typescript inheritance using static method

Question

Is there a way to obtain proper type using approach described below. I have tried make makeInstance() generic but I haven't obtained Extended type. Code below.

class Base {
  name = 'foo';

  static makeInstance() {
    return new this();
  }
}
class Extended extends Base {
  age = 10;
}

let base = Base.makeInstance() // is Base
let extended = Extended.makeInstance(); //should be Extended 

console.log(base.name);//ok
console.log(extended.age); //output ok; age doesn't exists
console.log(extended.name);// ok

Relevant GitHub issue: github.com/Microsoft/TypeScript/issues/5863 — jcalz
– jcalz, Commented Aug 17, 2018 at 16:29

Titian Cernicova-Dragomir · Accepted Answer · 2018-08-17 16:31:21Z

7

You can add a generic parameter to the static method to infer the class on which the method is invoked correctly :

class Base {
    name = 'foo';

    static makeInstance<T>(this: new () => T) {
        return new this();
    }
}
class Extended extends Base {
    age = 10;
}

let base = Base.makeInstance() // is Base
let extended = Extended.makeInstance(); //is Extended 

console.log(base.name);//ok
console.log(extended.age); //ok
console.log(extended.name);// ok

answered Aug 17, 2018 at 16:31

Titian Cernicova-Dragomir

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