JavaScriptにおいてはObject型が成功と失敗を同時に表す型。 つまり、値がnullまたはundefinedならば失敗を表し、それ以外なら成功を表す。 その前提でMaybeのモナド則を書くと。。。

maybe_monad_lows.js

var maybe = {
  'return': function (value) {
    return value;
  },
  'bind': function (value, f) {
    if (value == null) return null;
    return f(value);
  }
};

(function () {
  console.log('Monad law 1');
  var f = function(a) { return maybe.return(a * 3); };
  var lhs = maybe.bind(maybe.return(5), f);
  var rhs = f(5);
  if (lhs !== rhs) throw Error('Monad law 1');
}());


(function () {
  console.log('Monad law 2');
  var m = maybe.return(5);
  var lhs = maybe.bind(m, maybe.return);
  if (m !== lhs) throw Error('Monad law 2');
}());

(function () {
  console.log('Monad law 3');
  var f = function(a) { return maybe.return(a * 3); };
  var g = function(a) { return maybe.return(a * 5); };
  var m = maybe.return(7);
  var lhs = maybe.bind(maybe.bind(m, f), g);
  var rhs = maybe.bind(m, function(x) { return maybe.bind(f(x), g); });
  if (lhs !== rhs) throw Error('Monad law 3');
}());