# Native Modules — React Native

Source: https://www.skillbyai.com/en/react-native/v-native

> When JavaScript is not enough.

## Using and writing native modules

Most needs are covered by existing libraries; check the React Native Directory and Expo SDK first, and confirm a library supports the New Architecture. Libraries with native code require a **development build**, not Expo Go, and in Expo projects many include a **config plugin** that edits native project settings during prebuild. When you must write native code, the **Expo Modules API** lets you write modules in **Swift** and **Kotlin** with a small declarative DSL (`Name`, `Function`, `AsyncFunction`, `Events`, `View`) and works in Expo and bare apps. The core React Native alternative is a **Turbo Native Module** with a typed spec processed by Codegen. You can scaffold an Expo module with `npx create-expo-module`.

## A tiny Expo module in Swift

Expo Modules API sketch; check the docs for your version.

```typescript
// ios/BatteryModule.swift (Swift, shown as a comment block)
// import ExpoModulesCore
// public class BatteryModule: Module {
//   public func definition() -> ModuleDefinition {
//     Name("Battery")
//     Function("isLowPowerMode") { () -> Bool in
//       ProcessInfo.processInfo.isLowPowerModeEnabled
//     }
//   }
// }

// src/Battery.ts - calling it from TypeScript
import { requireNativeModule } from "expo-modules-core";

const Battery = requireNativeModule("Battery");

export function isLowPowerMode(): boolean {
  return Battery.isLowPowerMode();
}
```

## Keep the native surface small

Expose a few well-typed functions and do the platform work natively. A narrow API is easier to keep identical on iOS and Android.

## An adapter plug

A native module is a travel adapter: your JavaScript plug stays the same, and the adapter fits it into each country's socket.

**Quiz:** Which tool lets you write native modules in Swift and Kotlin with a declarative DSL?

- [x] The Expo Modules API
- [ ] AsyncStorage
- [ ] StyleSheet.create
- [ ] The Metro bundler config

*Answer:* The Expo Modules API. Expo modules work in Expo and bare React Native apps.