TypeScript generic function, unexpected behavior when generic is a class

Question

class Class {
}
const f0 = <T extends typeof Class> (c:T): T => {
  return c
}
const call0 = f0 (Class) //ok

const f1 = <T extends typeof Class> (c:T): T => {
  const a = new c()
  return a //TS2322: Type 'Class' is not assignable to type 'T'.   'T' could be instantiated with an arbitrary type which could be unrelated to 'Class'.
}
const call1 = f1 (Class)

const f2 = <T extends typeof Class> (c:T):InstanceType<T> => {
  const a = new c()
  return a //TS2322: Type 'Class' is not assignable to type 'InstanceType '.
}
const call2 = f1 (Class)

TypeScript Playground

The argument is typed as T, so why isn't that acceptable for the return type?

@pilchard The answer is hidden in there but it's very hard to understand it if you don't already know it. This question is using typeof Class to get a constructor which the question you mentioned is doesn't mention. Using typeof Class instead of a generic constructor type is what caused the problem — Ruan Mendes
– Ruan Mendes, Commented Dec 17, 2021 at 0:59
To those who closed this question. The accepted answer on the linked question is to use InstanceType<T> which the OP was already using. The linked answer does not answer the problem posted here which was how to create a generic constructor type. The solution to the OP's problem does not involve using InstanceType. Please don't close a question linking to another question whose accepted answer does not address this post's problem. — Ruan Mendes
– Ruan Mendes, Commented Dec 17, 2021 at 15:59

Ruan Mendes · Accepted Answer · 2021-12-17 01:44:50Z

0

Your first snippet doesn't make a lot of sense. It says that it takes something that is a constructor that returns a Class instance. That T extends new () => Class;

It also says it returns the same T (a constructor), but you are returning an instance; that doesn't make sense.

Solution

You were a lot closer in your second snippet

const f2 = <T extends typeof Class> (c:T): InstanceType<T> => {
  const a = new c()
  return a //TS2322: Type 'Class' is not assignable to type 'InstanceType '.
}

Again, typeof Class expands to {new() => Class} but what you really want is a generic constructor, so you can't just use typeof Class, you have to write your own type for the generic constructor of Class. Then your return type is a lot simpler.

class Class {  a = 1; }

class SubClass extends Class {  b = 2; }

class OtherClass {  c = 3; }

type ClassCtor<T extends Class> = new () => T;

const f = <C extends Class> (c: new() => C): C => {
  return new c();
}

// OR
const f = <C extends Class> (c: ClassCtor<C>): C => {
  return new c();
}



// It's a Class instance
const call1 = f(Class);

// It's a SubClass instance
const call2 = f(SubClass);

// Fails compilation
const call3 = f(OtherClass);

See live example

edited Dec 17, 2021 at 1:44

answered Dec 17, 2021 at 0:54

Ruan Mendes

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3 Comments

pilchard Over a year ago

Did you mean const f = <C extends Class> (c: ClassCtor<C>): C => {...? (you never use your constructor type). Nice explanation though.

Ruan Mendes Over a year ago

I just inlined the constructor type... But yes, that's how it would be used.

pilchard Over a year ago

I saw that, just thought it might clarify the concept to make use of the custom type.

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