Restore Metro log streaming via CLI flag (#49356)

Summary:
This change adds an opt-in to restore JavaScript log streaming via the Metro dev server, [removed from React Native core in 0.77](https://reactnative.dev/blog/2025/01/21/version-0.77#removal-of-consolelog-streaming-in-metro).

Users can opt into this legacy behaviour by adding the `--client-logs` flag to `npx react-native-community/cli start`.

- The default experience remains without streamed JS logs.
- The existing "JavaScript logs have moved! ..." notice is printed in all cases, and we do not advertise the new flag for new users.
- Under non-Community CLI dev servers (i.e. Expo), log streaming is restored implicitly.

We will clean up this functionality again when we eventually remove JS log streaming over `HMRClient`, tasked in T214991636.

**Implementation notes**

- Logs are always sent over `HMRClient` (previous status quo), even with log streaming off in the dev server. This is a necessary evil to be able to flag this functionality in a user-accessible place, and to move fast for 0.78.
- Necessarily, emitting `fusebox_console_notice` moves to the dev server itself, on first device (Fusebox) connection.

Changelog:
[General][Added] - Add opt in for legacy Metro log streaming via `--client-logs` flag

Author: huntie

Diff for: packages/community-cli-plugin/README.md

@@ -15,7 +15,7 @@ Start the React Native development server.
 #### Usage
 
 ```sh
-npx react-native start [options]
+npx @react-native-community/cli start [options]
 ```
 
 #### Options
@@ -37,6 +37,7 @@ npx react-native start [options]
 | `--cert <path>` | Specify path to a custom SSL cert. |
 | `--config <string>` | Path to the CLI configuration file. |
 | `--no-interactive` | Disable interactive mode. |
+| `--client-logs` | **[Deprecated]** Enable plain text JavaScript log streaming for all connected apps. |
 
 ### `bundle`
 
@@ -45,7 +46,7 @@ Build the bundle for the provided JavaScript entry file.
 #### Usage
 
 ```sh
-npx react-native bundle --entry-file <path> [options]
+npx @react-native-community/cli bundle --entry-file <path> [options]
 ```
 
 #### Options

Diff for: packages/community-cli-plugin/src/commands/start/index.js

@@ -95,6 +95,14 @@ const startCommand: Command = {
       name: '--no-interactive',
       description: 'Disables interactive mode',
     },
+    {
+      name: '--client-logs',
+      description:
+        '[Deprecated] Enable plain text JavaScript log streaming for all ' +
+        'connected apps. This feature is deprecated and will be removed in ' +
+        'future.',
+      default: false,
+    },
   ],
 };
 

Diff for: packages/community-cli-plugin/src/commands/start/runServer.js

@@ -44,6 +44,7 @@ export type StartCommandArgs = {
   config?: string,
   projectRoot?: string,
   interactive: boolean,
+  clientLogs: boolean,
 };
 
 async function runServer(
@@ -96,6 +97,11 @@ async function runServer(
       require.resolve(plugin),
     );
   }
+  // TODO(T214991636): Remove legacy Metro log forwarding
+  if (!args.clientLogs) {
+    // $FlowIgnore[cannot-write] Assigning to readonly property
+    metroConfig.server.forwardClientLogs = false;
+  }
 
   let reportEvent: (event: TerminalReportableEvent) => void;
   const terminal = new Terminal(process.stdout);

Diff for: packages/dev-middleware/src/createDevMiddleware.js

@@ -11,7 +11,7 @@
 
 import type {CreateCustomMessageHandlerFn} from './inspector-proxy/CustomMessageHandler';
 import type {BrowserLauncher} from './types/BrowserLauncher';
-import type {EventReporter} from './types/EventReporter';
+import type {EventReporter, ReportableEvent} from './types/EventReporter';
 import type {Experiments, ExperimentsConfig} from './types/Experiments';
 import type {Logger} from './types/Logger';
 import type {NextHandleFunction} from 'connect';
@@ -81,11 +81,15 @@ export default function createDevMiddleware({
   unstable_customInspectorMessageHandler,
 }: Options): DevMiddlewareAPI {
   const experiments = getExperiments(experimentConfig);
+  const eventReporter = createWrappedEventReporter(
+    unstable_eventReporter,
+    logger,
+  );
 
   const inspectorProxy = new InspectorProxy(
     projectRoot,
     serverBaseUrl,
-    unstable_eventReporter,
+    eventReporter,
     experiments,
     unstable_customInspectorMessageHandler,
   );
@@ -97,7 +101,7 @@ export default function createDevMiddleware({
         serverBaseUrl,
         inspectorProxy,
         browserLauncher: unstable_browserLauncher,
-        eventReporter: unstable_eventReporter,
+        eventReporter,
         experiments,
         logger,
       }),
@@ -129,3 +133,32 @@ function getExperiments(config: ExperimentsConfig): Experiments {
     enableNetworkInspector: config.enableNetworkInspector ?? false,
   };
 }
+
+/**
+ * Creates a wrapped EventReporter that locally intercepts events to
+ * log to the terminal.
+ */
+function createWrappedEventReporter(
+  reporter: ?EventReporter,
+  logger: ?Logger,
+): EventReporter {
+  return {
+    logEvent(event: ReportableEvent) {
+      switch (event.type) {
+        case 'fusebox_console_notice':
+          logger?.info(
+            '\n' +
+              '\u001B[7m' +
+              ' \u001B[1m💡 JavaScript logs have moved!\u001B[22m They can now be ' +
+              'viewed in React Native DevTools. Tip: Type \u001B[1mj\u001B[22m in ' +
+              'the terminal to open (requires Google Chrome or Microsoft Edge).' +
+              '\u001B[27m' +
+              '\n',
+          );
+          break;
+      }
+
+      reporter?.logEvent(event);
+    },
+  };
+}

