hermes-android-config.md

Configuring Hermes on Android

The jsEngine value in app config (app.json or app.config.js) is the single source to determine the JavaScript executor for your Expo app, but if you use the bare workflow for your project, please make sure the Android native project is set up as following patches.

This configuration is automatic for projects initialized with SDK 42 and higher.

Patches for the Android native project

Add expo-jsEngine=hermes in android/gradle.properties

--- a/android/gradle.properties
+++ b/android/gradle.properties
@@ -27,3 +27,7 @@ android.enableJetifier=true

 # Version of flipper SDK to use with React Native
 FLIPPER_VERSION=0.54.0
+
+# The hosted JavaScript engine
+# Supported values: expo.jsEngine = "hermes" | "jsc"
+expo.jsEngine=hermes

Make enableHermes reference the expo.jsEngine gradle property in android/app/build.gradle

--- a/android/app/build.gradle
+++ b/android/app/build.gradle
@@ -78,7 +78,7 @@ import com.android.build.OutputFile
  */

 project.ext.react = [
-    enableHermes: false,
+    enableHermes: (findProperty('expo.jsEngine') ?: "jsc") == "hermes",
 ]

 apply from: '../../node_modules/react-native-unimodules/gradle.groovy'

How this works for standalone apps on EAS Build

During the "prebuild" stage for managed apps, config plugins will read jsEngine value in app config and replace expo.jsEngine value in android/gradle.properties. This makes gradle build apps with the specified JavaScript engine.

How this works for publishing updates

Both expo publish and expo export reference jsEngine value in app config. If the jsEngine is set to "hermes", then the generated JavaScript bundle will be a Hermes bytecode bundle.

For bare workflow apps, please make sure the jsEngine value is consistent between app config (app.json or app.config.js) and android/gradle.properties.