Tooling & release — Expo, EAS, OTA, Metro, native builds

Expo vs bare — when each

• CNG (Continuous Native Generation): ios/ android/ are build output, regenerated by npx expo prebuild (--clean) from app.json/app.config.ts + config plugins (withInfoPlist, withEntitlementsPlist, withAndroidManifest). Native tweaks live in a plugin — a hand edit is lost on the next prebuild. Upgrades become “regenerate”.
• Expo Go = a prebuilt sandbox with one SDK’s modules: no custom native code. A dev build (expo-dev-client) is your binary + dev menu — needed once any native library is added.
• Bare (npx @react-native-community/cli init): you own the Xcode/Gradle projects. Choose it for brownfield (RN inside an existing native app) or when a native team owns the projects. Default for new apps: Expo (the RN docs recommend a framework). Expo ≠ “no native code”.

EAS · OTA updates

• Build: cloud builders, eas.json profiles, managed certificates / provisioning profiles / keystore (--local to build on your Mac). Submit: eas submit -p ios → App Store Connect / TestFlight; Android → a Play track.
• Update: a build carries a channel; a channel points at a branch of updates. Promote/roll back by re-pointing the channel or republishing an older update. expo-updates checks at launch, downloads, applies on the next cold start (checkForUpdateAsync / fetchUpdateAsync / reloadAsync for urgent fixes); a launch crash falls back to the previous update.
• runtimeVersion policies: appVersion, nativeVersion, fingerprint (hash of native inputs — safest), or a manual string.
• Apple: downloaded JS is allowed if it does not change the app’s primary purpose, add a storefront, or bypass security — fixes and tweaks, not a different app. CodePush died with App Center (retired 31 Mar 2025).

Example — eas.json + app.json

{ "build": {                       // eas.json
    "development": { "developmentClient": true },
    "preview":     { "channel": "preview" },
    "production":  { "channel": "production" } } }
// app.json > expo:
"runtimeVersion": { "policy": "fingerprint" },
"updates": { "url": "https://u.expo.dev/<project-id>" }

Metro · env · upgrades · debugging

• Metro resolves .ios.tsx/.android.tsx/.native.tsx first, caches transforms (stale after a Babel/env change: npx expo start -c · --reset-cache); monorepos need watchFolders. Debug loads from Metro (:8081).
• Fast Refresh re-runs the edited module, keeps component state; a file exporting non-components remounts.
• Secrets: the bundle is public. EXPO_PUBLIC_* / react-native-config values are inlined. Keys stay on a server.
• Upgrades hurt: the native template moves (AppDelegate, Podfile, AGP, Kotlin, min iOS), every native dep must support the new RN, Xcode bumps too. Upgrade Helper = template diff per version. Small steps; Expo: npx expo install --fix + npx expo-doctor.
• Debug: React Native DevTools (Chrome DevTools on Hermes) — default since 0.76; Flipper left the template in 0.74. Native: Xcode, Instruments.

iOS · Android specifics

• iOS: Podfile use_react_native! + use_native_modules!; bundle exec pod install; open the .xcworkspace. Envs = schemes/configurations + xcconfig (own bundle ID). Build phase “Bundle React Native code and images” embeds main.jsbundle in Release.
• Android: gradle.properties (newArchEnabled, hermesEnabled); envs = productFlavors; ship an AAB with Play App Signing; R8 (enableProguardInReleaseBuilds) shrinks and obfuscates — reflection-using libs need keep rules.

Interview traps

• OTA JS needing a new native module → crash; use fingerprint.
• Hand-editing ios/ in a CNG project — gone after prebuild.
• API key in .env “hidden” — it is in the bundle.
• No source maps for an OTA update → unreadable prod stacks.

Remember

Native ⇒ store · JS ⇒ OTA · runtimeVersion is the contract.

Likely questions

1. What can OTA change? — JS + assets, same runtimeVersion.
2. Expo or bare? — Expo + CNG + EAS; bare for brownfield.
3. Why do upgrades hurt? — template, deps, toolchains move.
4. Symbolicate a JS crash? — source maps per build and update.