Hermes Engine
Hermes is a JavaScript engine developed by Meta specifically optimized for React Native, offering faster startup times, reduced memory usage, and a smaller app size compared to using standard JavaScriptCore.
What is Hermes?
Hermes is designed from the ground up for mobile applications, prioritizing fast startup performance by precompiling JavaScript into efficient bytecode ahead of time, rather than parsing and compiling it at runtime.
Key Benefits of Hermes
- Faster app startup, or time-to-interactive
- Reduced memory consumption
- Smaller APK size on Android
- Improved garbage collection performance
Enabling Hermes
Modern React Native versions enable Hermes by default, but for older projects, it can be enabled explicitly in the project configuration.
Verifying Hermes is Active
Bytecode Precompilation
Hermes precompiles JavaScript into bytecode during the build process, meaning the app does not need to parse raw JavaScript at startup, significantly reducing time-to-interactive.
Debugging with Hermes
Hermes includes its own debugger support, accessible through Flipper or Chrome DevTools, allowing developers to inspect and debug JavaScript code running specifically on the Hermes engine.
Considerations When Using Hermes
Some JavaScript features or third-party libraries relying on specific JavaScriptCore behaviors may require verification for full compatibility with Hermes, though support has become extremely comprehensive in modern versions.
Conclusion
Hermes provides significant startup performance and memory improvements for React Native applications, making it the recommended default JavaScript engine for nearly all modern React Native projects.
