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Hermes on Windows + macOS

Hermes

The Hermes engine is an open source JavaScript engine created by Facebook to optimize building and running React Native applications.

To learn more about what it is and how to use it, check out the React Native documentation for it.

Hermes on Windows:

Hermes is experimentally supported on Windows, generally providing better performance characteristics than the default Chakra engine.

Enabling Hermes for new projects

The easiest way to enable Hermes is to pass the --useHermes flag to react-native-windows-init when creating a new project. This will set up your project to use the Hermes engine and to generate bundles as Hermes bytecode instead of JavaScript.

Using Hermes in an existing project

Hermes can be enabled in existing projects with a few edits.

First, set the UseHermes property to true in the ExperimentalFeatures.props file in your project's windows directory:

<PropertyGroup Label="Microsoft.ReactNative Experimental Features">
  ...
  <UseHermes>true</UseHermes>
</PropertyGroup>

Second, add the Hermes NuGet package to packages.config, found next to your projects vcxproj file:

<packages>
  ...
  <package id="ReactNative.Hermes.Windows" version="0.7.2" targetFramework="native" />
</packages>

Finally, edit the ExtensionTargets ItemGroup in your project's vcxproj file to import targets from the NuGet package:

<ImportGroup Label="ExtensionTargets">
  ...
  <Import Project="..\packages\ReactNative.Hermes.Windows.0.7.2\build\native\ReactNative.Hermes.Windows.targets" Condition="Exists('..\packages\ReactNative.Hermes.Windows.0.7.2\build\native\ReactNative.Hermes.Windows.targets')" />
</ImportGroup>

Disabling Hermes

If you've built a project using Hermes and want to opt-out, the above instructions may be reversed.

Known limitations

• Hermes is not yet supported when using C# projects
• Hermes debugger is not yet supported
• Enabling Hermes leads to a very large download on initial build. This will become smaller in the future
• Hermes is not yet compatible with other experimental features, like NuGet distribution or WinUI 3
• The binary for Hermes is not yet signed by Microsoft
• Debug builds of an app that uses Hermes are very slow

Hermes on macOS

Hermes is available on React Native for macOS, provided you are using version 0.62 or higher. To learn how to upgrade to the latest version, check out the Upgrading section of the macOS Getting Started guide.

After you have upgraded to the latest version of React Native for macOS, install and add the following:

1. Install the npm package yarn add 'hermes-engine-darwin@^0.4.3'
2. Add (or uncomment) the following pod dependencies to your macOS target in your Podfile:
   

pod 'React-Core/Hermes', :path => '../node_modules/react-native-macos/'
pod 'hermes', :path => '../node_modules/hermes-engine-darwin'
pod 'libevent', :podspec => '../node_modules/react-native-macos/third-party-podspecs/libevent.podspec'

3. Run pod install

Be sure to set your target's deployment target to at least 10.14 before running pod install