It's a common mistake to make if you forget that I've actually declared that variable up top, but wait a minute, I know any time that I use a variable, I have to declare it.

这是一个通常犯的错误，如果你忘了,之前在前面已经声明了这个变量，等等，我知道在任何时候,我使用一个变量，我必须声明它。

哈佛公开课 - 计算机科学课程节选