Maybe Monad sample implementation

Maybe-ramda-fantasy.js

//Showing relevant parts from the Maybe implementation from ramda-fantasy lib
//See https://github.com/ramda/ramda-fantasy/blob/master/src/Maybe.js for full source

function Maybe(x) { //<-- main constructor that returns Maybe of Just or Nothing
  return x == null ? _nothing : Maybe.Just(x);
}

function Just(x) {
  this.value = x;
}
util.extend(Just, Maybe);

Just.prototype.isJust = true;
Just.prototype.isNothing = false;

function Nothing() {}
util.extend(Nothing, Maybe);

Nothing.prototype.isNothing = true;
Nothing.prototype.isJust = false;

var _nothing = new Nothing();

Maybe.Nothing = function() {
  return _nothing;
};

Maybe.Just = function(x) {
  return new Just(x);
};

Maybe.of = Maybe.Just;

Maybe.prototype.of = Maybe.Just;


// functor
Just.prototype.map = function(f) { //Doing "map" on Just runs the func and returns Just out of the result
  return this.of(f(this.value));
};

Nothing.prototype.map = util.returnThis; // <-- Doing "Map" on Nothing doesnt do anything

Just.prototype.getOrElse = function() {
  return this.value;
};

Nothing.prototype.getOrElse = function(a) {
  return a;
};

module.exports = Maybe;