title: Javascript: Making a textbox display if a dropdown option is selected
body: <p>Here's a screenshot of what I'm trying to accomplish:
  <img src="https://i.stack.imgur.com/iszN9.jpg" alt="enter image description here"></p>
  
  <p>Basically, if the user chooses "SFTP", a textbox should be displayed.</p>
  
  <p>Here's the code I have:</p>
  
  <pre><code>&lt;strong class="heading"&gt;Image Hosting&lt;/strong&gt;
  &lt;div id="imagehosting"&gt;
      &lt;form name="test"&gt;
          Does the client require our company to host their images? &lt;br /&gt;
          &lt;em&gt;If you select this option, you'll have to specify what they will be using&lt;/em&gt;
          &lt;input type="checkbox" name="category_level" onchange="categorychanged(this.checked)" /&gt;
  
          &lt;select name="category_parent" style="display:none" onchange="if(this.selectedIndex==SFTP){this.form['box'].style.visibility='visible'}else {this.form['box'].style.visibility='hidden'};"&gt;
              &lt;option value="1"&gt;Library&lt;/option&gt;
              &lt;option value="2"&gt;SFTP&lt;/option&gt;
          &lt;/select&gt;
          &lt;input style="visibility:hidden;" type="text" name="box"&gt;
      &lt;/form&gt;
  &lt;br /&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>The function, "categorychanged", is handled by the following Javascript code:</p>
  
  <pre><code>&lt;script language="JavaScript" type="text/javascript"&gt;
  &lt;!--
  function categorychanged(enable) 
  {
      if (enable) 
      {
          document.test.category_parent.style.display="block";
      }
      else 
      {
          document.test.category_parent.style.display="none";
      }
  }
  //--&gt;
  &lt;/script&gt;
  </code></pre>
  
  <p>Right now, if I choose "SFTP", nothing happens. The first part works fine, i.e. dropdown displayed when the checkbox is selected.</p>
  
  <p>What am I doing wrong here?
  Thanks</p>