TypeScript generic's type inference fails if generic is used as a callback's argument

Question

This compiles just fine:

class Foo<F> {
  bar: Array<F> = [];

  static create<F, T extends Foo<F>>(this: new () => T): T {
    return new this();
  }
}

class Foo2 extends Foo<string> {}

const foo = Foo2.create();

But if I convert Array<F> to Array<(a: F) => void>, the same code fails:

class Foo<F> {
  bar: Array<(a: F) => void> = [];

  static create<F, T extends Foo<F>>(this: new () => T): T {
    return new this();
  }
}

class Foo2 extends Foo<string> {}

const foo = Foo2.create();
      ^^^
The 'this' context of type 'typeof Foo2' is not assignable to method's 'this' of type 'new () => Foo<unknown>'.
  Types of construct signatures are incompatible.
    Type 'new () => Foo2' is not assignable to type 'new () => Foo<unknown>'.
      Type 'Foo2' is not assignable to type 'Foo<unknown>'.
        Type 'unknown' is not assignable to type 'string'. ts(2684)

It works if I type it manually:

const foo = Foo2.create<string, Foo2>();

So for some reason TypeScript can't infer the string type if it's used in a function argument. Why is this?

In static methods it's reserved for this very purpose I believe @BoussadjraBrahim — Mahi
– Mahi, Commented Sep 19, 2022 at 8:15
@MarkusMeskanen it looks like what you want already exists in TypeScript as ThisType. — Meligy
– Meligy, Commented Sep 19, 2022 at 8:33

jcalz · Accepted Answer · 2022-09-19 16:46:25Z

1

I'm not sure why TypeScript is not figuring the type out, but I think you had a few mistakes that you got away with anyway, particularly the F type in the method declaration overriding the F in the class itself.

Your code can be simplified as follows:

class Foo<F> {
  bar: Array<(a: F) => void> = [];

  static create<T>(this: new () => T) {
    return new this();
  }
}

class Foo2 extends Foo<string> {}


const foo = Foo2.create();

And it'll work fine:

Playground Link

Update: Sorry, I misread the question originally. The answer has since been updated.

edited Sep 19, 2022 at 16:46

jcalz

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answered Sep 19, 2022 at 8:06

Meligy

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8 Comments

Mahi Over a year ago

You seem to have tried the first code that works for me as well, my question is about why the second one doesn't work

Meligy Over a year ago

Sorry about that, @MarkusMeskanen. Please see the updated answer.

Mahi Over a year ago

Thank you very much, it seems to work wonderfully! I had never heard of ThisType before, glad to see it exists :)

k.tten Over a year ago

This is not using ThisType. This is just a generic parameter named ThisType...

Meligy Over a year ago

I checked the solution for this and found a lot of solutions that work, but none of them support when the parent is generic. Even the creative ones like stackoverflow.com/a/52519934/146656. The thing is that in TypeScript you cannot access generic arguments from static methods.

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