javascript:; 和javascript:void(0);
javascript:; 和javascript:void(0);1、“javascript:void(0);”要去执行一次表达式“0”，然后返回undefined。
2、“javascript:;”直接返回undefined。
3、例如：
<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>
上面三个都是等价的