Define Object Methods inside class vs. added to prototype

oo.js

/**
*  Define a method inside of a JavaScript Object.  In this instance, the method gets
*  recreated with each Class Instance.  Sometimes desirable, sometimes not.
*/
function Wizard () {
  this.name = "The Wiz";
  this.level = 1;
  this.shoot = function () {
    alert('Imma Shoot You!');
  };
}

/**
*  Alternatively, you can add a method to the object's Prototype.  This is an
*  more inexpensive way to add a method as the method gets added only once
*  to the protype of the object's constructor function.  The method will
*  still be available to all class instances.
*/
function Wizard () {
  this.name = "The Wiz";
  this.level = 1;
}

Wizard.prototype.shoot = function () {
  alert("Imma Shoot You!");
};

When would the first example be desirable over the prototype approach?