关于javascript中event is not defined 异常的疑问

关于javascript中event is not defined 错误的疑问？
这个就是表格移上去当前行高亮显示的效果。

效果可以实现，但是firebug老报错。

JScript code

<script>var highlightcolor = '#c1ebff';var clickcolor = '#51b2f6';function changeto() {    source = event.srcElement;    if (source.tagName == "TR" || source.tagName == "TABLE")        return;    while (source.tagName != "TD")        source = source.parentElement;    source = source.parentElement;    cs = source.children;    //alert(cs.length);    if (cs[1].style.backgroundColor != highlightcolor && source.id != "nc"            && cs[1].style.backgroundColor != clickcolor)        for (i = 0; i < cs.length; i++) {            cs[i].style.backgroundColor = highlightcolor;        }}function changeback() {    if (event.fromElement.contains(event.toElement)            || source.contains(event.toElement) || source.id == "nc")        returnif  (event.toElement!=source&&cs[1].style.backgroundColor!=clickcolor)//source.style.backgroundColor=originalcolorfor(i=0;i<cs.length;i++){    cs[i].style.backgroundColor="";}}function  clickto(){source=event.srcElement;if  (source.tagName=="TR"||source.tagName=="TABLE")return;while(source.tagName!="TD")source=source.parentElement;source=source.parentElement;cs  =  source.children;//alert(cs.length);if  (cs[1].style.backgroundColor!=clickcolor&&source.id!="nc")for(i=0;i<cs.length;i++){    cs[i].style.backgroundColor=clickcolor;}elsefor(i=0;i<cs.length;i++){    cs[i].style.backgroundColor="";}}</script>