老师看下哪里不对

来源：3-16 编程练习

一心励志当码农

2021-05-10 10:59:41

<!DOCTYPE html>
<html>

<head lang="en">
    <meta charset="UTF-8">
    <title>toUpperCase和toLowerCase</title>
</head>

<body>
    <script>
        var str = "Never deter till tomorrow that which you can do today";
        var index = str.indexOf('tomorrow');//补充代码
        var strNew = str.substr(index,8);//补充代码
        var upperStr =strNew.toUpperCase;//补充代码
        document.write(str.substring(0, index) + upperStr + str.substr(index + 8));
    </script>
</body>

</html>