关于模态层中数值读取的问题

我想问，为什么每次弹出的a的值都是空的，就是无法从模态层中获取text中的值，请问该
怎么解决。。
<!doctype html>

Modal Login Window Demo

Welcome to the Site!

Just click the login link below to expand the modal window. This is only a demo so the input form will not load anything, but it is easy to connect into a backend system.

Modal Login

用户登录

用户名

  <div class="center"><input type="button" name="loginbtn" id="loginbtn" class="flatbtn-blu hidemodal" value="确定" tabindex="3" onclick="javascript:hah();"></div>
</form>

$(function(){ $('#loginform').submit(function(e){ return false; }); $('#modaltrigger').leanModal({ top: 110, overlay: 0.45, closeButton: ".hidemodal" }); }); function hah(){ var a=document.getElementById("username_1"); alert(a); }