feat(types): IsNever, IfNever

Signed-off-by: Lexus Drumgold <[email protected]>

src/types/__tests__/if-never.spec-d.ts

@@ -0,0 +1,19 @@
+/**
+ * @file Type Tests - IfNever
+ * @module tutils/types/tests/unit-d/IfNever
+ */
+
+import type TestSubject from '../if-never'
+
+describe('unit-d:types/IfNever', () => {
+  type False = false
+  type True = true
+
+  it('should equal False if IsNever<T> extends false', () => {
+    expectTypeOf<TestSubject<never[], True, False>>().toEqualTypeOf<False>()
+  })
+
+  it('should equal True if IsNever<T> extends true', () => {
+    expectTypeOf<TestSubject<never, True, False>>().toEqualTypeOf<True>()
+  })
+})

src/types/__tests__/is-never.spec-d.ts

@@ -0,0 +1,16 @@
+/**
+ * @file Type Tests - IsNever
+ * @module tutils/types/tests/unit-d/IsNever
+ */
+
+import type TestSubject from '../is-never'
+
+describe('unit-d:types/IsNever', () => {
+  it('should equal false if [T] does not extend [never]', () => {
+    expectTypeOf<TestSubject<bigint>>().toEqualTypeOf<false>()
+  })
+
+  it('should equal true if [T] extends [never]', () => {
+    expectTypeOf<TestSubject<never>>().toEqualTypeOf<true>()
+  })
+})

src/types/if-never.ts

@@ -0,0 +1,20 @@
+/**
+ * @file Type Definitions - IfNever
+ * @module tutils/types/IfNever
+ */
+
+import type IsNever from './is-never'
+
+/**
+ * Conditional type that resolves depending on whether or not `T` is type
+ * `never`.
+ *
+ * @see {@linkcode IsNever}
+ *
+ * @template T - Type to evaluate
+ * @template True - Type if `T` is type `never`
+ * @template False - Type if `T` is not type `never`
+ */
+type IfNever<T, True, False> = IsNever<T> extends true ? True : False
+
+export type { IfNever as default }

src/types/index.ts

@@ -28,13 +28,15 @@ export type { default as IfBigInt } from './if-big-int'
 export type { default as IfBoolean } from './if-boolean'
 export type { default as IfFunction } from './if-function'
 export type { default as IfJsonPrimitive } from './if-json-primitive'
+export type { default as IfNever } from './if-never'
 export type { default as IndexSignature } from './index-signature'
 export type { default as IsAny } from './is-any'
 export type { default as IsArray } from './is-array'
 export type { default as IsBigInt } from './is-big-int'
 export type { default as IsBoolean } from './is-boolean'
 export type { default as IsFunction } from './is-function'
 export type { default as IsJsonPrimitive } from './is-json-primitive'
+export type { default as IsNever } from './is-never'
 export type { default as IsTuple } from './is-tuple'
 export type { default as Join } from './join'
 export type { default as JsonArray } from './json-array'

src/types/is-never.ts

@@ -0,0 +1,13 @@
+/**
+ * @file Type Definitions - IsNever
+ * @module tutils/types/IsNever
+ */
+
+/**
+ * Returns a boolean indicating if `T` is type `never`.
+ *
+ * @template T - Type to evaluate
+ */
+type IsNever<T> = [T] extends [never] ? true : false
+
+export type { IsNever as default }