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!