title: jQuery/Javascript Invalid left-hand side in assignment
body: <p>I am using this relatively simple code:</p>
  
  <pre><code>var height = help ? 'minus' : 'plus';
  var prop = $('#properties');
  
  if(height == 'minus'){
      prop.height(prop.height() -= 206);
  } else {
      prop.height(prop.height() += 206);
  }
  </code></pre>
  
  <p>It fails on both lines that do the adding/subtracting! Any ideas?</p>