在iframe中获取父页面的元素

a.html

<!DOCTYPE html>
<html>
<head>
    <title></title>
</head>
<body>
<div id="a">hello world!!!</div>
<div id="b">
    <iframe id="frm" name="frm" src="b.html"></iframe>
</div>
</body>
</html>

b.html

<!DOCTYPE html>
<html>
<head>
    <title></title>
    <script type="text/javascript" src="jquery.js" ></script>
    <script type="text/javascript">
    function aa(){
        //获取本页面的元素
        //var a = document.getElementById("a12").innerHTML;
        //获取父页面的元素
        //方法一
        //var a = $(window.parent.document).find("#a").text();
        //方法二
        var a = $("#a",window.parent.document).text();
        alert(a);
    }
    </script>
</head>
<body>
<div id="a12">哈哈</div>
<div>
<div id="btn" onclick="aa()">点击</div>
</div>
</body>
</html>