Having this example:

class A {
  fooA() {
    console.log("fooA");
  }

  printFooBFromA() {
    const b = new this.B();
    b.fooB();
  }
}

class B {
  fooB() {
    console.log("fooB");
  }
}

A.prototype.B = B;

const a = new A();
a.printFooBFromA();

What is the use case of such scenario? Why would one want to insert another class retrospectively?