React-Native: Hermes is not enabled

React Native: Hermes is not Enabled

Hermes is a JavaScript engine optimized for React Native applications. It can significantly improve performance, especially on Android. When you encounter the “Hermes is not enabled” message, it indicates that your React Native app is not using Hermes for JavaScript execution. This article will guide you on understanding and resolving this issue.

Understanding Hermes

What is Hermes?

Hermes is a lightweight and fast JavaScript engine specifically designed for React Native. It offers various benefits, including:

• Improved startup time
• Faster code execution
• Reduced memory usage
• Enhanced performance on Android

Why is Hermes Not Enabled?

There are several reasons why Hermes might not be enabled in your React Native project:

• Project Configuration: Hermes might not be explicitly configured in your project’s build settings.
• Hermes Version Compatibility: There might be compatibility issues between the Hermes version and your React Native or Android version.
• Hermes Installation Errors: The Hermes installation process might have encountered errors, preventing it from being activated.

Troubleshooting Hermes Not Enabled

1. Check Project Configuration

Ensure Hermes is enabled in your React Native project’s configuration files:

• Android: Modify the `android/app/build.gradle` file to include Hermes:

android {
  ...
  buildTypes {
    release {
      minifyEnabled true
      shrinkResources true
      proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
      // Enable Hermes
      isEnableHermes true
      hermesEnabled true
    }
  }
  ...
}

• iOS: Hermes is typically automatically enabled for iOS projects. However, you can check the `Podfile` file to ensure the Hermes dependency is correctly defined:

use_frameworks!

target 'YourProjectName' do
  pod 'React-native-hermes'
end

2. Verify Hermes Version Compatibility

Check the compatibility between Hermes, React Native, and Android versions. Refer to the official documentation for supported versions:

• React Native documentation: https://reactnative.dev/docs/hermes
• Hermes documentation: https://hermesengine.dev/

3. Reinstall Hermes

If configuration is correct but Hermes is still not enabled, try reinstalling Hermes:

npx react-native start --reset-cache

4. Check for Errors

Inspect your project’s build logs or console output for any error messages related to Hermes installation or activation. This might provide clues about the root cause of the issue.

Comparison Table

Below is a table comparing JavaScript engines used in React Native:

Conclusion

Enabling Hermes in your React Native application can significantly improve performance, especially on Android. By understanding the reasons behind Hermes not being enabled and following the troubleshooting steps outlined in this article, you can resolve this issue and benefit from Hermes’ advantages.