Calc.js

(function(window,undefined){

  var Calc = function(){
    this.sum = 0;
    return this;
  }

  Calc.prototype.equals = function(){
    return this.sum;
  }

  Calc.prototype.add = function(n){
   this.sum = this.sum + n;
   return this;
  }

  Calc.prototype.subtract = function(n){
   this.sum = this.sum - n;
   return this;
  }

  Calc.prototype.multiply = function(n){
   this.sum = this.sum * n;
   return this;
  }

  Calc.prototype.divide = function(n){
   this.sum = this.sum / n;
   return this;
  }

  window.Calc = Calc;

})(window);