What does this block of obfuscated JavaScript do? 
<p>I've seen a block of JavaScript code, that return an alert message with 1 on page load. Here is the code</p>

<pre><code>($=[$=[]][(__=!$+$)[_=-~-~-~$]+({}+$)[_/_]+
($$=($_=!''+$)[_/_]+$_[+$])])()[__[_/_]+__
[_+~$]+$_[_]+$$](_/_)    
</code></pre>

<p>Can someone please explain what's going on here?</p>
