Question 1

Would you choose React Native, Flutter, or native (Swift/Kotlin) for a new app needing iOS and Android support with a team of 3? Explain your decision based on specific trade-offs.

Accepted Answer

There is no single right answer - what matters is the reasoning: (1) React Native - team has JS/React background, strong ecosystem, but bridge overhead and JS thread; (2) Flutter - Dart learning curve but near-native performance, truly single codebase (including UI rendering), great for UI-heavy apps; (3) Native - best performance, full API access, but 2 codebases = double effort for a small team. With a 3-person team and limited budget: Flutter or React Native is more realistic. Strong candidates will ask about the app type (game, e-commerce, fintech), required features (camera, biometrics, BLE), and team background before deciding. Avoid candidates who only say 'I'm familiar with React Native so I'll choose it'.

Question 2

Your app has users complaining about lag when scrolling long lists (FlatList with 1000+ items). How would you debug and optimize this?

Accepted Answer

Debug before optimizing: (1) Use React DevTools Profiler + Flipper to identify unnecessary re-renders, (2) Check FlatList config: `getItemLayout` to avoid dynamic measurement, correct `keyExtractor`, `windowSize` and `maxToRenderPerBatch`, (3) Optimize render: `React.memo` for list item component, avoid inline functions in render, avoid anonymous objects in props, (4) If still slow: consider FlashList (Shopify) instead of FlatList, or custom virtualization. Good candidates will mention `getItemLayout` as the first step since it is usually the biggest cause of FlatList scroll jank. Ask how they measure 'lag' - by frame rate (Perf Monitor in dev mode) or just by feel?

Question 3

You are asked to build a food ordering app that needs to work offline when the network is unavailable. Design an offline-first architecture for this app.

Accepted Answer

Offline-first architecture: (1) Local database - SQLite (via WatermelonDB or expo-sqlite) or AsyncStorage for small data; (2) Sync engine - conflict resolution strategy (last-write-wins vs. CRDT), queue operations when offline; (3) Network layer - retry logic with exponential backoff, optimistic UI updates; (4) UX considerations - display sync status, handle conflicts when a user adds an item offline that was then deleted on the server. The differentiator for strong candidates is conflict handling. Ask: 'If a user adds a product to the cart while offline, but when they go back online the product is out of stock, how do you handle it?' A strong answer will discuss optimistic UI and rollback.

Question 4

Have you implemented push notifications on mobile? Describe the architecture from server to device and the edge cases you need to handle.

Accepted Answer

Architecture: Server → FCM (Firebase Cloud Messaging)/APNs → Device. Details: (1) Token management - device token changes on reinstall, needs server sync; (2) Permission flow - iOS requires explicit permission, Android 13+ too; (3) Background vs. foreground handling differs; (4) Notification payload types: notification-only, data-only (background processing), notification+data; (5) Edge cases: app force-killed, device offline (how long does FCM queue?), notification grouping, deep linking on tap. Deep linking on notification tap is a point many developers overlook. Ask specifically: 'When a user taps a notification and the app is closed, how do you handle navigation?' - requires handling initialNotification in React Native Firebase.

Question 5

Have you set up CI/CD for a mobile app? Describe your pipeline from code commit to the app landing on TestFlight/Firebase App Distribution.

Accepted Answer

Typical pipeline: (1) Trigger: PR merge to develop/main, (2) Lint + Unit tests (Jest), (3) Build: Fastlane to automate signing (match for iOS certificates), (4) iOS: xcodebuild → .ipa → TestFlight via Fastlane pilot; Android: gradle → .aab → Firebase App Distribution or Play Console internal track; (5) Notify: Slack message with build status and download link, (6) Platform: GitHub Actions, Bitrise, or EAS Build (Expo). iOS certificate management is the biggest pain point in mobile CI/CD. Strong candidates will mention Fastlane Match (cert management) or Xcode Cloud. Ask how they handle when a certificate expires in production.

Question 6

What problems have you encountered with displaying and entering Vietnamese text in a mobile app? What are the specific technical challenges with fonts, virtual keyboards, and diacritics?

Accepted Answer

Specific challenges: (1) Fonts - need full Unicode Vietnamese support (Noto Sans, Inter). Some fonts are missing glyphs, especially compound diacritics like ộ, ướ; (2) Keyboard height - Vietnamese keyboards (Telex, VNI) take more space, need proper KeyboardAvoidingView handling; (3) String length - 'Việt Nam' is more bytes than visible characters, affects character limits and text truncation; (4) Search/sorting - normalize to non-diacritic form for search, use `localeCompare` with `vi` locale for sorting. This is a very Vietnam-specific practical question. Candidates with experience building apps for the Vietnamese market will answer in detail. Ask further about how they test with long Vietnamese names like 'Nguyễn Thị Thanh Hương' in short input fields.

Question 7

How have you handled state management in a mobile app? Compare Redux, Zustand, Jotai, and Context API - when would you choose each?

Accepted Answer

Decision framework based on scale and team size: (1) Context API - suitable for simple global state (theme, language, auth user), not suitable for frequently updated state (re-renders all consumers); (2) Zustand - sweet spot for most apps: minimal boilerplate, performant, selector pattern, easy devtools; (3) Redux Toolkit - justified for large teams needing strict patterns, time-travel debugging; (4) Jotai/Recoil - atom-based, great for fine-grained reactivity. For React Native: avoid over-engineering, Zustand is usually the optimal choice. Look for candidates who understand why Context API causes performance issues (every time the value changes, the entire component tree using that context re-renders) and how to solve it with context splitting or memo.