Tuple to Nested Object

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题目

Given a tuple type T that only contains string type, and a type U, build an object recursively.

type a = TupleToNestedObject<["a"], string>; // {a: string}
type b = TupleToNestedObject<["a", "b"], number>; // {a: {b: number}}
type c = TupleToNestedObject<[], boolean>; // boolean. if the tuple is empty, just return the U type

解答

type TupleToNestedObject<T, U> = T extends [infer First, ...infer Rest]
  ? {
      [K in First extends string ? First : never]: TupleToNestedObject<Rest, U>;
    }
  : U;

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[K in First extends string ? First : never]

可以简写为

[K in First & string]

type TupleToNestedObject<T, U> = T extends [infer F, ...infer R]
  ? {
      [K in F & string]: TupleToNestedObject<R, U>;
    }
  : U;