feat(typescript-types): Add IsNever<T>

packages/typescript-types/README.md

@@ -9,7 +9,9 @@ The types included in this library are categorized by their purpose.
 #### Test Types
 
 | Type             | Description                                                                                 |
-| -------------- | ---------------------------------------------------------------------------------- |
+| ---------------- | ------------------------------------------------------------------------------------------- |
 | [`IsAny<T>`][]   | `true` if `T` is `any`, `false` otherwise (`null`, `undefined` also yield `false`)          |
+| [`IsNever<T>`][] | `true` if `T` is `never`, `false` otherwise (`null`, `undefined`, `any` also yield `false`) |
 
 [`IsAny<T>`]: src/is-any.ts
+[`IsNever<T>`]: src/is-never.ts

packages/typescript-types/src/is-never.ts

@@ -0,0 +1,4 @@
+// See: https://github.com/microsoft/TypeScript/issues/31751#issuecomment-498526919
+//      https://stackoverflow.com/questions/53984650/typescript-never-type-inconsistently-matched-in-conditional-type/53984913#53984913
+//      https://www.zhenghao.io/posts/ts-never
+export type IsNever<T> = [T] extends [never] ? true : false

packages/typescript-types/test/is-never.tst.ts

@@ -0,0 +1,11 @@
+import type { IsNever } from '@/is-never.js'
+import { expect } from 'tstyche'
+
+expect<IsNever<never>>().type.toBe<true>()
+
+expect<IsNever<1>>().type.toBe<false>()
+expect<IsNever<'somestring'>>().type.toBe<false>()
+expect<IsNever<null>>().type.toBe<false>()
+expect<IsNever<undefined>>().type.toBe<false>()
+expect<IsNever<unknown>>().type.toBe<false>()
+expect<IsNever<any>>().type.toBe<false>()