在页面的input框中输入用户名，为什么username获取不到值？
下面这种写法，username不是一个全局变量吗？
但是把username移入到click事件中就可以获取到值。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>尚硅谷会员注册页面</title>
<link type="text/css" rel="stylesheet" href="../../static/css/style.css" >
<script type="text/javascript" src="../../static/script/jquery-1.7.2.js"></script>
<style type="text/css">
.login_form{
height:420px;
margin-top: 25px;
}
</style>
<script type="text/javascript">
$(function () {
var username = $("#username").val();
$("#sub_btn").click(function () {
alert(username);
});
});
</script>
</head>
<body>
<div id="login_header">
<img class="logo_img" alt="" src="../../static/img/logo.gif" >
</div>
<div class="login_banner">
<div id="l_content">
<span class="login_word">注册</span>
</div>
<div id="content">
<div class="login_form">
<div class="login_box">
<div class="tit">
<h1>注册尚硅谷会员</h1>
<span class="errorMsg"></span>
</div>
<div class="form">
<form action="regist_success.html">
<label>用户名称：</label>
<input class="itxt" type="text" placeholder="请输入用户名" autocomplete="off" tabindex="1" name="username" id="username" />
<br />
<br />
<label>用户密码：</label>
<input class="itxt" type="password" placeholder="请输入密码" autocomplete="off" tabindex="1" name="password" id="password" />
<br />
<br />
<label>确认密码：</label>
<input class="itxt" type="password" placeholder="确认密码" autocomplete="off" tabindex="1" name="repwd" id="repwd" />
<br />
<br />
<label>电子邮件：</label>
<input class="itxt" type="text" placeholder="请输入邮箱地址" autocomplete="off" tabindex="1" name="email" id="email" />
<br />
<br />
<label>验证码：</label>
<input class="itxt" type="text" style="width: 150px;" id="code"/>
<img alt="" src="../../static/img/code.bmp" style="float: right; margin-right: 40px">
<br />
<br />
<input type="submit" value="注册" id="sub_btn" />
</form>
</div>
</div>
</div>
</div>
</div>
<div id="bottom">
<span>
尚硅谷书城.Copyright ©2015
</span>
</div>
</body>
</html>
