Get Elements By Class Name

Javascript Code

<script>
var getByClassName = function(className, parentElement) {
  if (typeof parentElement == 'string'){
    parentElement = document.getElementById(parentElement);
  } else if (typeof parentElement != 'object' || typeof parentElement.tagName != 'string') {
    parentElement = document.body;
  }
  
  var children = parentElement.getElementsByTagName('*');
  var re = new RegExp('\\b' + className + '\\b');
  var element, elements = [];
  var i = 0;
  while ( (element = children[i++]) ){
    if ( ellement.className && re.test(element.className)){
      elements.push(element);
    }
  }
  return elements;
}
<script>

Markup

	<div id="some_id">
		<div class="first">Message 1</div>
		<div class="first">Message 2</div>
	</div>

Usage:

var divs1 = getByClassName('first');
var divs2 = getByClassName('first','some_id');
var divs3 = getByClassName('first',document.getElementById('some_id'));