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1097 - IsUnion #26845

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smileboyi opened this issue Apr 25, 2023 · 0 comments

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smileboyi commented Apr 25, 2023 •

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type IsUnion<T, U = T> = (T extends T ? U extends T ? true : unknown : never) extends true ? false : true

思路：

T extends T可以判断T是否为never；
U extends T，extends可以拆分联合类型分别判断，再将每个子结果再联合起来；
如果T是联合类型，那么子结果就会含有unknown ，括号最终的结果是一个含有unknown的联合类型；
Union extends true，Union含有unknown就是一个联合类型，只含有never或true就不是一个联合类型，可以通过extends true来判断。

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