Hermes is a JavaScript engine optimized for React Native. By compiling JavaScript into bytecode ahead of time, Hermes can improve your app start-up time. The binary size of Hermes is also smaller than other JavaScript engines, such as JavaScriptCore (JSC). It also uses less memory at runtime, which is particularly valuable on lower-end Android devices.

A limitation with JavaScriptCore is that the debugger does not work with modules built on top of JSI. This means that if your app uses react-native-reanimated version 2, for example, remote JS debugging will not work. Hermes makes it possible to debug your app even when using JSI modules.

To get started, open your app.json and add jsEngine field under the android section:

{
  "expo": {
    "android": {
      "jsEngine": "hermes"
    }
  }
}

Now you can build an APK or AAB through eas build and your app will run with Hermes instead of JavaScriptCore.

Publishing updates with both expo publish and expo export will generate Hermes bytecode bundles and their sourcemaps.

Please note that the Hermes bytecode format may change between different versions of hermes-engine — an update produced for a specific version of Hermes will not run on a different version of Hermes. Updating the Hermes version can be thought of in the same way as updating any other native module, and so if you update the hermes-engine version you should also update the runtimeVersion in app.json. If you don't do this, your app may crash on launch because the update may be loaded by an existing binary that uses an older version of hermes-engine that is incompatible with the updated bytecode format. See "Update Compatibility" for more information.

To use Hermes inspector for JavaScript debugging, we recommend following the instructions from the React Native docs.

iOS is not supported yet — Hermes support was added to React Native for iOS in react-native@0.64 and will likely to be added to Expo projects when it has been used successfully in production more broadly.

The classic build system isn't flexible enough to support using Hermes for some apps and not for others. You will need to use the new build system, EAS Build, to use Hermes in your standalone apps.