href="javascript:"和href="javascript:void(0);"

Javascript代码  

<a onclick="fn()">Does not appear as a link, because there's no href</a>
<a href="javascript:void(0)" onclick="fn()">fn is called</a>
<a href="javascript:undefined" onclick="fn()">fn is called</a>
<a href="javascript:" onclick="fn()">fn is called too!</a>  这三个是等价的。