为啥我的className 报错是没有定义的

function getByClass(clsName,parent) {

var oParent=parent?document.getElementById(parent):document,

eles=[],

elements=oParent.getElementsByTagName('*');

for(var i=0;l=elements.length;i++){

if(elements[i].className==clsName){

eles.push(elements[i]);

}

}

return eles;

}

window.onload=drag;

function drag() {

var oTitle=getByClass('login_logo_webqq','loginPanel')[0];

oTitle.onmosedown=fnDown;

}

function fnDown(){

var oDrag=document.getElementById('loginPanel')

document.onmousemove=function(event){

event=event||window.event;

document.title=event.clientX+','+event.clientY;

oDrag.style.left=event.clientX+'px';

oDrag.style.top=event.clientY+'px';

}

}