Parametric Polymorphism ... in my JavaScript?

index.js

// @flow


// The Identity Monad
class Container<A> {
  value: A;
  constructor(x: A) {
    this.value = x;
  }
  map<B>(f: (x: A) => B): Container<B> {
    return Container.of(f(this.value));
  }
  get(): A {
    return this.value;
  }
  static of<B>(x: B): Container<B> {
    return new Container(x);
  }
}

// Types of a and b are infered by Flow
let a = Container.of(23)
  .map(x => x * 2)
  .map(x => x.toString())
  .get();

let b = Container.of('hello')
  .map(x => x.length)
  .get();