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Last

介绍​

实现一个Last<T>泛型，它接受一个数组T并返回其最后一个元素的类型。

例如

ts
  type arr1 = ['a', 'b', 'c']
  type arr2 = [3, 2, 1]
  type tail1 = Last<arr1> // 应推导出 'c'
  type tail2 = Last<arr2> // 应推导出 1
ts
  type arr1 = ['a', 'b', 'c']
  type arr2 = [3, 2, 1]
  type tail1 = Last<arr1> // 应推导出 'c'
  type tail2 = Last<arr2> // 应推导出 1
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start point​

ts
/* _____________ Your Code Here _____________ */
 
type Last<T> = any
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<Last<[]>, never>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[2]>, 2>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[3, 2, 1]>, 1>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[() => 123, { a: string }]>, { a: string }>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
]
 
 
 
ts
/* _____________ Your Code Here _____________ */
 
type Last<T> = any
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<Last<[]>, never>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[2]>, 2>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[3, 2, 1]>, 1>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<Last<[() => 123, { a: string }]>, { a: string }>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
]
 
 
 
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my solutions​

Spoiler warning // Click to reveal answer
ts
//方案1
type Last<T extends any[]> = T['length'] extends 0 ? never : T extends [...infer R, infer L] ? L : never 
 
ts
//方案1
type Last<T extends any[]> = T['length'] extends 0 ? never : T extends [...infer R, infer L] ? L : never 
 
ts
// 方案2
type Last<T extends any[]> = T extends [...infer R, infer L] ? L : never
 
ts
// 方案2
type Last<T extends any[]> = T extends [...infer R, infer L] ? L : never
 
ts
// 方案3 most popular
 
type Last<T extends any[]> = [never, ...T][T['length']]
 
ts
// 方案3 most popular
 
type Last<T extends any[]> = [never, ...T][T['length']]
 
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