title: Interactive HTML Form Using Javascript
body: <p>How can I make interactive HTML form, example :</p>
  
  <pre><code>&lt;select name="command"&gt;
      &lt;option value="send"&gt;Send&lt;/option&gt;
      &lt;option value="cancel"&gt;Cancel&lt;/option&gt;
  &lt;/select&gt;
  
  &lt;!-- 
      I want the following text input hidden by default,
      but active only if option "Cancel" is selected
  --&gt;
  &lt;input type="text" name="cancel_reason" id="needreason"&gt;
  </code></pre>
  
  <p>I want the "cancel_reason" input field is hidden by default, and shown if dropdown-option "Cancel" is selected before, otherwise it should remain hidden.</p>