IsNever

microsoft/TypeScript#31751 (comment)

Author: ytakhs

src/isNever.ts

@@ -0,0 +1,46 @@
+/*
+  1042 - IsNever
+  -------
+  by hiroya iizuka (@hiroyaiizuka) #medium #union #utils
+
+  ### Question
+
+  Implement a type IsNever, which takes input type `T`.
+  If the type of resolves to `never`, return `true`, otherwise `false`.
+
+  For example:
+
+  ```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
+  ```
+
+  > View on GitHub: https://tsch.js.org/1042
+*/
+
+/* _____________ Your Code Here _____________ */
+
+type IsNever<T> = [T] extends [never] ? true : false;
+
+/* _____________ Test Cases _____________ */
+import type { Equal, Expect } from "@type-challenges/utils";
+
+type cases = [
+  Expect<Equal<IsNever<never>, true>>,
+  Expect<Equal<IsNever<never | string>, false>>,
+  Expect<Equal<IsNever<"">, false>>,
+  Expect<Equal<IsNever<undefined>, false>>,
+  Expect<Equal<IsNever<null>, false>>,
+  Expect<Equal<IsNever<[]>, false>>,
+  Expect<Equal<IsNever<{}>, false>>
+];
+
+/* _____________ Further Steps _____________ */
+/*
+  > Share your solutions: https://tsch.js.org/1042/answer
+  > View solutions: https://tsch.js.org/1042/solutions
+  > More Challenges: https://tsch.js.org
+*/