求助代码看不懂

addstr赋值为strarr的倒数第二个元素的值

var str = document.getElementById("up").innerHTML;
var strarr = str.split("<BR>");
var addstr = strarr[strarr.length - 2];
addstr = addstr.replace("id=FileUpload" + (strarr.length - 1).toString(), "id=FileUpload" + (strarr.length).toString());
addstr = addstr.replace("name=FileUpload" + (strarr.length - 1).toString(), "name=FileUpload" + (strarr.length).toString());
document.getElementById("up").innerHTML = str + addstr + "<BR>";
}
还是看不懂~

恩谢谢大家了 刚才我忘记了设置断点了alert没执行决定义个变量打印一次了对楼上几位表示感谢