Question

I’ve been trying to understand Samy Kamkar’s evercookie code and he does at least one thing I simply don’t understand. The relevant code can be seen online at http://code.google.com/p/beef/source/browse/trunk/modules/beefjs/lib/evercookie.js?spec=svn604&r=604.

My question is about how this code sets evercookie itself. The relevant construct:

var evercookie = (function () {
  this._class = function() { 
      ... 
  }; 
  return _class;
})();

I understand that evercookie is intended to be a constructor (despite not following the convention of capitalizing constructors) and I understand that evercookie is being set to the return of an anonymous, self-executing function. Unless I am more confused than I think I am, evercookie should be set to the value _class has in the scope of the outer (anonymous) function. What I don’t follow: where does _class get a value? _class is not the same thing as this._class, and there is no other reference to _class.

Answer 1

In that function _class is the same as this._class because this references the window object in the automatically executed function. It’s essentially making the _class function a global variable.

Here’s the basic idea: http://jsfiddle.net/hYQab/