new Cat());

Hooray!

Now, with bounded generics, they can reference one another to define the bounds of the relationship they have with each other:

function someFunction <T extends U, U> (t: T, u: U): T {
  return t;
}

const dog = someFunction(new Dog(), new Pet());
const cow = someFunction(new Cow(), new Animal());
const BOOM = someFunction(new Cat(), new Dog()); // *BEWM!*