请老师检查下代码，谢谢。

来源：3-16 编程练习

不厌_

2021-06-03 22:47:41

相关代码：

<!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');
    var strNew=str.substr(index,8);
    var upperStr=str.substring(0,index) + strNew.toUpperCase() +str.slice(index+8);
    console.log(upperStr)
    document.write(upperStr)
</script>
</body>
</html>