js为元素动态增加class样式

div{ border:1px solid #ccc; width:200px; height:200px;} 
.a{ background:#900; } 
.b{ font-size:30px; font-weight:bold;} 

function addClass(elements, value) 

{ 
if (!elements.className) { 
elements.className = value; 
} 
else 
{ 
newClass = elements.className; 
newClass += " "; 
newClass += value; 
elements.className = newClass; 
} 
} 
window.onload = function () 
{ 
var test = document.getElementById('test'); 
alert(test.className); 
addClass(test, 'a b'); 
//addClass(test, 'b'); 

} 

<div id="test">这里是测试层</div>