Option Monad in JavaScript

Option.js

/**
 * Option Monad in JavaScript
 */
function Option() {
}

function Some(value) {
  this.value = value;
}

function None() {
}

Option.unit = function(value) {
  if (value !== undefined && this.value !== null) {
    return new Some(value);
  } else {
    return new None();
  }
};

Option.prototype.bind = function(func) {
  if (this instanceof Some) {
    return Option.unit(func(this.value))
  } else {
    return None.unit();
  }
};

Option.prototype.get = function() {
  if (this instanceof Some) {
    return this.value;
  } else {
    return null;
  }
};

Some.prototype = None.prototype = Object.create(Option.prototype);