‘document.getElementById（...）'为空或不是对象。但是又确实存在这些对象，而且ID都是对的
<script type="text/javascript">
var flag=Request("y_name");
if(flag!=null && flag!="")
{
if(flag=="cl")
{
document.getElementById("clxt").className="affairlab1";
document.getElementById("gdxt").className="affairlab2";
document.getElementById("zhjkxt").className="affairlab2";
document.getElementById("gdaoxt").className="affairlab2";
document.getElementById("pbmxt").className="affairlab2";
document.getElementById("sjpxt").className="affairlab2";
document.getElementById("sdtfxt").className="affairlab2";
document.getElementById("xhxt").className="affairlab2";
document.getElementById("txxt").className="affairlab2";
document.getElementById("dtxt").className="affairlab2";
document.getElementById("zdftxt").className="affairlab2";

}
else if(flag=="gd")
{
document.getElementById("clxt").className="affairlab2";
document.getElementById("gdxt").className="affairlab1";
document.getElementById("zhjkxt").className="affairlab2";
document.getElementById("gdaoxt").className="affairlab2";
document.getElementById("pbmxt").className="affairlab2";
document.getElementById("sjpxt").className="affairlab2";
document.getElementById("sdtfxt").className="affairlab2";
document.getElementById("xhxt").className="affairlab2";
document.getElementById("txxt").className="affairlab2";
document.getElementById("dtxt").className="affairlab2";
document.getElementById("zdftxt").className="affairlab2";

}
else if(flag=="pbm")
{
document.getElementById("clxt").className="affairlab2";
document.getElementById("gdxt").className="affairlab2";
document.getElementById("zhjkxt").className="affairlab2";
document.getElementById("gdaoxt").className="affairlab2";
document.getElementById("pbmxt").className="affairlab1";
document.getElementById("sjpxt").className="affairlab2";
document.getElementById("sdtfxt").className="affairlab2";
document.getElementById("xhxt").className="affairlab2";
document.getElementById("txxt").className="affairlab2";
document.getElementById("dtxt").className="affairlab2";
document.getElementById("zdftxt").className="affairlab2";

}
else if(flag=="sjp")
{
document.getElementById("clxt").className="affairlab2";
document.getElementById("gdxt").className="affairlab2";
document.getElementById("zhjkxt").className="affairlab2";
document.getElementById("gdaoxt").className="affairlab2";

document.getElementById("pbmxt").className="affairlab2";
document.getElementById("sjpxt").className="affairlab1";
document.getElementById("sdtfxt").className="affairlab2";
document.getElementById("xhxt").className="affairlab2";
document.getElementById("txxt").className="affairlab2";
document.getElementById("dtxt").className="affairlab2";
document.getElementById("zdftxt").className="affairlab2";
}
</script>
我加了这段代码后 在页面的左下角 就报 错误‘document.getElementById（...）'为空或不是对象。但是又确实存在这些对象，而且ID都是对的

[解决办法]
把这段代码放到最后
或
<script type="text/javascript" defer>