Why is Javascript's "in" operator consistently slower than strict member comparison to undefined?
<p>See <a href="http://jsperf.com/in-vs-member-object-access" rel="nofollow">http://jsperf.com/in-vs-member-object-access</a></p>

<p>Essentially, why is checking <code>if ('bar' in foo) {}</code> significantly slower than <code>if (foo.bar !== undefined) {}</code>?</p>
