function factory() {
  var count = 0;

  return function() {
    function fx() {
      count += 1;
      return fx;
    }

    fx.valueOf = function() {
      return count;
    };

    return fx();
  }
}

var f1 = factory();
console.log(f1()()); // 2
console.log(f1()()); // 4
console.log(f1()()); // 6

var f2 = factory();
console.log(f2());     // 1
console.log(f2()());   // 3
console.log(f2()()()); // 6

var f3 = factory();
console.log(f3()()()); // 3
console.log(f3()()()); // 6
console.log(f3()()()); // 9