feat(typescript-types): Add If<Test,TrueBranch,FalseBranch>

packages/typescript-types/README.md

@@ -8,10 +8,14 @@ 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`) |
+| 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`) |
+| [`If<Test, TrueBranch, FalseBranch>`][] | Returns `TrueBranch` if `Test` is `true`, `FalseBranch` otherwise[^if_remark].              |
+
+[^if_remark]: Note the special behavior, if `boolean` is passed as `Test` the return value is a union of both branches, i.e. `TrueBranch | FalseBranch`.
 
 [`IsAny<T>`]: src/is-any.ts
 [`IsNever<T>`]: src/is-never.ts
+[`If<Test, TrueBranch, FalseBranch>`]: src/if.ts

packages/typescript-types/src/if.ts

@@ -0,0 +1,13 @@
+import type { IsAny } from './is-any.js'
+import type { IsNever } from './is-never.js'
+
+// Note: Returns a union of TrueBranch and FalseBranch if Test is any or boolean
+// Note: Returns FalseBranch if Test is never
+export type If<Test, TrueBranch, FalseBranch> =
+	IsAny<Test> extends true
+		? FalseBranch
+		: IsNever<Test> extends true
+			? FalseBranch
+			: Test extends true
+				? TrueBranch
+				: FalseBranch

packages/typescript-types/test/if.tst.ts

@@ -0,0 +1,10 @@
+import type { If } from '@/if.js'
+import { expect } from 'tstyche'
+
+expect<If<true, 'Y', 'N'>>().type.toBe<'Y'>()
+
+expect<If<false, 'Y', 'N'>>().type.toBe<'N'>()
+expect<If<never, 'Y', 'N'>>().type.toBe<'N'>()
+expect<If<any, 'Y', 'N'>>().type.toBe<'N'>()
+
+expect<If<boolean, 'Y', 'N'>>().type.toBe<'Y' | 'N'>()