js弹出div窗口并返回数据

直接上代码：

<style>
        *{
            margin:0;
            padding: 0;
        }
        #bg{
            width: 100%;
            height: 100%;
            z-index: 999;
            position: absolute;
            left: 0%;
            top: 0%;
            background-color: rgba(0,0,0,0.8);
            filter: alpha(opacity=80);
            display: none;
        }
        #content{

            z-index: 1000;
            width: 50%;
            height: 50%;
            position: absolute;
            left: 20%;
            top: 20%;
            background: white;
            padding: 30px;
            display: none;
        }
    </style>

<div id="bg">
</div>
<div id="content">
    <input type="text" value="" id="txt">
    <input type="submit" onclick="document.getElementById('bg').style.display='none';document.getElementById('content').style.display='none';var txt=document.getElementById('txt').value;document.getElementById('back').innerText=txt; ">
</div>
<div id="index"><a href="javascript:void(0);" onclick="document.getElementById('bg').style.display='block';document.getElementById('content').style.display='block'; ">点击弹出窗口</a></div>
<div id="back"></div>