document.getElementsByClassName returns an array of elements that have the given CSS class.

Syntax

var elementList = document.getElementsByClassName("classname");

Example

<div class="special">first element</div>
<div>second element</div>
<div class="special">third element</div>

<a href="#" onclick="alert('elements with classname special: ' + document.getElementsByClassName('special').length);">Count elements</a>