feat: ✨ Add IsNullish type - Check if a type is either undefined or null

Author: CRBroughton

.changeset/heavy-rats-juggle.md

@@ -0,0 +1,5 @@
+---
+"@crbroughton/ts-test-utils": minor
+---
+
+Add IsNullish type - Check if a type is either undefined or null

README.md

@@ -16,6 +16,7 @@ A collection of helper TypeScript types to test other TypeScript types. This col
 - IsNonNullable - Check if a type is not nullable
 - IsUndefined - Check if a type is undefined
 - IsNonUndefined - Check if a type is not undefined
+- IsNullish - Check if a type is either undefined or null
 
 ## Installation
 

index.ts

@@ -30,3 +30,9 @@ export type IsNonNullable<T> = IsNullable<T> extends true ? false : true
 export type IsUndefined<T> = undefined extends T ? true : false
 
 export type IsNonUndefined<T> = IsUndefined<T> extends true ? false : true
+
+export type IsNullish<T> =
+  IsNullable<T> extends true ?
+    true
+    : IsUndefined<T> extends true ?
+      true : false

tests/IsNullish.test.ts

@@ -0,0 +1,19 @@
+/* eslint-disable unused-imports/no-unused-vars */
+import { describe, it } from 'bun:test'
+import type { Expect, IsNullish } from '../index'
+
+describe('IsNullable tests', () => {
+  it('Passes the IsNullable test when the type is nullable', () => {
+    type ResultNull = Expect<IsNullish<{ id: number } | null>>
+    //   ^?
+    type ResultUndefined = Expect<IsNullish<{ id: number } | undefined>>
+    //   ^?
+    type ResultBoth = Expect<IsNullish<{ id: number } | null | undefined>>
+    //   ^?
+  })
+  it('Failed the IsNullable test when the type is not nullable', () => {
+    // @ts-expect-error - Fails the exclusion
+    type Result = Expect<IsNullish<{ id: string }>>
+    //   ^?
+  })
+})