CODEWITHSUNDEEP
typescript
jayatubi
jayatubi

Reputation: 2212

What is the difference between `extends A` and `extends typeof A` in TypeScript?

I was make a class decorator and I wanted to limit this decorator could only be applied to certain classes so I did:

@decorator()
class A {
    foo:string;
}
@decorator()
class B extends A {

}
@decorator()
class C {}

function decorator () {
  // This makes decorators work on class A and B except class C
  return function<T extends typeof A> (target:T) {}
  // This makes decorators become error on all the classes
  // return function<T extends A> (target:T) {}
}

If I changed the function<T extends typeof A> (target:T) {} into function<T extends A> (target:T) {} then all the decorators would became errors.

I'm not sure why do I have to use extends typeof A instead of extends A? And what's the difference between them?

Upvotes: 2

Views: 396

Answers (1)

Alex Wayne
Alex Wayne

Reputation: 187144

The name of class when used as a type means instances of that class. To get the type of a class constructor, you use typeof.

So:

class A {}
const instance: A = new A()

const aConstructor: typeof A = A
const anotherInstance: A = new aConstructor()

So the reason that your decorator requires typeof A is because it operates on a class constructor, and not instances.

Upvotes: 5

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