Simulate getter in JavaScript by valueOf and toString method

function Foo(a, b) {

    this.a = a;

    this.b = b;



    // simulate getter via valueOf and toString method

    this.sum = {

        valueOf: function () {

            return a + b

        },

        toString: function () {

            return a + b

        }

    }

}

alert(new Foo(2, 3).sum);