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IsNever

介绍​

实现一个类型IsNever, 接收类型T. 如果类型解析为never, 返回true, 否则返回false.

例如：

ts
type A = IsNever<never> // expected to be true
type B = IsNever<undefined> // expected to be false
type C = IsNever<null> // expected to be false
type D = IsNever<[]> // expected to be false
type E = IsNever<number> // expected to be false
ts
type A = IsNever<never> // expected to be true
type B = IsNever<undefined> // expected to be false
type C = IsNever<null> // expected to be false
type D = IsNever<[]> // expected to be false
type E = IsNever<number> // expected to be false
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起点​

ts
/* _____________ Your Code Here _____________ */
 
type IsNever<T> = any
 
/* _____________ Test Cases _____________ */
 
 
type cases = [
  Expect<Equal<IsNever<never>, true>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<never | string>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<''>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<undefined>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<null>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<[]>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<{}>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
]
 
ts
/* _____________ Your Code Here _____________ */
 
type IsNever<T> = any
 
/* _____________ Test Cases _____________ */
 
 
type cases = [
  Expect<Equal<IsNever<never>, true>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<never | string>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<''>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<undefined>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<null>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<[]>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
  Expect<Equal<IsNever<{}>, false>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.
]
 
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解决方案​

Spoiler warning // Click to reveal answer
ts
// most popular 
 
type IsNever<T> = [T] extends [never] ? true : false 
 
 
ts
// most popular 
 
type IsNever<T> = [T] extends [never] ? true : false 
 
 
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