Changing a large image with JavaScript when clicking a thumbnail image -

i have used following html/javascript code make webpage can change larger image clicking thumbnail image:

<script type="text/javascript">     function changeimage(a) {     document.getelementbyid("viewer").src = a;     } </script>  <section>     <img class="thumbnail" src="/images/thumbnails/formalheadshot1.jpg" onclick="changeimage("/images/formalheadshot1.jpg");">     <img class="thumbnail" src="/images/thumbnails/formalheadshot2.jpg" onclick="changeimage("/images/formalheadshot2.jpg");">     <img class="thumbnail" src="/images/thumbnails/performing.jpg" onclick="changeimage("/images/performing.jpg");">     <img class="thumbnail" src="/images/thumbnails/playing.jpg" onclick="changeimage("/images/playing.jpg");">     <img class="thumbnail" src="/images/thumbnails/informalheadshot.jpg" onclick="changeimage("/images/informalheadshot.jpg");">      <br>      <img id="viewer" src="/images/formalheadshot1.jpg">  </section> 

any idea why when click thumbnail images nothing happens large picture? kinda new javascript, may missing obvious, research i've done hasn't seemed solve problem.

try putting js code below html.