react-native/packages/dev-middleware

@react-native/dev-middleware

Dev server middleware supporting core React Native development features. This package is preconfigured in all React Native projects.

Usage

Middleware can be attached to a dev server (e.g. Metro) using the createDevMiddleware API.

import { createDevMiddleware } from '@react-native/dev-middleware';

function myDevServerImpl(args) {
  ...

  const {middleware, websocketEndpoints} = createDevMiddleware({
    projectRoot: metroConfig.projectRoot,
    serverBaseUrl: new URL(`http://${args.host}:${args.port}`),
    logger,
  });

  await Metro.runServer(metroConfig, {
    host: args.host,
    ...,
    unstable_extraMiddleware: [
      middleware,
      // Optionally extend with additional HTTP middleware
    ],
    websocketEndpoints: {
      ...websocketEndpoints,
      // Optionally extend with additional WebSocket endpoints
    },
  });
}

Included middleware

@react-native/dev-middleware is designed for integrators such as @expo/dev-server and @react-native/community-cli-plugin. It provides a common default implementation for core React Native dev server responsibilities.

We intend to keep this to a narrow set of functionality, based around:

• Debugging — The Chrome DevTools protocol (CDP) endpoints supported by React Native, including the Inspector Proxy, which facilitates connections with multiple devices.
• Dev actions — Endpoints implementing core Dev Menu actions, e.g. reloading the app, opening the debugger frontend.

HTTP endpoints

DevMiddlewareAPI.middleware

These are exposed as a connect middleware handler, assignable to Metro.runServer or other compatible HTTP servers.

GET /json/list, /json (CDP)

Returns the list of available WebSocket targets for all connected React Native app sessions.

GET /json/version (CDP)

Returns version metadata used by Chrome DevTools.

GET /debugger-frontend

Subpaths of this endpoint are reserved to serve the JavaScript debugger frontend.

POST /open-debugger

Open the JavaScript debugger for a given CDP target. Must be provided with one of the following query params:

• device‌ — An ID unique to a combination of device and app, stable across installs. Implemented by getInspectorDeviceId on each native platform.
• target — The target page ID as returned by /json/list for the current dev server session.
• appId (deprecated, legacy only) — The application bundle identifier to match (non-unique across multiple connected devices). This param will only match legacy Hermes debugger targets.

Example

curl -X POST 'http://localhost:8081/open-debugger?target=<targetId>'

WebSocket endpoints

DevMiddlewareAPI.websocketEndpoints

/inspector/device

WebSocket handler for registering device connections.

/inspector/debug

WebSocket handler that proxies CDP messages to/from the corresponding device.

Experimental features

React Native frameworks may pass an unstable_experiments option to createDevMiddleware to configure experimental features. Note that these features might not work correctly, and they may change or be removed in the future without notice. Some of the experiment flags available are documented below.

unstable_experiments.enableStandaloneFuseboxShell

Enables launching the debugger frontend in a standalone app shell (provided by the @react-native/debugger-shell package) rather than in a browser window. Since React Native 0.83 this defaults to true, and may be disabled by explicitly passing false.

The shell is powered by a separate binary that is downloaded and cached in the background (immediately after the call to createDevMiddleware). If there is a problem downloading or invoking this binary for the first time, the debugger frontend will revert to launching in a browser window until the next time createDevMiddleware is called (typically, on the next dev server start).

Contributing

Changes to this package can be made locally and tested against the rn-tester app, per the Contributing guide. During development, this package is automatically run from source with no build step.