Metro RAM Bundles: Faster Startup with Incremental Loading

Insight

Metro's default bundle is a single large file, which means the JavaScript engine must parse and evaluate everything before the first screen appears. RAM bundles break the bundle into many small modules that are fetched lazily as the app navigates, cutting the startup time dramatically, especially on low‑end devices.

Example

// metro.config.js
module.exports = {
  transformer: {
    // Enable RAM bundles
    ramBundle: true,
  },
  serializer: {
    // Optional: create a separate bundle for each entry point
    createModuleIdFactory: () => {
      const projectRoot = __dirname;
      return path => path.replace(projectRoot, "");
    },
  },
};

Add the config, rebuild with eas build or react-native run-android, and you’ll see the bundle split into many *.jsbundle chunks.

Takeaway

Enable ramBundle in Metro to let the runtime load only the code it needs when it needs it—resulting in a snappier launch and a smoother experience on all devices.