title: Add css class using document.getElementsByClassName
body: <p>I am trying to add css class using javascript but its not working   </p>
  
  <pre><code>var x = document.getElementsByClassName('oldclassname');
  var i;
  for (i = 0; i &lt; x.length; i++) {
    x[i].className += 'newclassname';
  }
  </code></pre>
  
  <p>but when I tried changing background it works</p>
  
  <pre><code>var x = document.getElementsByClassName("oldclassname");
  var i;
  for (i = 0; i &lt; x.length; i++) {
  
    x[i].style.backgroundColor = "red";
  }
  </code></pre>
  
  <p>Am I doing anything wrong while adding css file</p>