Pass default values for feature flags to overrides providers (#46533)

Summary:
Pull Request resolved: #46533

Changelog: [internal]

When defining overrides for feature flags, it's common to have this pattern:

```javascript
myFeatureFlag: () => someCondition ? value : defaultValueForFlag
```

But it's not always obvious how to get the default value defined in the feature flag system. This modifies the API so we receive the default value as a parameter to simplify the logic:

```javascript
myFeatureFlag: (defaultValueForFlag) => someCondition ? value : defaultValueForFlag
```

Reviewed By: mdvacca, rshest

Differential Revision: D62853299

fbshipit-source-id: 9421e31d00662e2a23f1e3fb43bdc9f5cf03fdf3

packages/react-native/scripts/featureflags/templates/js/ReactNativeFeatureFlags.js-template.js

@@ -28,6 +28,7 @@ ${DO_NOT_MODIFY_COMMENT}
 
 import {
  type Getter,
+ type OverridesFor,
  createJavaScriptFlagGetter,
  createNativeFlagGetter,
  setOverrides,
@@ -42,7 +43,7 @@ ${Object.entries(definitions.jsOnly)
  .join('\n')}
 };
 
-export type ReactNativeFeatureFlagsJsOnlyOverrides = Partial<ReactNativeFeatureFlagsJsOnly>;
+export type ReactNativeFeatureFlagsJsOnlyOverrides = OverridesFor<ReactNativeFeatureFlagsJsOnly>;
 
 export type ReactNativeFeatureFlags = {
  ...ReactNativeFeatureFlagsJsOnly,

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

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<08cb1ef37f80dbd53e1aa9ff5ba97286>>
+ * @generated SignedSource<<a1d731250e59d99ac390c9d01c4dbd2f>>
  * @flow strict
  */
 
@@ -20,6 +20,7 @@
 
 import {
  type Getter,
+ type OverridesFor,
  createJavaScriptFlagGetter,
  createNativeFlagGetter,
  setOverrides,
@@ -43,7 +44,7 @@ export type ReactNativeFeatureFlagsJsOnly = {
  useRefsForTextInputState: Getter<boolean>,
 };
 
-export type ReactNativeFeatureFlagsJsOnlyOverrides = Partial<ReactNativeFeatureFlagsJsOnly>;
+export type ReactNativeFeatureFlagsJsOnlyOverrides = OverridesFor<ReactNativeFeatureFlagsJsOnly>;
 
 export type ReactNativeFeatureFlags = {
  ...ReactNativeFeatureFlagsJsOnly,

packages/react-native/src/private/featureflags/ReactNativeFeatureFlagsBase.js

@@ -20,6 +20,12 @@ let overrides: ?ReactNativeFeatureFlagsJsOnlyOverrides;
 
 export type Getter<T> = () => T;
 
+// This defines the types for the overrides object, whose methods also receive
+// the default value as a parameter.
+export type OverridesFor<T> = Partial<{
+ [key in keyof T]: (ReturnType<T[key]>) => ReturnType<T[key]>,
+}>;
+
 function createGetter<T: boolean | number | string>(
  configName: string,
  customValueGetter: Getter<?T>,
@@ -45,7 +51,7 @@ export function createJavaScriptFlagGetter<
  configName,
  () => {
  accessedFeatureFlags.add(configName);
- return overrides?.[configName]?.();
+ return overrides?.[configName]?.(defaultValue);
  },
  defaultValue,
  );

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

@@ -70,7 +70,7 @@ describe('ReactNativeFeatureFlags', () => {
  it('should access and cache overridden JS-only flags', () => {
  const ReactNativeFeatureFlags = require('../ReactNativeFeatureFlags');
 
- const jsOnlyTestFlagFn = jest.fn(() => true);
+ const jsOnlyTestFlagFn = jest.fn((defaultValue: boolean) => true);
  ReactNativeFeatureFlags.override({
  jsOnlyTestFlag: jsOnlyTestFlagFn,
  });
@@ -125,4 +125,17 @@ describe('ReactNativeFeatureFlags', () => {
  }),
  ).toThrow('Feature flags cannot be overridden more than once');
  });
+
+ it('should pass the default value of the feature flag to the function that provides its override', () => {
+ const ReactNativeFeatureFlags = require('../ReactNativeFeatureFlags');
+
+ ReactNativeFeatureFlags.override({
+ jsOnlyTestFlag: (defaultValue: boolean) => {
+ expect(defaultValue).toBe(false);
+ return true;
+ },
+ });
+
+ expect(ReactNativeFeatureFlags.jsOnlyTestFlag()).toBe(true);
+ });
 });