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React Native Integration

Using a custom Hermes build in a React Native app#

To make Hermes usable in a React Native app we need to perform the following steps:

  1. Package a Hermes build into a npm package structure.
  2. Register this npm package with Yarn for use in other local projects.
  3. Link the npm package registered with Yarn into the React Native tree for our app.

Making a Hermes npm package#

A Hermes npm package combines:

  • Hermes CLI tools in binary form. These are used to compile JavaScript to bytecode when packaging up an app release.
  • Pre-compiled release and debug versions of the native Hermes library for Android targets.

The interface between Hermes and React Native changes over time, so if you are not using the main branch of both, you may want to consider matching the Hermes version with your app's React Native version. E.g. git checkout v0.5.0 to build for [email protected].


To make a Hermes npm package, first follow the instructions on building Hermes to create a Release Build of Hermes. This will be the source of CLI binaries for use in the npm.

As we will be compiling native libraries for Android, we first need to setup a suitable development environment. This subject is covered in more detail elsewhere, for example in the guide on building React Native From Source. However, a rough guide to the external dependencies for macOS/Linux is as follows:

  • Node.js
  • JDK (1.8+)
  • Yarn
  • The Android Native Development Kit (NDK)
  • The Android Software Development Kit (SDK)

Note the Android SDK is typically downloaded and installed automatically as part of the flow in Android Studio. If you are following this common route, be sure to note the location Android Studio uses to install the SDK.

Build environment#

Ensure the following variables are present and set correctly in your environment:

  • ANDROID_NDK - the root of your Android NDK install. E.g. /opt/android_ndk/r15c.
  • ANDROID_SDK, ANDROID_HOME, and ANDROID_SDK_ROOT - the root of your Android SDK install. E.g. /opt/android_sdk.
  • JAVA_HOME - the root of your Java Runtime Environment (JRE) install. Note a JRE instance is typically available in a sub-directory of a JDK install. E.g. /opt/jdk-1.8/jre
  • HERMES_WS_DIR - The root of your workspace where the hermes git checkout directory and build_release Release Build directory should already be subdirectories. E.g. $HOME/workspace.

Make sure the node and yarn binaries are available in your system PATH.

Package build#

To make a Hermes npm package, check that you already followed the Building Hermes instructions for a Release Build. The build_release directory will be the source of CLI binaries for use in the npm.

# Package CLI binaries for the current platform
(cd $HERMES_WS_DIR/build_release && ninja github-cli-release)
# Package Android libraries
(cd $HERMES_WS_DIR/hermes/android && ./gradlew githubRelease)
# Copy the above to the NPM directory
cp $HERMES_WS_DIR/build_android/distributions/hermes-runtime-android-v*.tar.gz $HERMES_WS_DIR/hermes/npm
cp $HERMES_WS_DIR/build_release/github/hermes-cli-*-v*.tar.gz $HERMES_WS_DIR/hermes/npm
# Create NPMs
(cd $HERMES_WS_DIR/hermes/npm && yarn install && yarn run unpack-builds-dev && yarn run create-npms-dev)

The NPM bundles can now be found in $HERMES_WS_DIR/hermes/npm/*.tgz

Linking Hermes into a React Native app#

To use your custom Hermes npm package in an app, first make sure the app works with a normal release of Hermes by following instructions in the React Native docs.

Next, install the Hermes npm package into your app as a dependency, and (if running react-native from source) into react-native as well. For example, assuming your project is in the directory $AWESOME_PROJECT you would run this command:

# Install the Hermes NPM to be used by the project
( cd ${AWESOME_PROJECT?} && yarn add $HERMES_WS_DIR/hermes/npm/hermes-engine-v*.tgz )
# If running react-native from source, install it there as well
( cd ${AWESOME_PROJECT?}/node_modules/react-native && yarn add $HERMES_WS_DIR/hermes/npm/hermes-engine-v*.tgz )

You can now develop your app in the normal way.

Note: if you are making changes to the compiler in Hermes, be sure to make sure you test your app in release mode as this enables bytecode compilation in advance.

Reporting native crashes#

If Hermes causes a native crash in your application, a stack trace is critical for us to be able to understand where the crash occurred. If you have a native crash to report, please be aware that in most cases, it is not feasible for us to debug crashes in earlier versions of Hermes. Please update your app to a hermes-engine npm versions 0.5.1, 0.5.2-rc1, or later. Version 0.5.0 is too old. This may require you to update React Native as well.

At a minimum, you should include in your bug report the native stack trace as described in the the ndk-stack documentation. documentation

Including 10-20 lines from the logs before the line of asterisks can also help us understand what went wrong.

Additionally, you can also symbolicate the stack trace in order to indicate where in the Hermes source the failure is occurring. Symbolication will only work with the newer versions of Hermes listed above. It will not work with 0.5.0 or any earlier version.

The necessary unstripped libraries can be found in the GitHub release corresponding to the Hermes NPM you used to build your application. Download the hermes-runtime-android-vX.Y.Z.tar.gz file for your version. Unpack the tar file, then run ndk-stack using the contained directory corresponding to the build flavor and architecture for your app. For example, for a release arm64 build:

$ANDROID_NDK/ndk-stack -sym .../unstripped-release/0/lib/arm64-v8a < crash.txt

Including the symbolicated stack trace will make it easier for us to address your bug report more quickly.

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