Debug: React Native app crashes on startup

React Native app crashes immediately on startup without showing any UI. Metro bundler may or may not show errors.

Systematic debugging:

1. Check Metro bundler for errors:

npx react-native start --reset-cache

1. View native crash logs:

# iOS:
npx react-native log-ios
# Android:
adb logcat *:E | grep -i 'react\|fatal\|crash'

1. Common causes:

a) Native module not linked:
cd ios && pod install && cd ..
# Android: clean build
cd android && ./gradlew clean && cd ..

b) Missing native dependency:
npx react-native doctor # Check environment

c) JavaScript syntax error:
# Check Metro output for red error screen
# If app crashes before error screen shows:
# Add try/catch in index.js

d) Version mismatch:
# React Native, React, and native modules must be compatible
npx react-native info # Show versions

e) Android SDK/Gradle issues:
# Check android/build.gradle for correct versions
# Ensure ANDROID_HOME is set

f) iOS certificate/provisioning:
# Xcode > Product > Clean Build Folder
# Delete DerivedData

1. Nuclear options:

rm -rf node_modules && npm install
cd ios && rm -rf Pods && pod install
npx react-native start --reset-cache