feat: ✨ Add GetArrayUnion type

.changeset/dry-months-scream.md

@@ -0,0 +1,5 @@
+---
+"@crbroughton/ts-test-utils": minor
+---
+
+Add GetArrayUnion type

index.ts

@@ -58,3 +58,5 @@ export type IsNonVoid<Actual> = IsVoid<Actual> extends true ? false : true
 export type NonVoid<Actual> = Actual extends void ? never : Actual
 
 export type GetParameters<T> = T extends (...args: infer P) => any ? P : never
+
+export type GetArrayUnion<T> = T extends readonly (infer U)[] ? U : never

tests/GetArrayUnion.ts

@@ -0,0 +1,18 @@
+/* eslint-disable unused-imports/no-unused-vars */
+import type { Equals, Expect, GetArrayUnion } from '..'
+
+// Passes the GetArrayUnion test when the resulting type matches the number array
+const testArray = [1, 2, 3]
+type Result = Expect<Equals<GetArrayUnion<typeof testArray>, number>>
+//   ^?
+
+// Passes the GetArrayUnion test when the resulting type matches the array with const
+const testArrayConst = [1, 2, 'stringHere'] as const
+type ResultConst = Expect<Equals<GetArrayUnion<typeof testArrayConst>, 1 | 2 | 'stringHere'>>
+//   ^?
+
+// Failed the GetArrayUnion test when the resulting type does equal the expectation
+const testArrayFail = [1, 2, 'stringHere'] as const
+// @ts-expect-error - GetParameters failing the equality checker
+type ResultFailure = Expect<Equals<GetParameters<typeof testArrayFail>, number | string>>
+//   ^?