feat(Conditionals): Add IsUnion and corresponding Ifs

BHC-IT · Jun 27, 2023 · 66701bc · 66701bc
1 parent 9899345
commit 66701bc
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Showing 2 changed files with 22 additions and 1 deletion.
diff --git a/tests/Util.test.d.ts b/tests/Util.test.d.ts
@@ -1,4 +1,4 @@
-import { Equal, Expect, PartialExcept } from '../types'
+import { Equal, Expect, ExpectFalse, ExpectTrue, IsUnion, PartialExcept } from '../types'
 
 declare namespace PartialExcept {
   interface Dummy {
@@ -19,4 +19,14 @@ type __TEST__ = {
     >,
     Expect<Equal<PartialExcept<PartialExcept.Dummy, never>, Partial<PartialExcept.Dummy>>>
   ]
+  IsUnion: [
+    ExpectTrue<IsUnion<'sauce' | 'blanche'>>,
+    ExpectTrue<IsUnion<string | number>>,
+    ExpectFalse<IsUnion<never>>,
+    ExpectFalse<IsUnion<string>>,
+    // `string | 'a literal'` evaluates to string
+    ExpectFalse<IsUnion<string | 'sauce'>>,
+    // `string | never` evalutates to string
+    ExpectFalse<IsUnion<string | never>>
+  ]
 }
diff --git a/types/Conditionals.d.ts b/types/Conditionals.d.ts
@@ -63,3 +63,14 @@ export type IfAny<Type, OnIsAny, OnIsNotAny = never> = If<IsAny<Type>, OnIsAny,
 export type IfNotAny<Type, OnIsNotAny, OnIsAny = never> = If<IsNotAny<Type>, OnIsNotAny, OnIsAny>
 
 export type IsObject<Type> = Extends<Type, object>
+
+type __IsUnion<T, U> = IfNotNever<T, T extends U ? ([U] extends [T] ? false : true) : never, false>
+export type IsUnion<T> = __IsUnion<T, T>
+export type IsNotUnion<T> = Not<IsUnion<T>>
+
+export type IfUnion<T, OnIsUnion, OnIsNotUnion = never> = If<IsUnion<T>, OnIsUnion, OnIsNotUnion>
+export type IfNotUnion<T, OnIsNotUnion, OnIsUnion = never> = If<
+  IsNotUnion<T>,
+  OnIsNotUnion,
+  OnIsUnion
+>