title: jQuery if do nothing else do something
body: <p>I have a snippit to alter the <code>margin-left</code> of a node based on a click:</p>
  
  <pre><code>$("#left").click(function(){
      var slidewidth = $('#slide_box').attr('width'),
          slideleft = $('#slide_box').attr('margin-left');
      if (slideleft == 0) {
      }
      else {
          $("#slide_box").animate({"margin-left": "+=135px"}, "fast");
      } 
  });
  </code></pre>
  
  <p>Now, if <code>slideleft == 0</code>, it shouldn't do anything.  But it does.  However, when the <code>var slidewidth</code> is calculated, will it return <code>0</code> or <code>0px</code>?  How would I define that value in the <code>if</code> argument?  When I tried <code>if (slideleft == 0px)</code> it just killed the script.</p>