A step-by-step guide on properly specifying the return value of a factory method

Question

I have a base class with the following structure:

class Base {
    protected constructor() {}

    static create() {
         return new this;
    }
}

However, when I extend this class and attempt to initialize it, a TypeScript error occurs.

class Bar extends Base {
    magic() {}
}

let bar: Bar;
bar = Bar.create(); // <-- Typescript error

The error message states:

  Property 'magic' is missing in type 'Base' but required in type 'Bar'

It appears that TypeScript assumes the returned value of create is an instance of Base, rather than Bar.

To resolve this issue, we can make the necessary adjustments as shown below:

class Base {
    protected constructor() {}

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

And then assign the correct type like so:

bar = Bar.create<Bar>();

It may seem unnecessary to explicitly specify the return type since it should be clear, but how can we inform TypeScript about the exact return type?

Answer 1

Answer №1

Your issue stems from the fact that you have not overridden the create method in the subclass.

static create() {
    return new this;
}

This method is implicitly returning type Base.

In a non-static method, you could return this, but it's not feasible in this context.

The problem of using Polymorphic this for static methods has been recognized as a long-standing issue

Answer 2

Your issue stems from the fact that you have not overridden the create method in the subclass.

static create() {
    return new this;
}

This method is implicitly returning type Base.

In a non-static method, you could return this, but it's not feasible in this context.

The problem of using Polymorphic this for static methods has been recognized as a long-standing issue