How to define a generic constructor in typescript under inheritance?

Question

I am using typescript and want to write a factory function to create a new object instance and provide an array of properties to be filled. Basically a converter from an existing object to a new instance of another (related) type. It should be used like this

const newA = A.from({ a: 1 });
const newC = C.from({ a: 1, b: 2, c: 9 });

I have this working (member variables omitted for clarity and Object.create could be simpler, see below for the reason)

class A {
  static from(data: Partial<A>) : A {
    const o = Object.create(A);
    Object.assign(o, data);
    return o;
  }
}

This works nicely as long as I inherit from this class. I do not want to recreate this function anytime like so

class A {
  static from(data: Partial<A>) : A {
    const o = Object.create(A);
    Object.assign(o, data);
    return o;
  }
}

class B extends A {
static from(data: Partial<B>) : B {
    const o = Object.create(B);
    Object.assign(o, data);
    return o;
  }
}

The following snippet works, but I need to specify the class when I call it twice

class A {
  ...
  static from<T extends A>(data: Partial<T>) : T {
    const o = Object.create(this);
    Object.assign(o, data);
    return o;
  }
}

const newC = C.from<C>({ ... });

and want to use some form of generic, i.e. I imagine something like this.

class A {
  static from<T extends this>(data: Partial<T>) : A {
    const o = Object.create(this);
    Object.assign(o, data);
    return o;
  }
}

const new C = C.from({ ... });

I would guess that this is theoretically possible, since at compile time the context of the static function is known and you could infer the class type, but I did not figure out on how to use this inherited type in the signature definition.

Is this a good (acceptable) pattern and if not, what should I do?

Any help is appreciated!

jcalz · Accepted Answer · 2020-01-16 19:33:08Z

3

You're looking for polymorphic this types for static methods, but they are not currently supported in TypeScript. There's an open feature request for it, microsoft/TypeScript#5863 but I don't know if it will ever be implemented.

Luckily there are workarounds, one of which is mentioned here: use a generic method in which the this parameter depends on the generic. For example:

class A {
    static from<T extends A>(this: new () => T, data: Partial<T>): T {
        const o = new this(); // no-arg constructor
        Object.assign(o, data);
        return o;
    }
}

Here we will only allow from() to be called on objects which are no-arg constructors of some subtype of A, and they will return this subtype. I changed the implementation from Object.create(this) to new this() just to show that we care about the "no-arg-ness" (after all, a subclass called with Object.create(this) that required a constructor argument is likely to fail to construct a valid instance).

So then this works, since B is a no-arg constructor of B instances:

class B extends A {
    foo = "bar";
}
const b = B.from({}); // B
console.log(b.foo.toUpperCase()); // BAR

but this fails, since Z is not a no-arg constructor of Z instances:

class Z extends A {
    constructor(public bar: string) {
        super();
    }
}
const z = Z.from({}); // error!
// -----> ~
// typeof Z is not assignable to new () => Z
console.log(z.bar.toUpperCase());  // error at runtime, z.bar is undefined

Okay, hope that helps; good luck!

Playground link to code

answered Jan 16, 2020 at 19:33

jcalz

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2 Comments

jhp Over a year ago

That was insanely enlighting! Thank you. I am still struggeling as to why you can add the this parameter to the signature and it is ignored in the function call? Is this a special type of parameter?

jcalz Over a year ago

Yes, exactly. It’s called a “this parameter” and I’ve linked to the relevant documentation in my answer. Here’s the link again: this parameters

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