js如何实现控制 input 不让空值成功提交

<script language="javascript">
function test(){
if (document.form1.username.value==""){
alert("请输入用户名！");
document.form1.username.focus();
return false;
}
if(document.form1.username1.value==""){
alert("请输入密码！");
document.form1.username1.focus();
return false;
}
}
</script>

<div class="user">
<td>用 户 名：</td><input name="username" id="username" type="text" value="" />
</div>
<div id="user_f" style="display:none">请填写用户名</div>
<div class="pwd" onclick="if(document.getElementById('username').value==''){document.getElementById('user_f').style.display='';}">
<td>密码</td><input name="username1" id="username1" type="text" value="" />
</div>本回答被提问者采纳