javascript页面验证数据的合法性

<script language="javascript">
	function on_submit() { //验证数据的合法性
		if (form1.username.value == "") {
			alert("用户名不能为空，请输入用户名！");
			return false;
		}
		if (form1.userpassword.value == "") {
			alert("用户密码不能为空，请输入密码！");
			form1.userpassword.focus();
			return false;
		}
		if (form1.reuserpassword.value == "") {
			alert("用户确认密码不能为空，请输入密码！");
			form1.reuserpassword.focus();
			return false;
		}
		if (form1.userpassword.value != form1.reuserpassword.value) {
			alert("密码与确认密码不同");
			form1.userpassword.focus();
			return false;
		}
		if (form1.email.value.length != 0) {
			for (i = 0; i < form1.email.value.length; i++)
				if (form1.email.value.charAt(i) == "@")
					break;
			if (i == form1.email.value.length) {
				alert("非法EMail地址！");
				form1.email.focus();
				return false;
			}
		} else {
			alert("请输入Email！");
			form1.email.focus();
			return false;

		}
	}
</script>