第2个回答 推荐于2016-10-17
<form name="free"method="POST"action=""onsubmit="return check()">
<p align="center">
<input type="text"name="t1"size="20">
<input type="submit"value="提交"name="B1">
</p>
</form>
<script LANGUAGE="javascript">
function check()
{
if(document.free.t1.value.length==0){
alert("输入框不能为空!");
document.free.t1.focus();
return false;
}
}
</script>本回答被提问者和网友采纳