function get_perviousSibling(m){

var si=m.perviousSibling;

while (si && si.nodeType !=1){

si=si.perviousSibling;

}return si;

}

var si=document.getElementsByTagName("li")[1];

document.write("<br>"+si.nodeName);

document.write("=");

document.write(si.innerHTML);

var zi=get_perviousSibling(si);//这里是调用函数，将上面输出的节点属性进行判断

if(zi !=null){   

document.write("<br/>previousSbiling: ");

document.write(zi.nodeName);

document.write("=");

document.write(zi.innerHTML);

}else{

document.write("<br>已经是第一个节点");

}