Mobile Framework Expo

Expo Development Patterns

Quick Guide: Build production-ready React Native apps with Expo. Use managed workflow with Continuous Native Generation for most projects, Expo Router for file-based navigation, and EAS for builds/updates. Development builds replace Expo Go for production testing.

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use development builds for production testing - Expo Go is for prototyping only)

(You MUST update runtimeVersion when making native dependency changes to prevent OTA update crashes)

(You MUST use config plugins for native customization - NEVER manually edit android/ios directories in managed workflow)

(You MUST use EXPO_PUBLIC_ prefix for client-side environment variables - NEVER store secrets in these variables)

Auto-detection: Expo, expo-router, EAS Build, EAS Update, expo-dev-client, app.config.js, app.json, expo prebuild, npx expo, eas.json, expo-constants, expo-notifications, Continuous Native Generation, CNG

When to use:

• Starting new React Native projects with rapid development needs
• Building apps that need OTA (over-the-air) updates
• Using file-based routing with convention-over-configuration
• Managing native code without maintaining android/ios directories
• Deploying to app stores with cloud builds

Key patterns covered:

• Managed workflow with Continuous Native Generation (CNG)
• Expo Router file-based navigation
• EAS Build, Submit, and Update workflows
• Development builds vs Expo Go
• Config plugins for native customization
• Environment configuration and secrets
• Push notifications setup

When NOT to use:

• Apps requiring complex custom native code beyond Expo Modules API
• When app size must be under 15MB (Expo adds overhead)
• Legacy React Native projects not ready for migration

Philosophy

Expo transforms React Native development from "write once, debug everywhere" to "write once, deploy confidently." The key insight is that most apps don't need direct native access - they need well-maintained native modules with consistent APIs.

Core principles:

1. Managed by default - Let Expo handle native complexity; prebuild only when necessary
2. Continuous Native Generation - Treat android/ios as build artifacts, not source code
3. Development builds for truth - Expo Go is for learning; development builds show production reality
4. OTA for velocity - Ship JavaScript updates without app store delays
5. Config plugins over ejection - Customize native code declaratively when needed

Mental model:

Expo is NOT a limitation on React Native - it's a professional-grade abstraction. You can always drop down to native code via Expo Modules API or prebuild, but most apps never need to.

Core Patterns

Pattern 1: Dynamic Configuration with app.config.ts

Use app.config.ts for environment-specific builds. Use named constants for SDK versions and build numbers.

// app.config.ts - Environment-aware config
const IS_PRODUCTION = process.env.APP_ENV === "production";
const BUILD_NUMBER = 1;

export default ({ config }: ConfigContext): ExpoConfig => ({
  ...config,
  name: IS_PRODUCTION ? "MyApp" : "MyApp (Dev)",
  ios: {
    bundleIdentifier: IS_PRODUCTION ? "com.app" : "com.app.dev",
    buildNumber: String(BUILD_NUMBER),
  },
  android: {
    package: IS_PRODUCTION ? "com.app" : "com.app.dev",
    versionCode: BUILD_NUMBER,
  },
});

Full examples: examples/core.md - App Configuration section

Pattern 2: Config Plugins for Native Customization

Modify native code declaratively -- changes survive expo prebuild --clean. Use config plugins for permissions, SDK versions, and native settings.

// app.config.ts plugins array
plugins: [
  [
    "expo-camera",
    { cameraPermission: "Allow $(PRODUCT_NAME) to access your camera." },
  ],
  [
    "expo-build-properties",
    { android: { minSdkVersion: 24 }, ios: { deploymentTarget: "15.1" } },
  ],
];

Full examples: examples/core.md - Config Plugins section

Pattern 3: Environment Variables

Use EXPO_PUBLIC_ prefix for client-side variables. Metro requires direct property access -- destructuring and bracket notation don't work.

// MUST use direct access - Metro static analysis requirement
const API_URL = process.env.EXPO_PUBLIC_API_URL; // Works
// const { EXPO_PUBLIC_API_URL } = process.env;  // BROKEN - undefined at runtime

Full examples: examples/core.md - Environment Variables section

Pattern 4: Development Builds

Use expo-dev-client for production-accurate testing. Expo Go is for prototyping only -- it lacks your native dependencies, push notifications, and accurate splash screens.

# Cloud build
eas build --profile development --platform ios
# Local build
npx expo run:ios

Full configuration: examples/eas.md - Development Builds section

Pattern 5: Asset Management

Block splash screen while loading fonts, use expo-image for remote images with blur hash placeholders and disk caching.

SplashScreen.preventAutoHideAsync();
// Load fonts, then call SplashScreen.hideAsync() when ready

Full examples: examples/core.md - Font Loading and Image Handling sections

RED FLAGS

• Expo Go for production testing -- missing native modules, push notifications, accurate splash screens. Always use development builds.
• Not updating runtimeVersion after native changes -- OTA updates crash on apps with incompatible native code. Use "fingerprint" policy for automatic detection.
• Storing secrets in EXPO_PUBLIC_ variables -- embedded in JS bundle, visible to anyone who decompiles. Use EAS Secrets and backend proxies.
• Manually editing android/ios directories -- changes lost on expo prebuild --clean. Use config plugins.
• Destructuring process.env -- Metro requires direct property access (process.env.EXPO_PUBLIC_*). Destructuring and bracket notation produce undefined.
• Using expo-av -- removed in SDK 55. Migrate to expo-video and expo-audio.
• Legacy Architecture -- removed after SDK 54. React Native 0.82+ requires New Architecture.

Full anti-patterns and gotchas: reference.md

Detailed Resources:

• examples/core.md - Project config, environment variables, fonts, images
• examples/router.md - File-based routing, tabs, auth flows, modals
• examples/eas.md - Cloud builds, app store submission, OTA updates
• reference.md - Decision frameworks, SDK compatibility, anti-patterns

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST use development builds for production testing - Expo Go is for prototyping only)

(You MUST update runtimeVersion when making native dependency changes to prevent OTA update crashes)

(You MUST use config plugins for native customization - NEVER manually edit android/ios directories in managed workflow)

(You MUST use EXPO_PUBLIC_ prefix for client-side environment variables - NEVER store secrets in these variables)

Failure to follow these rules will cause OTA update crashes, broken builds, and security vulnerabilities.

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