Add Fantom integration test for ReactNativeFeatureFlags

Summary:
Converts the Jest-only flag tests to Fantom integration tests. Uses
dangerouslyResetForTesting() between tests to replace jest.resetModules().
The 5 tests that mock the native module (jest.doMock) are omitted as
module mocking is unavailable in Fantom.

Changelog: [Internal]

Differential Revision: D94887602

Author: sammy-SC

packages/react-native/src/private/featureflags/__tests__/ReactNativeFeatureFlags-itest.js

@@ -0,0 +1,107 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
+
+import * as ReactNativeFeatureFlags from '../ReactNativeFeatureFlags';
+import * as ReactNativeFeatureFlagsBase from '../ReactNativeFeatureFlagsBase';
+
+/**
+ * These tests cover the JS-only feature flag behavior that does not require
+ * jest.mock() / jest.doMock() / jest.fn() and is therefore convertible to
+ * Fantom integration tests.
+ *
+ * Tests that are NOT convertible (require jest module mocking):
+ *  - "should provide default values for common flags and NOT log an error if
+ *     the native module is available but the method is NOT defined"
+ *  - "should access and cache common flags from the native module if it is available"
+ *  - "when the native module is NOT available" (all sub-cases)
+ */
+
+beforeEach(() => {
+  ReactNativeFeatureFlagsBase.dangerouslyResetForTesting();
+});
+
+test('should provide default values for JS-only flags', () => {
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(false);
+});
+
+test('should access and cache overridden JS-only flags', () => {
+  let callCount = 0;
+  ReactNativeFeatureFlags.override({
+    jsOnlyTestFlag: () => {
+      callCount++;
+      return true;
+    },
+  });
+
+  expect(callCount).toBe(0);
+
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(true);
+  expect(callCount).toBe(1);
+
+  // Second call should return cached value without calling the override again.
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(true);
+  expect(callCount).toBe(1);
+});
+
+test('should throw an error if any of the JS flags has been accessed before overriding', () => {
+  ReactNativeFeatureFlags.jsOnlyTestFlag();
+
+  expect(() =>
+    ReactNativeFeatureFlags.override({
+      jsOnlyTestFlag: () => true,
+    }),
+  ).toThrow(
+    'Feature flags were accessed before being overridden: jsOnlyTestFlag',
+  );
+});
+
+test('should NOT throw an error if any of the common flags has been accessed before overriding', () => {
+  ReactNativeFeatureFlags.commonTestFlag();
+
+  expect(() => {
+    ReactNativeFeatureFlags.override({
+      jsOnlyTestFlag: () => true,
+    });
+  }).not.toThrow();
+
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(true);
+});
+
+test('should throw an error when trying to set overrides twice', () => {
+  ReactNativeFeatureFlags.override({
+    jsOnlyTestFlag: () => true,
+  });
+
+  expect(() =>
+    ReactNativeFeatureFlags.override({
+      jsOnlyTestFlag: () => false,
+    }),
+  ).toThrow('Feature flags cannot be overridden more than once');
+});
+
+test('should evaluate to default value if the override returns null', () => {
+  ReactNativeFeatureFlags.override({
+    // $FlowExpectedError[incompatible-call]
+    jsOnlyTestFlag: () => null,
+  });
+
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(false);
+});
+
+test('should evaluate to default value if the override returns undefined', () => {
+  ReactNativeFeatureFlags.override({
+    // $FlowExpectedError[incompatible-call]
+    jsOnlyTestFlag: () => undefined,
+  });
+
+  expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(false);
+});