Option<?A> in JavaScript

index.js

type Maybe = Some | None;

class Option<A> {
  value: A;
  constructor(x: ?A) {
    if (x == null) {
      return new None();
    } else {
      return new Some(x);
    }
  }
  static of(x: ?A): Maybe {
    return new Option(x);
  }
}

class Some<A> {
  value: A;
  constructor(x: A) {
    this.value = x;
  }
  map<B>(f: (x: A) => ?B): Maybe {
    const result: ?B = f(this.value);
    if (result === null || result === undefined) {
      return new None();
    } else {
      return new Some(result);
    }
  }
  isNothing(): boolean {
    return false;
  }
  getOrElse<B>(other: B): A {
    return this.value;
  }
}

class None {
  constructor() { }
  isNothing(): boolean {
    return true;
  }
  map<B>(f: (x: any) => B): None {
    return this;
  }
  getOrElse<B>(other: B): B {
    return other;
  }
}

Option.of(233)                // Some<number>
  .map(x => x * 42)           // Some<number>
  .map(x => x + 'hello')      // Some<string>
  .map(x => null)             // None
  .map(x => x.length)         // None
  .getOrElse('yo');           // => 'yo'