Js Maybe type

maybe.js

const VALUE = Symbol('Value');

class Container {
  constructor(x) {
    this[VALUE] = x
  }

  map(f) {
    return f(this[VALUE]);
  }
}

class Functor extends Container {
  static of(x) {
    return new Functor(x);
  }

  map(f) {
    return Functor.of(f(this[VALUE]));
  }
}

class Nothing extends Functor {
  isNothing() {
      return true;
  }
  
  map(f) {
    return this;
  }
}

class Just extends Functor {
  isNothing() {
    return false;
  }

  map(f) {
    return Maybe.of(f(this[VALUE]));
  }
}

class Maybe extends Functor {
  constructor(x) {
    return x === undefined || x === null
      ? new Nothing()
      : new Just(x)
  }

  static of(x) {
    return new Maybe(x)
  }
}

module.exports = {
  Nothing,
  Just,
  Maybe
}