为什么这个代码一提交就卡了

//前一个节点

    function get_previousSibling(n){

        var a=n.previousSibling;

        while (a && a.nodeType!=1){

            a=n.previousSibling;

        }

        return a;

    }

    var a = document.getElementsByTagName("li")[4];

    document.write(a.nodeName);

    document.write(" = ");

    document.write(a.innerHTML);

    var b = get_previousSibling(a);

    if(b!=null){

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

        document.write(b.nodeName);

        document.write(" = ");

        document.write(b.innerHTML+'<br>');

    }