Simulating private instance methods.

private-instance-methods1.js

// This is probably the cleanest approach, but forces you to use 
// privateMethod.call(this) syntax.

// MyClass module
(typeof module !== 'undefined' ? module.exports : window).MyClass = (function() {
  
	// private
	var sayHi = function() {
    return 'Hi ' + this.name;
	};

  // define constructor
	var MyClass = function() {
		this.name = 'Alexa';
	};

	// public prototype
	MyClass.prototype.happyHi = function() {
    return sayHi.call(this) + '!!!';
	};

	// return constructor
	return MyClass;

})();


// TESTS
var a = new MyClass('Alexa'); 
console.log(a.name, 'Alexa');
console.log(a.sayHi, undefined, 'Private methods are undefined');
console.log(a.happyHi(), 'Hi Alexa!!!', 'Public methods can access private methods with the correct context');

a.name = 'Bob';
console.log(a.happyHi(), 'Hi Bob!!!', 'Methods reference the actual instance variables, not copies.');

var b = new MyClass('Anna'); 
b.name = 'Satya';
console.log(a.happyHi(), 'Hi Bob!!!', 'Instances have their own instance variables as you\'d expect.');

private-instance-methods2.js

// This is trick to enable normal this.privateMethod syntax by creating a 
// hidden context object that hosts all the public and private methods and sets
// its prototype to the actual instance. This will break in some circumstances
// where methods expect to interact witht the actual host object instead of
// a fake prototype.

// MyClass module
(typeof module !== 'undefined' ? module.exports : window).MyClass = (function() {

	var extend = function() {
		var host = arguments[0] || {};
		for(var i=1; i<arguments.length; i++) {
			for(var key in arguments[i]) {
				host[key] = arguments[i][key];
			}
		}
    return host;
	};

  
	// private methods
	var priv = {
		// private function accessing instance variable
		sayHi: function() {
      return 'Hi ' + this.name;
		}
	};

	// public methods
	var pub = {
		happyHi: function() {
      return this.sayHi() + '!!!';
		}
	};


  // define constructor
	var MyClass = function(name) {
		this.name = name;

		// create a hidden context with the new instance as its prototype object
		var hiddenContext = Object.create(this);
		extend(hiddenContext, priv, pub);

		// this could be done with _.bindAll
		for(var methodName in pub) {
			this[methodName] = pub[methodName].bind(hiddenContext);
		}
	};


	// return constructor
	return MyClass;

})();

// TESTS
var a = new MyClass('Alexa'); 
console.log(a.name, 'Alexa');
console.log(a.sayHi, undefined, 'Private methods are undefined');
console.log(a.happyHi(), 'Hi Alexa!!!', 'Public methods can access private methods with the correct context');

a.name = 'Bob';
console.log(a.happyHi(), 'Hi Bob!!!', 'Methods reference the actual instance variables, not copies.');

var b = new MyClass('Anna'); 
b.name = 'Satya';
console.log(a.happyHi(), 'Hi Bob!!!', 'Instances have their own instance variables as you\'d expect.');