Javascript "for" syntax
<p>So in <a href="http://stackoverflow.com/questions/15833756/what-was-used-to-compress-this-code">this question</a> I asked about how some Javascript was compressed. The question was answered yet the following snippet confuses me so much that I had to ask another question. Here it is:</p>

<pre><code>for (Y = 0; $ = 'zxqj`_^ZWVUQONMKJIHGCA@8$  ' [Y++];) 
     with(_.split($)) _ = join(pop());
eval(_) 
</code></pre>

<p>Where does this syntax come from? I kind of understand what the third for operation is (<code>[Y++]</code>), but why is it in brackets and why isn't there any semicolon before it? Also, at line 2: it looks to me like <code>_</code> is a variable being declared, but what kind of variable type is <code>with(_.split($))</code>?</p>
