Nodejs inheritance

adder.js

/* main class */
var Adder = function(a, b){
    this.a = a || 1;
    this.b = b || 2;
};

Adder.prototype.add = function() {
    return this.a + this.b;
};

/* class inheritance */
var Adder2 = function() {
    Adder2.super_.apply(this, arguments);
    this.b = 4;
};

require('util').inherits(Adder2, Adder);

/* usage */
var adder = new Adder2(5);
console.log(adder.add());