Private variables in ES5. Technique adapted from http://fitzgeraldnick.com/weblog/53/

SomeClass.js

var SomeClass = (function() {
	// We'll store in an object to be as light as possible
	var privates = {};

	// Each instance will get its own ID
	var nextId = 0;

	function SomeClass() {
		// Store a non-writable property called _
		// This will contain our instance's ID
		Object.defineProperty(this, '_', {
			value: nextId++,
			writable: false
		});

		privates[this._] = {
			// Private stuff here
			privateString: 'This string is private!'
		};
	}

	SomeClass.prototype.destruct = function() {
		// Cleanup after ourselves
		delete privates[this._];
	};

	SomeClass.prototype.someMethod = function() {
		// Get a private variable
		var privateString = privates[this._].privateString;
		console.log(privateString);
	};

	return SomeClass;
}());