Typescript generics with factory function

Question

I have a class that I want to extend with a factory class that will return the correct class based on the input.

class Base<S> {
  prop: S;
  base() { }
}

class A<S> extends Base<S> {
  a() { }
}

class B<S> extends Base<S> {
  b() { }
}


interface TypeA {
  propA: any;
}

interface TypeB {}

function isTypeA(params: TypeA | TypeB): params is TypeA {
  return (params as TypeA).propA !== undefined;
}

function factory(params: TypeA): typeof A;
function factory(params: TypeB): typeof B;
function factory(params: TypeA | TypeB): typeof A | typeof B {
  if (isTypeA(params)) {
    return A;
  } else {
    return B;
  }
}

const param: TypeA = { propA: {} };
const param2: TypeB = {};

class Test extends factory(param) { }

In addition to that, I need to pass a generics all the way up. How can I pass the generic from the factory function to the Base class?

Oscar Paz · Accepted Answer · 2018-04-05 10:15:16Z

2

Do this:

class Test<T> extends factory(param)<T> { }

Or, if you don't want Test to be generic:

class Test extends factory(param)<string>()

answered Apr 5, 2018 at 10:15

Oscar Paz

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3 Comments

Titian Cernicova-Dragomir Over a year ago

I think the point is that factory should return typeof A<typeof param> which is syntactically incorrect in typescript, but that is the desired effect. ie class Test extends factory(param)<typeof param> { }

Oscar Paz Over a year ago

You think so? I didn't understand it that way. I think that the type of param and the type parameter of A (or B) are not related, but of course, he/she can clarify that

Titian Cernicova-Dragomir Over a year ago

I might have misunderstood, OP should clarify :)

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