var box = document.getElementById("box")<html>
    <head>
        <meta charset=utf-8>
    </head>
    <body>
        <div class='box'></div>
    </body>
    <script type="text/javascript">
        function modifyText(){
            // 补全代码
            document.getElementsByClassName('box')[0].innerHTML='欢迎来到牛客网'
        }
    </script>
</html>