Infer "this" from extended classes

[eliottrobson]

TypeScript Version: 2.4.0

Code

class B {
    static make() {
        return new this;
    }
}

class A extends B {
    id = 1;
}

console.log(typeof A.make().id);

Expected behavior:
A.make().id can be inferred to be the id (1)

Actual behavior:
A.make() is typehinted to return "B" where it returns "A" at runtime.

[aluanhaddad]

Give this a try

class B {
    static make<T extends typeof B>(this: T): T['prototype'] {
        return new this();
    }
}

class A extends B {
    id = 1;
}

console.log(A.make().id);

It is a bit ugly, and there may be a more concise way to write it, but you only need to specify it once.

Note that T is inferred as A because it is the type of the this argument.

[eliottrobson]

I've never seen that syntax before. It works, thanks a lot!

[DanielRosenwasser]

Dupe of #5863.