// "class" Square (capitalize by convention)
function Square(side_length)  {
  this.side = side_length;    // "instance variable"
  this.area = function() {    // "instance method"
     return this.side*this.side;
  }
}
// "instance"
var someSquare = new Square(3);

// better, since keeps instance methods separate and avoids having N copies of them
function Square(side_length) {
  this.side = side_length;
}
// .prototype is a special property defined only on functions.  The 'new'
// constructor sets __proto__ property of new object from .prototype's value.
// Also, __proto__ is not writable in IE's implementation of JScript.
Square.prototype.area = function() {
  return this.side * this.side;
}