gistfile1.txt

function Base(x) {
  // override properties in prototype if necessary
  if (arguments.length == 1) {
    this._x = x;
  }
}

Base.prototype = {
  _x: 1
};

function Derived(x) {
  // override properties in prototype if necessary
  if (arguments.length == 1) {
    Base.call(this, x);
  }
}

Derived.prototype = new Base(3); // default value of _x is 3 for Derived