test(utils): add test for isArrayMember

packages/utils/tests/isArrayMember.test.ts

@@ -0,0 +1,27 @@
+import { isArrayMember } from '../src/isArrayMember';
+
+describe(isArrayMember.name, () => {
+    it('determines if value is contained in an array', () => {
+        const arr = ['a', 'b', 'c'] as const;
+        expect(isArrayMember('a' as string, arr)).toBe(true);
+        expect(isArrayMember('d' as any, arr)).toBe(false);
+        expect(isArrayMember(undefined as any, arr)).toBe(false);
+    });
+
+    it('correctly narrows down value type (typescript only)', () => {
+        type Variant = 'a' | 'b' | 'c' | 'd';
+        const skippedVariants = ['a', 'b'] satisfies Variant[];
+        const variant = 'a' as Variant;
+
+        // the unit test is trivial here, but type-check verifies that the type narrowing works as supposed to
+        if (isArrayMember(variant, skippedVariants)) {
+            expect(variant === 'a').toBe(true);
+            // @ts-expect-error TS2367: variant was narrowed down to 'a' | 'b', so it has no overlap with 'c'
+            expect(variant === 'c').toBe(false);
+        } else {
+            expect(variant === 'c').toBe(false);
+            // @ts-expect-error TS2367: variant was narrowed down to 'c' | 'd', so it has no overlap with 'a'
+            expect(variant === 'a').toBe(true);
+        }
+    });
+});