Expo

Cross-platform mobile with Expo managed workflow. Setup, native features (EAS Build/Update), deployment with TypeScript and React Native.

When to Use

Cross-platform mobile (iOS + Android)
Expo managed workflow
EAS Build/Update config
Native features via Expo SDK
OTA updates

Don’t use for:

Bare React Native (use react-native skill)
Web-only apps
Unsupported custom native modules

Critical Patterns

✅ REQUIRED: Use Expo SDK for Native Features

// ✅ CORRECT: Expo SDK modules
import * as Location from "expo-location";
import { Camera } from "expo-camera";

// ❌ WRONG: Direct React Native linking (use Expo modules)
import { NativeModules } from "react-native";

✅ REQUIRED: Handle Permissions Properly

// ✅ CORRECT: Request permissions before using
const { status } = await Location.requestForegroundPermissionsAsync();
if (status === "granted") {
  const location = await Location.getCurrentPositionAsync();
}

// ❌ WRONG: No permission check
const location = await Location.getCurrentPositionAsync(); // May crash

✅ REQUIRED: Use TypeScript

// ✅ CORRECT: Typed props and state
interface Props {
  title: string;
}

const Component: React.FC<Props> = ({ title }) => {
  const [count, setCount] = useState<number>(0);
};

✅ REQUIRED: EAS Build Configuration

Configure build profiles before submitting to app stores.

{
  "build": {
    "development": { "developmentClient": true, "distribution": "internal" },
    "preview": { "distribution": "internal" },
    "production": {}
  }
}

# Build for all platforms (production)
eas build --platform all --profile production
# Run development build on device
eas build --platform ios --profile development

✅ REQUIRED: Over-the-Air Updates (EAS Update)

Configure OTA updates in app.json. Match runtimeVersion to native changes.

{
  "expo": {
    "updates": { "url": "https://u.expo.dev/<project-id>" },
    "runtimeVersion": { "policy": "appVersion" }
  }
}

# Publish JS-only update (no native change needed)
eas update --branch production --message "Fix login crash"

❌ NEVER: Eject Without a Clear Reason

// ❌ WRONG: Ejecting to bare workflow for minor native customization
// Loses: Expo Go dev testing, managed OTA updates, EAS Build simplicity

// ✅ CORRECT: Config plugins handle most native customizations
const { withInfoPlist } = require('@expo/config-plugins');
module.exports = withInfoPlist(config, (config) => {
  config.modResults.NSCameraUsageDescription = "For QR scanning";
  return config;
});
// Only eject when config plugins cannot satisfy the requirement

Decision Tree

Native feature?
  → Check Expo SDK first

Custom native code?
  → Config plugins or eject

Store builds?
  → EAS Build (cloud)

OTA updates?
  → EAS Update

Platform code?
  → Platform.select() or .ios.tsx/.android.tsx

Testing?
  → Expo Go (dev), devices/simulators (final)

Navigation?
  → Expo Router or React Navigation

Example

import { StatusBar } from 'expo-status-bar';
import { StyleSheet, Text, View } from 'react-native';

export default function App() {
  return (
    <View style={styles.container}>
      <Text>Welcome to Expo</Text>
      <StatusBar style="auto" />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
});

Edge Cases

Platform-specific code: Use Platform.select() for inline differences; platform-specific file extensions (.ios.tsx, .android.tsx) for large divergences between platforms.
Permission denial: Always handle the denied state — show an explanation UI and guide the user to device settings. Never assume permission is granted on subsequent launches.
OTA update conflicts: JS-only changes publish via eas update. Any native code change (new SDK module, config plugin change) requires a new app store build with a matching runtimeVersion bump.
Offline mode: Use @react-native-community/netinfo to detect connectivity. Cache data with AsyncStorage or MMKV. Show a visible offline indicator and gracefully block network-dependent actions.
Custom native modules: Use Expo config plugins first. Use a development client (expo-dev-client) for modules not available in Expo Go. Only eject to bare workflow when config plugins cannot satisfy the requirement.

Checklist

eas init run — project linked to EAS
eas.json configured with development, preview, production profiles
Permissions declared in app.json (android.permissions, ios.infoPlist)
OTA updates configured (updates.url, runtimeVersion.policy)
Platform-specific code uses Platform.select() or platform file extensions
Offline state handled (network detection + cached data fallback)
TypeScript strict mode enabled
All native features use Expo SDK modules (not direct React Native linking)
Expo Go used for dev, physical device/simulator for final testing