A.js

const P = require("P");

class A extends P {
}

B.js

const P = require("P");

class B extends P {
}

P.js

const A = require("A");
const B = require("B");

class P {
  static function factory(param){
    if (param.isA()) {
      return new A();
    } else {
      return new B();
    }
  }
}