Puoi usare
className (N maiuscola)

codice:
<div id="foo" class="fooclass"></div>

<script>
alert(document.getElementById('foo').className)
document.getElementById('foo').className='nuovaclasse';
alert(document.getElementById('foo').className)
</script>
Oppure
codice:
<input type="text" id="foo" value="ciao" class="fooclass" />

prova

<script>
function boh(){
if(document.getElementById('foo').className=='fooclass'){document.getElementById('foo').value='arrivederci';}
return false;
}
</script>