title: change background color if elements are empty with jquery
body: <p>I have 2 textboxs, 1 select and 1 file.</p>
  
  <p>How can I chage their background color with jquery if they are empty.</p>
  
  <p>I want to check all of them with a for loop.</p>
  
  <pre><code>&lt;input type="text"&gt;
  &lt;input type="text"&gt;
  
  &lt;select&gt;
      &lt;option&gt;&lt;/option&gt;
      &lt;option&gt;A&lt;/option&gt;
      &lt;option&gt;B&lt;/option&gt;
      &lt;option&gt;C&lt;/option&gt;
  &lt;/select&gt;
  
  &lt;input type="file"&gt;
  </code></pre>