Cannot set property 'onmousedown' of undefined QQ面板拖拽请问什么原因引起的

Question

function getByClass(clsName,parent){ var oParent=parent?document.getElementById(parent):document, eles=[], elements=oParent.getElementsByTagName('*'); for(var i=0,l=elements.length;i<l;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.onmousedown=fnDown;}function fnDown(){ document.onmousemove=function(event){ event=event||window.event; document.title=event.clientX+","+event.clientY; }}