class.js

var Baby = (function() {
  function Baby(name) { // Constructor
    this.name = name;
  }

  Baby.prototype = { // Public
    introduce: function() {
      alert("Hello! My name is " + name);
    },
    play: function() {
      run();
      jump();
      scream();
    }
  };

  // Private
  function run() {
    alert("Run baby, run!");
  }

  function jump() {
    alert("Beowm, beowm, beowm!");
  }

  function scream() {
    alert("A-A-A-A-A-A!");
  }

  return Baby;
})();

var alice = new Baby("Alice");

alice.introduce();
alice.play();
alice.jump(); // Will fail