Feat is never

README.md

@@ -949,7 +949,23 @@ test('Conditions can be based on XOR', t => {
 
 ### IsNever
 
-
+```ts
+test('Can ask if T is of type "never"', t => {
+    assert<IsNever<'hi'>, False>(t);
+    assert<IsNever<any>, False>(t);
+    assert<IsNever<object>, False>(t);
+    assert<IsNever<Function>, False>(t);
+    assert<IsNever<() => string>, False>(t);
+    assert<IsNever<number>, False>(t);
+    assert<IsNever<boolean>, False>(t);
+    assert<IsNever<string | number>, False>(t);
+    assert<IsNever<unknown>, False>(t);
+    assert<IsNever<null>, False>(t);
+    assert<IsNever<undefined>, False>(t);
+
+    assert<IsNever<never>, True>(t);
+});
+```
 
 ### IsNil
 

scripts/testTsVersions.sh

@@ -1,6 +1,6 @@
 set -e
 
-for v in 3.0.3 3.1.6 3.2.2 next; do
+for v in 3.5.3 next; do
     npm install --no-save typescript@$v
     npm test
 done

src/types/predicates.ts

@@ -15,7 +15,7 @@ export type ObjectPrototypeKeys = keyof Object;
 /** no-doc */
 export type FunctionPrototypeKeys = keyof Function;
 
-export type IsNever<S extends string> = Not<(Record<S, True> & Record<string, False>)[S]>;
+export type IsNever<T> = keyof any extends keyof T ? Not<IsAny<T>> : False;
 export type IsType<T, X> = X extends T ? True : False;
 export type IsArray<T> = T extends unknown[] ? True : False;
 export type IsNumber<T> = T extends number ? True : False;

test/predicates/IsNever.test.ts

@@ -0,0 +1,20 @@
+import test from 'ava';
+import { assert } from '../helpers/assert';
+
+import { False, True, IsNever } from '../../src';
+
+test('Can ask if T is of type "never"', t => {
+    assert<IsNever<'hi'>, False>(t);
+    assert<IsNever<any>, False>(t);
+    assert<IsNever<object>, False>(t);
+    assert<IsNever<Function>, False>(t);
+    assert<IsNever<() => string>, False>(t);
+    assert<IsNever<number>, False>(t);
+    assert<IsNever<boolean>, False>(t);
+    assert<IsNever<string | number>, False>(t);
+    assert<IsNever<unknown>, False>(t);
+    assert<IsNever<null>, False>(t);
+    assert<IsNever<undefined>, False>(t);
+
+    assert<IsNever<never>, True>(t);
+});