Code:
  somevar = 5;
  delete window.somevar;
  $H.displayln(typeof somevar) //=> somevar is deleted!

  var someother = 5;
  delete window.someother;
  $H.displayln(typeof someother) //=> number, someother is NOT deleted

  // first block can also be written as:
  with (this){//this === window
    somemore = 5;
  }
  delete somemore;
  $H.displayln('typeof somemore: '+typeof somemore); //=> undefined, deleted

  // this is equivalent to the second block: 
  with (this){ //this === window
    var somemore2 = 5;
  }
  delete somemore2;
  $H.displayln('typeof somemore2: '+typeof somemore2); //=> number, NOT deleted

  // from within a function: same behavior
  (function(){
    somemore3 = 5;
  })();
  delete somemore3;
  $H.displayln('typeof somemore3: '+typeof somemore3); //=> number!