js改变或添加className

 <style type="text/css">

 .newDiv {

     font-weight: bold;

 }

 </style>

js和html

<script>

function clk() {

    var parent = document.getElementById("parent");

   // parent.className = parent.className + " newDiv";  //添加classname  既html中的div 的class变为 class = "google newDiv"

   parent.className = " newDiv";  //改变classname  既html中的div 的class变为 class = "newDiv"

}

</script>

</head>

<body>

<input type="button" value="click" onclick="clk();"/>

<div id="parent" class="google">

    <div >child</div>

</div>

</body>