不用再检查默认的复合值，您也可以捕获对未定义变量的访问。

Instead of checking for a default compound value, you can also catch the access to an undefined variable. E.g.

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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.

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如果代码的改变在被重构的方法内部引发了问题——变量未定义，或者类型不匹配——重构操作将对这些问题进行标记。

If the changes cause problems in the method being refactored — because it leaves undefined variables or mismatched types — the refactoring operations will flag these.

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