它只保存一段数据：一个字符串、一个数字、未定义的值以及对另一个变量的引用。
They hold only one piece of data: a string, a number, the undefined value, a reference to another variable.
它还让用户可以显示应用程序中任意未定义的值。
It also allows the user to display any undefined values in the application.
该函数也会正确地处理未定义的值，尽管在警告打开时，常规表达式在匹配未定义的值时会输出警告信息。
It works correctly with undefined values as well, whereas with warnings turned on, a regular expression match on an undefined value will print a warning.
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