Javascript objects properties are typically implemented with a hashtable. Setting to null leaves the key in the hashtable pointing to a null value, while delete eliminates both the key and the value.
The main observable difference between the two is that if you iterate over the keys with a for..in loop, deleting keys results in fewer entries seen by the iteration.
I would suggest preferring deletion in general, unless you are going to be repeatedly setting and clearing the same keys, which would argue for leaving the hash table structure in place. However, any performance difference between the two techniques is going to be immeasurably small in any typical case.
-m@
