Typescript generic factory function including parameters for created object

Question

I am trying to create a factory class in Typescript. The factory should be set up to create a specific class of object, including default parameters set up at factory initialization.

Here's what I've come up with so far:

type Constructor<T> = new(...args: any) => T;

class Factory<T> {
    private params: ConstructorParameters<Constructor<T>>;
    private ctor: Constructor<T>;
    init<TCtor extends Constructor<T>>(tConstructor: TCtor,
        ...params: ConstructorParameters<TCtor>)
    {
        this.ctor = tConstructor;
        this.params = params;
    }

    create(): T {
        return new this.ctor(...this.params);
    }
}

class ClassWithParams { constructor(a: number) { } }

const factory = new Factory<ClassWithParams>();
//factory.init(ClassWithParams);
// Compilation error - missing param a

factory.init(ClassWithParams, 1);
console.log(factory.create());
// instance of ClassWithParams

The above code works well (Typescript playground). However, it requires an init() call for the factory directly after construction. I can't move the init() code into the constructor because init() relies on a generic parameter TCtor in order to properly typecheck the parameter list.

I could make TCtor a generic parameter of the class instead, but that would mean every time Factory is referenced generically an additional type parameter would need to be supplied for no reason.

Any ideas to remove that init() call?

I realized my answer had a mistake in it that caused type information to be lost. Please see the updated version — Aluan Haddad
– Aluan Haddad, Commented Jun 24, 2019 at 6:58
Oh interesting, it seemed to work for me - what type information was lost? — Daniel
– Daniel, Commented Jun 24, 2019 at 7:16
The return type of Factory.prototype.create was wrong. Try the new version — Aluan Haddad
– Aluan Haddad, Commented Jun 24, 2019 at 9:00
@AluanHaddad the new implementation is incorrect - there is no type error for incorrect parameters. Did you mean tConstructor: C instead of tConstructor: Constructor<T> ? — Daniel
– Daniel, Commented Jun 25, 2019 at 4:46

Aluan Haddad · Accepted Answer · 2019-06-24 06:57:30Z

1

What's wrong with the following code

type Constructor<T> = new (...args: any[]) => T;

class Factory<T, C extends Constructor<T> = Constructor<T>> {
    params: ConstructorParameters<C>;
    ctor: Constructor<T>;
    constructor(tConstructor: Constructor<T>,
        ...params: ConstructorParameters<C>) {
        this.ctor = tConstructor;
        this.params = params;
    }

    create(): T {
        return new (this.ctor)(...this.params)
    }
}
class ClassWithParams { constructor(a: number) { this.a = a; } a: number } 

const factory1 = new Factory(ClassWithParams);

const factory2 = new Factory(ClassWithParams, '1');

const factory3 = new Factory(ClassWithParams, 0);


const value = new Factory(ClassWithParams, 0).create().a;

As seen in this playground it's convenient and correct.

edited Jun 24, 2019 at 6:57

answered Jun 24, 2019 at 2:34

Aluan Haddad

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