TypeScript Object Factory

Question

I'm trying to build an object factory in TypeScript where the objects generated must have a common base type, but I haven't figured out how to encode it properly.

Here's my current attempt which is incorrect because TypeScript says T is not guaranteed to be of type Base.

class Base {
    constructor() {}
}


class User extends Base {
    constructor() {
        super()
    }
}

class Post extends Base {
    constructor() {
        super()
    }
}

function buildObject<T extends Base>(type: typeof Base): T {
    return new type()
}

I thought I could just create a new type like type BaseInstance<T> = T extends Base to use here instead:

function buildObject<T>(type: typeof BaseInstance<T>): BaseInstance<T> {
    return new type()
}

But this construct type BaseInstance<T> = T extends Base is invalid.

Use type: new() => T instead of type: typeof Base as buildObject parameter. — hoangdv
– hoangdv, Commented Sep 25, 2020 at 1:04

Linda Paiste · Accepted Answer · 2020-09-25 00:53:07Z

1

Try defining a type BaseConstructor which can be called via the new keyword and return an instance of Base. Your classes Base, User, and Post all fulfill this type definition.

type BaseConstructor<T extends Base> = new() => T;

function buildObject<T extends Base>(type: BaseConstructor<T>): T {
    return new type();
}

const myObj: Post = buildObject(Post);

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answered Sep 25, 2020 at 0:53

Linda Paiste

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