<script language="javascript">
    function checkedNull()
    {
        if (fm.ch.value=="")
        {
            alert("警告");
            return false;
        }
        else
        {
            return true;
        }
    }
</script>
<form name="fm" method="post" onsubmit="javascript:return checkedNull()">
<input type=text name="ch">
<input type=submit onclick="this.form.action='a.asp';" value="确定">
<input type="button" onclick="javascript:return checkedNull()" value="试用">
</form>