Combination

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题目

Given an array of strings, do Permutation & Combination. It's also useful for the prop types like video controlsList

// expected to be `"foo" | "bar" | "baz" | "foo bar" | "foo bar baz" | "foo baz" | "foo baz bar" | "bar foo" | "bar foo baz" | "bar baz" | "bar baz foo" | "baz foo" | "baz foo bar" | "baz bar" | "baz bar foo"`
type Keys = Combination<["foo", "bar", "baz"]>;

解答

type Combination<
  T extends string[],
  U extends string = T[number],
  P extends string = U
> = U extends U
  ? U | `${U} ${Combination<T, Exclude<P, U>, Exclude<P, U>>}`
  : never;

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全排列的问题已经做过几次了

由于入参是一个数组，我们没办法 extends 操作，需要转成联合类型，通过 extends 一次取一个

通过 ${I} ${Combination<[], Exclude<A, I>>} 来递归剩余区域的内容，非常巧妙的通过 I | 来返回所有联合类型

但是你可以会这么写，会有很多的报错，大概就是 A 和 U 的类型不对

type Combination<T extends string[], A = T[number], U = A> = U extends A
  ? U | `${U} ${Combination<[], Exclude<A, U>>}`
  : never;

我们需要通过 infer 来推一下 U

// 答案
type Combination<
  T extends string[],
  A = T[number],
  U = A
> = U extends infer I extends string
  ? I | `${I} ${Combination<[], Exclude<A, I>>}`
  : never;