Why is this JSON variable undefined? [closed]
<script>
function postUser() {
    var user = $('span input').serialize;
    alert(user.username);
}
</script>

<span>
  User name : <input type="text" name="username"><br />
  Password: <input type="password" name="password" /><br />
  First name: <input type="text" name="firstName" /><br />
  Last name: <input type="text" name="lastName" /><br />

  <button onclick="postUser()">Submit</button>
</span>

var user = $('span input').serialize();

var username = $('span input[name=username]').val();

var vals = {};

$('span input').each(function() {
    vals[this.name] = this.value;
});

var name = vals.username;

alert($('input[name="username"]').val());

<span>
  User name : <input type="text" name="username"><br />
  Password: <input type="password" name="password" /><br />
  First name: <input type="text" name="firstName" /><br />
  Last name: <input type="text" name="lastName" /><br />

  <button id="btn" onclick="postUser()">Submit</button>
</span>​

$("#btn").click(function(){
   alert($('input[name="username"]').val());
 });