I’ve had this discussion a couple of times now with several people and maybe It’s about time to make a post for it here 
The delete operator in ECMAScript-262 can only be used to delete dynamically created variables/properties. Having that said heres a code sample:
var foo = "test"; //executing this in the global window-scope will actually create window.foo and it's being considered as a dynamically created property on the window object. delete foo; //will return true, success alert(foo); //throws a javascript exception since foo is deleted, thus being undefined |
On the other hand…
(function(){ var foo ="test"; //local to the anonymous function, could as well be in a named function. delete foo; //will return false, failure to delete alert(foo); //will alert "test", delete had no affect })(); |