isFinite returns true when variable is undefined
<p>I'm using isFinite to determine if a key from an array is correct,</p>

<pre><code>for (x in selectList) {
        if (isFinite(x)) {
             $('#' + selectList[x])[0].selectedIndex = 0;
        }
}
</code></pre>

<p>I thought this was working correctly but now in Firefox isFinite is returning TRUE when x is  undefined. This doesn't seem right to me. Is this a bug?</p>
