Enabling Hermes in React Native for Faster Startup and Lower Memory

Enable Hermes in a React Native project

Desired outcome: faster app startup, lower memory usage, smaller Android APK.

Prerequisites

• React Native CLI project (version 0.60 or newer).
• Android Studio with Gradle plugin compatible.
• CocoaPods installed for iOS.
• Write access to project files.

Procedure

1. Android: Open android/app/build.gradle. Locate the project.ext.react block and set enableHermes: true.

project.ext.react = [
    enableHermes: true
]

2. Sync Gradle and rebuild: cd android && ./gradlew clean assembleDebug.
3. iOS: Open ios/Podfile. Add use_hermes! inside the target block.

target 'YourApp' do
  use_hermes!
end

4. Run pod install from the ios directory.
5. Verify that Info.plist contains hermes_enabled set to true.
6. Re‑build the iOS app: npx react-native run-ios or via Xcode.

Expected checks

• On Android, after launch run adb logcat | grep -i hermes. Look for a line similar to Hermes is enabled.
• On iOS, run xcrun simctl spawn booted log show --predicate 'process == "YourApp"' --last 5s and verify Hermes appears.
• Optional: inspect the generated bundle for the Hermes bytecode marker: npx react-native bundle --platform android --dev false --entry-file index.js --bundle-output /tmp/index.android.bundle then grep -a 'hermes' /tmp/index.android.bundle.

Limitations and risks

• Some native libraries that depend on JavaScriptCore‑specific APIs may fail; test all third‑party modules after switching.
• Chrome remote debugging is disabled; use Flipper or Hermes‑specific debugging tools.
• First build may take longer due to bytecode generation.

Recovery (rollback)

1. Android: set enableHermes: false or remove the line in android/app/build.gradle and rebuild.
2. iOS: remove use_hermes! from Podfile, run pod install again, and ensure hermes_enabled is false or absent in Info.plist.
3. Re‑build the app and check logs for the absence of Hermes messages.