hermes-ios-config.md

Configuring Hermes on iOS

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 native project is set up with the following patches.

This configuration is automatic for projects initialized with SDK 43 and higher.

Patches for the iOS native project

Create ios/Podfile.properties.json

{
  "expo.jsEngine": "hermes"
}

Update ios/Podfile to reference the Hermes setting from ios/Podfile.properties.json

--- a/ios/Podfile
+++ b/ios/Podfile
@@ -4,13 +4,16 @@ require File.join(File.dirname(`node --print "require.resolve('@react-native-com

 platform :ios, '12.0'

+require 'json'
+podfile_properties = JSON.parse(File.read('./Podfile.properties.json')) rescue {}
+
 target 'HelloWorld' do
   use_expo_modules!
   config = use_native_modules!

   use_react_native!(
     :path => config[:reactNativePath],
-    :hermes_enabled => false
+    :hermes_enabled => podfile_properties['expo.jsEngine'] == 'hermes'
   )

   # Uncomment to opt-in to using Flipper

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 ios/Podfile.properties.json. CocoaPods and Xcode will link with the specified JavaScript engine accordingly.

How this works for publishing updates

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

For bare workflow apps, please ensure the jsEngine value is consistent between app config (app.json or app.config.js) and ios/Podfile.properties.json.