<html>
<body>
<div class="foo">Check developer tool's console!</div>
<pre>console.log(document.getElementsByClassName('foo')[0].class);</pre>
<p>じゃなくて</p>
<pre>console.log(document.getElementsByClassName('foo')[0].className);</pre>
<p>という話</p>
<script>
var e = document.getElementsByClassName('foo')[0]
console.log(e.className);
e.className = 'bar';
</script>
</body>
</html>