ES4 Contextually Reserved Words
ECMAScript 4 introduces a new type of reserved words: ones that are only contextually reserved. That is, they’re only reserved at certain times and in certain places. For best results, though, we recommend that you not use the following words as variables names.
The good news, though, so far as ECMAScript 4 is concerned: they’ve gotten rid of the concept of future reserved words. Barring any changes (unlikely though that is), there shouldn’t be any more additions to this list in the future.