using closure.

NumberCount.example.js

var c = new Counter();

c.add();	// 1
c.get();	// 1
c.add();	// 2

var n;
n = c + 1;	// n == 3

NumberCounter.js

var Counter = function(){
	var count = 0;
	
	var add = function(){
		count += 1;
		return getCount();
	};
	
	var get = function(){
		return getCount();
	};
	
	var getCount = function(){
		return count ? count : 0;
	};
	
	return function(){
		this.add = add;
		this.get = get;
		this.valueOf = get;
	};
}();