1097 - IsUnion

1097 - IsUnion

by null (@bencor) #中等 #union

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

在 Github 上查看：https://tsch.js.org/1097/zh-CN

代码

/* _____________ 你的代码 _____________ */

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

测试用例

/* _____________ 测试用例 _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<IsUnion<string>, false>>,
  Expect<Equal<IsUnion<string | number>, true>>,
  Expect<Equal<IsUnion<'a' | 'b' | 'c' | 'd'>, true>>,
  Expect<Equal<IsUnion<undefined | null | void | ''>, true>>,
  Expect<Equal<IsUnion<{ a: string } | { a: number }>, true>>,
  Expect<Equal<IsUnion<{ a: string | number }>, false>>,
  Expect<Equal<IsUnion<[string | number]>, false>>,
  // Cases where T resolves to a non-union type.
  Expect<Equal<IsUnion<string | never>, false>>,
  Expect<Equal<IsUnion<string | unknown>, false>>,
  Expect<Equal<IsUnion<string | any>, false>>,
  Expect<Equal<IsUnion<string | 'a'>, false>>,
  Expect<Equal<IsUnion<never>, false>>,
]

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