DOM事件探秘-QQ拖曳效果

function getByName(clsName,parent) {

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

      eles = [],

      elements = oParent.getElementsByTagName('*');

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

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

        eles.push(elements[i]);

      }

      return eles;

  }

}

window.onload = drag;

function drag() {

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

  oTitle.onmousedown = fnDown;

}

function fnDown() {

  var oDrag = document.getElementById('loginPanel')

  document.onmousemove = function(event){

    event = event || window.event;

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

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

  }

}

DOM时间探秘里边照着视频打的，对了一遍没找到问题……控制台报错Cannot set property 'onmousedown' of undefined，不知道啥原因……望解答，谢谢！