5.6 文档之间共享函数

看例子是，iframe 页面通过 parent 修改其父文档的内容；
index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
</head>
<body>
<a href="index.html">点击</a>
<iframe src="iframe.html" id="myTargetId"></iframe>
</body>
</html>

iframe.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        window.onload = function () {
            parent.document.links[0].onclick = function () {
                alert("123");
            }
        };
    </script>
</head>
<body>
<button >点击</button>
</body>
</html>