// Create a new anonymous function, to use as a wrapper
  (function(){
      // The variable that would, normally, be global
      var msg = "Thanks for visiting!";
  
      // Binding a new function to a global object
      window.onunload = function(){
          // Which uses the 'hidden' variable
          alert( msg );
      };
  
  // Close off the anonymous function and execute it
  }());