Maybe monad

class Maybe{
  constructor(val){
    this._value = val;
  }
  static of = x => new Maybe(x);
  
  isNothing = () => (this.__value === null || this.__value === undefined);
  
  map = () => this.isNothing() ? Maybe.of(null) : Maybe.of(f(this.__value));
  
  ap = other => this.isNothing() ? Maybe.of(null) : other.map(this.__value);
  
  join = () => this.isNothing() ? Maybe.of(null) : this.__value;
}