Small improvement in types for feature flags definitions in JS (#47239)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47239

Changelog: [internal]

Small refactor of the types for feature flags in JS to make objects read-only.

Reviewed By: mdvacca

Differential Revision: D65058612

fbshipit-source-id: 0d72df2f4afebf0426b9ac76a8f8b195c74fea52

packages/react-native/scripts/featureflags/types.js

@@ -10,30 +10,30 @@
 
 export type FeatureFlagValue = boolean | number | string;
 
-export type FeatureFlagDefinitions = {
+export type FeatureFlagDefinitions = $ReadOnly<{
   common: CommonFeatureFlagList,
   jsOnly: JsOnlyFeatureFlagList,
-};
+}>;
 
-type CommonFeatureFlagList = {
-  [flagName: string]: {
+export type CommonFeatureFlagList = $ReadOnly<{
+  [flagName: string]: $ReadOnly<{
     defaultValue: FeatureFlagValue,
     metadata: FeatureFlagMetadata,
     // Indicates if this API should only be defined in JavaScript, only to
     // preserve backwards compatibility with existing native code temporarily.
     skipNativeAPI?: true,
-  },
-};
+  }>,
+}>;
 
-type JsOnlyFeatureFlagList = {
-  [flagName: string]: {
+export type JsOnlyFeatureFlagList = $ReadOnly<{
+  [flagName: string]: $ReadOnly<{
     defaultValue: FeatureFlagValue,
     metadata: FeatureFlagMetadata,
-  },
-};
+  }>,
+}>;
 
-type FeatureFlagMetadata =
-  | {
+export type FeatureFlagMetadata =
+  | $ReadOnly<{
       purpose: 'experimentation',
       /**
        * Aproximate date when the flag was added.
@@ -41,25 +41,25 @@ type FeatureFlagMetadata =
        */
       dateAdded: string,
       description: string,
-    }
-  | {
+    }>
+  | $ReadOnly<{
       purpose: 'operational' | 'release',
       description: string,
-    };
+    }>;
 
-export type GeneratorConfig = {
+export type GeneratorConfig = $ReadOnly<{
   featureFlagDefinitions: FeatureFlagDefinitions,
   jsPath: string,
   commonCxxPath: string,
   commonNativeModuleCxxPath: string,
   androidPath: string,
   androidJniPath: string,
-};
+}>;
 
-export type GeneratorOptions = {
+export type GeneratorOptions = $ReadOnly<{
   verifyUnchanged: boolean,
-};
+}>;
 
-export type GeneratorResult = {
+export type GeneratorResult = $ReadOnly<{
   [path: string]: string /* content */,
-};
+}>;