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# source.reactCompiler

- **Type:** `boolean | ReactCompilerOptions`
- **Default:** `undefined` (disabled)

Whether to enable [React Compiler](https://react.dev/learn/react-compiler). React Compiler is a build-time tool that optimizes re-rendering performance of React applications through automatic memoization.

Modern.js implements this capability based on the Rust-based React Compiler built into Rspack's `builtin:swc-loader` (equivalent to setting SWC's `jsc.transform.reactCompiler`), reusing Rspack's built-in SWC transform chain without introducing Babel.

:::tip
This option is disabled by default. It must be enabled explicitly for any React version, including React 19.
:::

## Example

### Enable React Compiler (React 19)

```ts title="modern.config.ts"
import { defineConfig } from '@modern-js/app-tools';

export default defineConfig({
  source: {
    reactCompiler: true,
  },
});
```

### Using with React 18

The compiled output targets React 19 by default. To use it in React 18 projects, you need to:

1. Install [react-compiler-runtime](https://www.npmjs.com/package/react-compiler-runtime) as a **runtime dependency** (the compiled output references it at runtime):

```bash
npm add react-compiler-runtime
```

2. Specify the React version via `target`:

```ts title="modern.config.ts"
import { defineConfig } from '@modern-js/app-tools';

export default defineConfig({
  source: {
    reactCompiler: {
      target: '18',
    },
  },
});
```

### Customize compilation behavior

When passing an object, the options are the same as Rspack's `jsc.transform.reactCompiler`. For example, use `compilationMode: 'annotation'` to only compile functions annotated with the `"use memo"` directive:

```ts title="modern.config.ts"
import { defineConfig } from '@modern-js/app-tools';

export default defineConfig({
  source: {
    reactCompiler: {
      compilationMode: 'annotation',
    },
  },
});
```

For the complete list of options, see [Rsbuild - reactCompiler](https://rsbuild.rs/plugins/list/plugin-react#reactcompiler) and the [React Compiler configuration docs](https://react.dev/reference/react-compiler/configuration).