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62. implement Unique<T extends any[]>

To implement the Unique type in TypeScript, we can leverage the power of conditional types and the keyof keyword to create a type that deduplicates an array of strings.

Here is the implementation of the Unique type:

type Unique<T extends Array<any>> = T extends [infer U, ...infer R]  ? U extends string    ? [U, ...Unique<{ [K in keyof R]: R[K] extends U ? never : R[K] }>]    : never  : []type A = Unique<['A', 'A', 'A', 'B', 'B', 'C']>// A resolves to ['A', 'B', 'C']

Let's break down the implementation step by step:

  1. The Unique type takes a type parameter T that extends an Array of any type T extends Array<any>.
  2. We use conditional types to check if the input array T can be deconstructed into a tuple [infer U, ...infer R].
  3. We then check if the first element U of the array is a string U extends string.
  4. If U is a string, we recursively call the Unique type on the rest of the array excluding any elements that are equal to U using a mapped type { [K in keyof R]: R[K] extends U ? never : R[K] }. This effectively removes any duplicate occurrences of the string U in the resulting array.
  5. If U is not a string, we return never to exclude it from the resulting array.
  6. Finally, we return an empty array [] when the input array T is empty.

When we use the Unique type with the example Unique<['A', 'A', 'A', 'B', 'B', 'C']>, the type A resolves to ['A', 'B', 'C'] which deduplicates the string array as expected.