为什么无法显示alert?
<html>
<script language="javascript">
var a=document.form1.username.value;
alert("aa");
</script>
<body>
<form name="form1">
<table>
<tr>
<td>用户名</td><td><input type="text" name="username" value="nimu"/></td></tr>
<tr><td>密码</td><td><input type="password" name="password" value="nihao" id="ps"/></td></tr>
<tr><td><input type="submit" value="提交" onsubmit="alert("year")"/></td></tr>
</table>
</form>
</body>
</html>
我把var a=document.form1.username.value;去掉就可以显示了为什么?