jquery/jquery — comment on commit 3ad0ba62f03ff3d7aec7d3dfb25492cdf33350a7
File:  line 0
By mikesherov:

@Krinkle, using strict comparison causes the function to be executed in IE... thus causing side effects. The fact that certain other parts of jQuery core may or may not conform to such standards (although the line you pointed do isn't an example of inconsistency in this case, it just looks like one) isn't really a good argument.

The three options are:
1. use strict comparison, and get unnecessary side effects in IE.
2. use "in", which is ridiculously slow.
3. use typeof !== "undefined"

3 is the clear winner here.