title: Select element by and classname in javascript
body: <p>I have an onChange event that needs to dynamically select an ID and then add a preset classname to pass to a function.</p>
  
  <pre><code>onChange = "show(document.getElementById(this.value). {select a class here? } );"
  </code></pre>
  
  <p>The equivalent in jquery</p>
  
  <pre><code>$('#'+this.value+'.myclassname').function();
  </code></pre>
  
  <p>So how do I select an ID+classname in javascript?  I know I'm being dense.</p>