feat: 🚀 isNever (#251)

* feat: 🚀 export IsUnknown

* docs: 📄 changeset

* test: 🧪 testIsUnknown

* feat: ➕ add IsNever

* test: 🧪 testIsNever

* docs: 📄 changeset

* docs: ➕ add IsUnknown 📄

* docs: ➕ add IsNever to README.md 📄

* fix: 🧪 typo: never => unknown

* docs: 🧪 – => -

.changeset/honest-olives-speak.md

@@ -0,0 +1,5 @@
+---
+"ts-essentials": minor
+---
+
+Add `IsNever` which returns true if it's `never`, otherwise false

README.md

@@ -36,6 +36,7 @@ If you use any [functions](https://github.com/krzkaczor/ts-essentials/blob/maste
   - [Dictionaries](#Dictionaries)
   - [Type checkers](#type-checkers)
     - `IsUnknown`
+    - `IsNever`
   - [Deep\* wrapper types](#Deep-wrapper-types)
     - DeepPartial
     - DeepRequired
@@ -147,6 +148,15 @@ type Test1 = IsUnknown<unknown>;
 type Test2 = IsUnknown<{ name: "Alexey" }>;
 ```
 
+- `IsNever` checks whether we get `never` or not. If so, we get `true`. Otherwise, `false`
+
+```typescript
+// ✅ true
+type Test1 = IsNever<never>;
+// ❌ false
+type Test2 = IsNever<{ name: "Alexey" }>;
+```
+
 ### Deep\* wrapper types
 
 - DeepPartial

lib/types.ts

@@ -5,6 +5,7 @@ export type IsTuple<T> = T extends any[] ? (any[] extends T ? never : T) : never
 type AnyRecord<T = any> = Record<PropertyKey, T>;
 // https://stackoverflow.com/questions/49927523/disallow-call-with-any/49928360#49928360
 type IsAny<T> = 0 extends 1 & T ? true : false;
+export type IsNever<T> = [T] extends [never] ? true : false;
 export type IsUnknown<T> = IsAny<T> extends true ? false : unknown extends T ? true : false;
 export type AnyArray<T = any> = Array<T> | ReadonlyArray<T>;
 

test/index.ts

@@ -48,6 +48,7 @@ import {
   OmitProperties,
   isExact,
   IsUnknown,
+  IsNever,
 } from "../lib";
 
 function testDictionary() {
@@ -1292,3 +1293,35 @@ function testIsUnknown() {
     Assert<IsExact<IsUnknown<any>, false>>,
   ];
 }
+
+function testIsNever() {
+  type cases = [
+    Assert<IsExact<IsNever<string>, false>>,
+    Assert<IsExact<IsNever<number>, false>>,
+    Assert<IsExact<IsNever<boolean>, false>>,
+    Assert<IsExact<IsNever<bigint>, false>>,
+    Assert<IsExact<IsNever<symbol>, false>>,
+    Assert<IsExact<IsNever<undefined>, false>>,
+    Assert<IsExact<IsNever<null>, false>>,
+    Assert<IsExact<IsNever<Function>, false>>,
+    Assert<IsExact<IsNever<Date>, false>>,
+    Assert<IsExact<IsNever<Error>, false>>,
+    Assert<IsExact<IsNever<RegExp>, false>>,
+    Assert<IsExact<IsNever<Map<string, unknown>>, false>>,
+    Assert<IsExact<IsNever<ReadonlyMap<string, unknown>>, false>>,
+    Assert<IsExact<IsNever<WeakMap<{ a: 1 }, unknown>>, false>>,
+    Assert<IsExact<IsNever<Set<string>>, false>>,
+    Assert<IsExact<IsNever<ReadonlySet<string>>, false>>,
+    Assert<IsExact<IsNever<WeakSet<{ a: 1 }>>, false>>,
+    Assert<IsExact<IsNever<{ a: 1 }>, false>>,
+    Assert<IsExact<IsNever<[]>, false>>,
+    Assert<IsExact<IsNever<[1]>, false>>,
+    Assert<IsExact<IsNever<readonly [1]>, false>>,
+    Assert<IsExact<IsNever<readonly number[]>, false>>,
+    Assert<IsExact<IsNever<number[]>, false>>,
+    Assert<IsExact<IsNever<Promise<number>>, false>>,
+    Assert<IsExact<IsNever<unknown>, false>>,
+    Assert<IsExact<IsNever<never>, true>>,
+    Assert<IsExact<IsNever<any>, false>>,
+  ];
+}