gistfile1.js

function Foo() {
	var my_private;	

	this.get_my_private = function() { return my_private; }
	this.set_my_private = function(newval) { my_private = newval; }
}

x = new Foo();

x.set_my_private("this is a test private value");
x.get_my_private();
x.my_private; // this is undefined