> For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt.

# HermesBytecode Plugin

This plugin is converts JavaScript chunks into [Hermes](https://hermesengine.dev/) bytecode for production-level performance optimization.
It also converts related source maps to be compatible with bytecode bundles.

:::warning Remember to exclude the main bundle!
If you enable Hermes in your project, your `index.bundle` file and it's source-map will be transformed by `react-native`.
You should exclude it from the plugin to avoid processing it twice.
:::

:::details Implementation details
This plugin will only transform assets that are emitted after the compilation. To ensure that asset is always emitted we disable the `compareBeforeEmit` option which is enabled by default in Rspack/webpack.

`compareBeforeEmit` option is used to skip emitting assets that are identical to the
ones present in build directory, which might result in transformation being
skipped when there is a untransformed bundle present in the build directory.

Since this plugin is only meant to be used as a production-level optimization, it should not affect your development experience.
:::

## Usage

```js title="rspack.config.cjs"
const Repack = require("@callstack/repack");

module.exports = {
  plugins: [
    new Repack.plugins.HermesBytecodePlugin({
      // options
    }),
  ],
};
```

## Options

### enabled

- Type: `boolean`
- Required: `true`

Use this to enable or disable the plugin. You should only enable this plugin when building for production environment.

### test

- Type: `string[] | RegExp | RegExp[]`
- Required: `true`

Matches all modules that match this resource, and will match against Resource (the absolute path without query and fragment).

### include

- Type: `string[] | RegExp | RegExp[]`
- Default: `undefined`

Matches all modules that match this condition against the absolute path of the resource (without query and fragment).

### exclude

- Type: `string[] | RegExp | RegExp[]`
- Default: `undefined`

Excludes all modules that match this condition and will match against the absolute path of the resource (without query and fragment).

### hermesCLIPath

- Type: `string`
- Default: `undefined`

Path to the Hermes command-line tool. If not specified, the plugin will try to find it automatically.

### reactNativePath

- Type: `string`
- Default: `undefined`

Use this to specify the path to the React Native package. If not specified, the plugin will try to find it automatically.

### compareBeforeEmit

- Type: `boolean`
- Default: `undefined`

Use this to force enable `compareBeforeEmit` Rspack/webpack's output option.

## Example

```js title="rspack.config.cjs"
const Repack = require("@callstack/repack");

module.exports = (env) => {
  const { mode } = env;

  return {
    plugins: [
      new Repack.plugins.HermesBytecodePlugin({
        enabled: mode === "production",
        test: /\.bundle$/,
        exclude: /index.bundle$/,
      }),
    ],
  };
};
```