Diff for: packages/dev-middleware/src/inspector-proxy/Device.js

@@ -41,6 +41,8 @@ const PAGES_POLLING_INTERVAL = 1000;
 // more details.
 const FILE_PREFIX = 'file://';
 
+let fuseboxConsoleNoticeLogged = false;
+
 type DebuggerConnection = {
   // Debugger web socket connection
   socket: WS,
@@ -513,6 +515,7 @@ export default class Device {
       // created instead of manually checking this on every getPages result.
       for (const page of this.#pages.values()) {
         if (this.#pageHasCapability(page, 'nativePageReloads')) {
+          this.#logFuseboxConsoleNotice();
           continue;
         }
 
@@ -1067,4 +1070,14 @@ export default class Device {
   dangerouslyGetSocket(): WS {
     return this.#deviceSocket;
   }
+
+  // TODO(T214991636): Remove notice
+  #logFuseboxConsoleNotice() {
+    if (fuseboxConsoleNoticeLogged) {
+      return;
+    }
+
+    this.#deviceEventReporter?.logFuseboxConsoleNotice();
+    fuseboxConsoleNoticeLogged = true;
+  }
 }

Diff for: packages/dev-middleware/src/inspector-proxy/DeviceEventReporter.js

@@ -212,6 +212,12 @@ class DeviceEventReporter {
     });
   }
 
+  logFuseboxConsoleNotice(): void {
+    this.#eventReporter.logEvent({
+      type: 'fusebox_console_notice',
+    });
+  }
+
   #logExpiredCommand(pendingCommand: PendingCommand): void {
     this.#eventReporter.logEvent({
       type: 'debugger_command',

Diff for: packages/dev-middleware/src/types/EventReporter.js

@@ -77,6 +77,9 @@ export type ReportableEvent =
             | 'PROTOCOL_ERROR',
           >,
     }
+  | {
+      type: 'fusebox_console_notice',
+    }
   | {
       type: 'proxy_error',
       status: 'error',

Diff for: packages/react-native/Libraries/Core/setUpDeveloperTools.js

@@ -42,9 +42,8 @@ if (__DEV__) {
   if (!Platform.isTesting) {
     const HMRClient = require('../Utilities/HMRClient');
 
-    if (global.__FUSEBOX_HAS_FULL_CONSOLE_SUPPORT__) {
-      HMRClient.unstable_notifyFuseboxConsoleEnabled();
-    } else if (console._isPolyfilled) {
+    // TODO(T214991636): Remove legacy Metro log forwarding
+    if (console._isPolyfilled) {
       // We assume full control over the console and send JavaScript logs to Metro.
       [
         'trace',

Diff for: packages/react-native/Libraries/Utilities/HMRClient.js

@@ -26,7 +26,6 @@ let hmrUnavailableReason: string | null = null;
 let currentCompileErrorMessage: string | null = null;
 let didConnect: boolean = false;
 let pendingLogs: Array<[LogLevel, $ReadOnlyArray<mixed>]> = [];
-let pendingFuseboxConsoleNotification = false;
 
 type LogLevel =
   | 'trace'
@@ -52,7 +51,6 @@ export type HMRClientNativeInterface = {|
     isEnabled: boolean,
     scheme?: string,
   ): void,
-  unstable_notifyFuseboxConsoleEnabled(): void,
 |};
 
 /**
@@ -142,29 +140,6 @@ const HMRClient: HMRClientNativeInterface = {
     }
   },
 
-  unstable_notifyFuseboxConsoleEnabled() {
-    if (!hmrClient) {
-      pendingFuseboxConsoleNotification = true;
-      return;
-    }
-    hmrClient.send(
-      JSON.stringify({
-        type: 'log',
-        level: 'info',
-        data: [
-          '\n' +
-            '\u001B[7m' +
-            ' \u001B[1m💡 JavaScript logs have moved!\u001B[22m They can now be ' +
-            'viewed in React Native DevTools. Tip: Type \u001B[1mj\u001B[22m in ' +
-            'the terminal to open (requires Google Chrome or Microsoft Edge).' +
-            '\u001B[27m' +
-            '\n',
-        ],
-      }),
-    );
-    pendingFuseboxConsoleNotification = false;
-  },
-
   // Called once by the bridge on startup, even if Fast Refresh is off.
   // It creates the HMR client but doesn't actually set up the socket yet.
   setup(
@@ -341,9 +316,6 @@ function flushEarlyLogs(client: MetroHMRClient) {
     pendingLogs.forEach(([level, data]) => {
       HMRClient.log(level, data);
     });
-    if (pendingFuseboxConsoleNotification) {
-      HMRClient.unstable_notifyFuseboxConsoleEnabled();
-    }
   } finally {
     pendingLogs.length = 0;
   }

Diff for: packages/react-native/Libraries/Utilities/HMRClientProdShim.js

@@ -25,7 +25,6 @@ const HMRClientProdShim: HMRClientNativeInterface = {
   disable() {},
   registerBundle() {},
   log() {},
-  unstable_notifyFuseboxConsoleEnabled() {},
 };
 
 module.exports = HMRClientProdShim;

Diff for: packages/react-native/Libraries/__tests__/__snapshots__/public-api-test.js.snap

@@ -8942,7 +8942,6 @@ export type HMRClientNativeInterface = {|
     isEnabled: boolean,
     scheme?: string
   ): void,
-  unstable_notifyFuseboxConsoleEnabled(): void,
 |};
 declare const HMRClient: HMRClientNativeInterface;
 declare module.exports: HMRClient;