calculator.js

var calculator = (function () {
  var result = 0;
  var calculator = {
    add: function (n) {
      if (typeof n === "number") {
        result += n;
      }
      return calculator;
    },
    subtract: function (n) {
      if (typeof n === "number") {
        result -= n;
      }
      return calculator;
    },
    multiply: function (n) {
      if (typeof n === "number") {
        result *= n;
      }
      return calculator;
    },
    Divide: function (n) {
      if (typeof n === "number" && n == 0) {
        result /= n;
      }
      return calculator;
    },
    getResult: function () {
      return result;
    }
  };

  return calculator;
})();

var product = calculator.add(2).add(3).getResult(),
    other;

console.log( product === 5 );

other = calculator.add(2).add(3).getResult();

console.log( other === 5 ); // false!!!



// Instead, use a closed over constructor declaration to create "private" data:


(function( scope ) {
  // Map instances to values
  var map = [ [], [] ],
      operators = {
        "add": "+",
        "subtract": "-",
        "multiply": "*",
        "divide": "/"
      };

  function Calculator( n ) {
    if ( !(this instanceof Calculator) ) {
      return new Calculator( n || 0 );
    }

    var value = n != null ? n : 0;

    map[0].push(this);
    map[1].push(value);
  }

  function update(key, val) {
    map[1][ map[0].indexOf(key) ] = val;
  }

  function operate() {
    var args = [].slice.call(arguments),
    operator = args.shift();

    // Ensure all values are numbers
    args.filter(function(val) {
      return +val;
    });

    // Safely eval the operation
    return new Function("return " + args.join(operator))();
  }

  Calculator.prototype = {
    get value() {
      return map[1][ map[0].indexOf(this) ];
    }
  };

  // Define operation methods for Calculator prototype
  Object.keys( operators ).forEach(function( opKey ) {
    Calculator.prototype[ opKey ] = function( n ) {
      update( this, operate( operators[ opKey ], this.value, n ) );
      return this;
    };
  });

  scope.calc = Calculator;

})( global );


var v1 = calc(0).add(2).add(3).value,
    v2;

console.log( v1 === 5 );

v2 = calc(0).add(2).add(3).value;

console.log( v2 === 5 ); // true