Is undefined a property of window/global?
<p>It seems that the <strong>undefined</strong> is a property of window/global :</p>

<p><img src="http://i.stack.imgur.com/WBMku.jpg" alt="enter image description here"></p>

<p>I always thought that <strong>undefined</strong> is, like <strong>null</strong>, a uniqe value in JavaScript.</p>

<p>But above code (<em>tested in Chrome</em>) make me confused.</p>

<p>Can some explain why</p>

<pre><code>undefined in window
</code></pre>

<p>evalute to true, while</p>

<pre><code>null in window
</code></pre>

<p>evaluate to false</p>
