title: How to Javascript change Label value from selecting Checkbox and drop-down list
body: <p>I have 2 checkboxes and one of it contains a drop-down list with 5 elements.</p>
  
  <ul>
  <li>Checkbox A</li>
  <li>Checkbox B: drop-down list</li>
  <li>Textbox</li>
  </ul>
  
  <p>First have to check if the checbox B is checked, then Textbox only can be showing value, otherwise is empty.
  Secondly, if checkbox B checked and a value from drop-down list been selected, then the Textbox must show value immidiately on the changes.</p>
  
  <pre><code>&lt;input type="checkbox" name="events" value="event02 /&gt;
  Event 01
  &lt;/input&gt;
  &lt;br/&gt;
  &lt;input type="checkbox" name="events" value="event02 /&gt;
  Event 02:
  &lt;select name="selectOne" onchange="amount()"&gt;
      &lt;option value="00" selected="selected"&gt;00&lt;/option&gt;
      &lt;option value="01"&gt;01&lt;/option&gt;
      &lt;option value="02"&gt;02&lt;/option&gt;
      &lt;option value="03"&gt;03&lt;/option&gt;
      &lt;option value="04"&gt;04&lt;/option&gt;
      &lt;option value="05"&gt;05&lt;/option&gt;
      &lt;option value="06"&gt;06&lt;/option&gt;
      &lt;option value="07"&gt;07&lt;/option&gt;
  &lt;/select&gt;
  &lt;br/&gt;
  &lt;input type="text" name="displayTxtBox" id="displayTxtBox" size="10" maxlength="50" value="" /&gt;
  </code></pre>
  
  <p>I have no idea in Javascript how to write code detect the Checkbox B detected and Drop-down List selected only display value on Textbox.</p>
  
  <p>I think it needs onChange and some function in Javascript.</p>
  
  <p>Please kindly guide me.</p>