Использование Prototype

var Calculator = function (eq) {
            this.eqCtl = document.getElementById(eq);
        };
        // using JSON
        Calculator.prototype =
        {
            add: function (x, y) {
                var val = x + y;
                this.eqCtl.innerHTML = val;
            },
            sub: function (x, y) {
                var val = x - y;
                this.eqCtl.innerHTML = val;
            }
 
 
        }
 
        $(function () {
            var c = new Calculator("eq");
            c.sub(2,4);
        });