What does [literal] in [array/object] mean?
<p>What does the following syntax mean:</p>

<pre><code>1 in [1,2,3,5]
</code></pre>

<p>I know it doesn't search for 1 in the array. But what does it do? </p>

<p>I've seen it used in loops:</p>

<pre><code>for (var i in testArray)
{

}
</code></pre>

<p>But have also seen this used by itself. Does it mean check if the literal is a valid index in the array or object that is the other operand?</p>
