Avoid using delete which can cause the backing JIT class to mutate

[Xon]

Mostly minor, as the flags aren't mutated that often.

[slact]

Can you link me to a reference for this optimization?

[Xon]

In a loop, there is a noticeable performance difference; https://jsperf.com/delete-vs-undefined-vs-null/16

A V8 developer mentions using delete will likely be slow; https://stackoverflow.com/questions/43594092/slow-delete-of-object-properties-in-js-in-v8 https://groups.google.com/forum/#!topic/v8-users/zE4cOHBkAnY

[slact]

Well, none of these deletes are in any loops, so this won't affect performance. But you're right, it does appear to be best practice to avoid delete if possible. Merge approved.