class in javascript

My Code:

function MyClass(){}

a = new MyClass();
a.text = "The sum is "
a.num1 = 10;
a.num2 = 20;

b = new MyClass();
b.firstname = "Jason";
b.lastname = "Bourne";

function X(p){
    if(p.num1!=null)
        alert(p.text +(p.num1 + p.num2));
    else
        alert(b.firstname+" "+b.lastname);    
}

X(a);
X(b);