IsUnion

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题目

Implement a type IsUnion, which takes an input type T and returns whether T resolves to a union type.

For example:

type case1 = IsUnion<string>; // false
type case2 = IsUnion<string | number>; // true
type case3 = IsUnion<[string | number]>; // false

解答

type IsUnionImpl<T, C extends T = T> = (
  T extends T ? (C extends T ? true : unknown) : never
) extends true
  ? false
  : true;

type IsUnion<T> = IsUnionImpl<T>;

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IsUnion<string|number>
=> IsUnionImpl<string|number, string|number>
=> (string|number extends string|number ? string|number extends string|number ? true : unknown : never) extends true ? false : true
=> (
  (string extends string|number ? string|number extends string ? true : unknown : never) |
  (number extends string|number ? string|number extends number ? true : unknown : never)
) extends true ? false : true
=> (
  (string|number extends string ? true : unknown) |
  (string|number extends number ? true : unknown)
) extends true ? false : true
=> (
  (
    (string extends string ? true : unknown) |
    (number extends string ? true : unknown)
  ) |
  (
    (string extends number ? true : unknown) |
    (number extends number ? true : unknown)
  )
) extends true ? false : true
=> (
  (
    (true) |
    (unknown)
  ) |
  (
    (unknown) |
    (true)
  )
) extends true ? false : true
=> (true|unknown) extends true ? false : true
=> (unknown) extends true ? false : true
=> true