Easy Mobile App Development Interview Questions

Show answer

Native apps are written in platform languages, Kotlin for Android and Swift for iOS, with full access to platform APIs and the best performance. Cross-platform frameworks like Flutter and React Native share one codebase and render native-feeling UI, saving time for two platforms. Hybrid apps, such as Ionic or Cordova, run web code inside a WebView. Choice depends on performance needs, team skills, budget and how much platform-specific UI is required.

When would you choose a cross-platform framework over native development?

Show answer

Choose cross-platform when you need both Android and iOS quickly with a small team, the app is mostly forms, lists, content and API-driven screens, and design consistency across platforms is desired. Choose native when the app needs heavy graphics, deep hardware integration, advanced background processing, newest platform features on day one, or very tight performance. Many companies mix both, using native modules inside a cross-platform app.

What is the difference between a debug and release build in mobile apps?

Show answer

Debug builds include debugging tools, hot reload, assertions and unoptimised code, so they are slower and larger. Release builds are compiled ahead of time, minified, optimised and signed with a release key. In Flutter, debug uses JIT while profile and release use AOT compilation; in React Native, release bundles minified JavaScript precompiled to Hermes bytecode. Always measure performance and test critical flows on release or profile builds.

What is hot reload and how is it different from hot restart?

Show answer

Hot reload injects changed code into the running app and rebuilds the UI while keeping the current state, so you see changes in about a second. Hot restart reloads the whole app code and resets state, needed when changing initialisation logic, global variables or app startup. React Native offers Fast Refresh, similar to hot reload. Native code changes, such as new plugins or platform files, require a full rebuild.

What is a WebView and when is it appropriate to use one?

Show answer

A WebView embeds a browser engine in your app to show web content. It suits help pages, terms, payment gateway pages or quickly reusing an existing web flow. It is not ideal for core app screens because it feels slower and less native. Restrict navigation to trusted domains, avoid exposing powerful JavaScript bridges to untrusted content, and handle back navigation. Flutter uses webview_flutter and React Native uses react-native-webview.

Your app works on Android but crashes immediately on iOS when the user taps the camera button. What is the likely cause?

Show answer

The most common cause is a missing NSCameraUsageDescription key in Info.plist. iOS terminates the app when it requests a protected resource without a usage description string. Add descriptions for camera, photo library, microphone and location as required by the plugin, written clearly for the user. Also verify the plugin's iOS setup steps, run pod install, and test on a real device since simulators lack cameras.

Why do you want to work in mobile app development?

Show answer

Answer with genuine motivation and proof. For example: I enjoy building things people use every day in their pocket, and mobile gives fast feedback from real users. I started by building an attendance app for my college, published it for classmates, and improved it from their reviews. That experience of seeing ratings and usage grow made me choose mobile development as my career.

What is the role of a mobile developer in an Agile team?

Show answer

A mobile developer turns user stories and designs into working app features, estimates tasks in sprint planning, raises technical risks early, and collaborates with backend, design and QA. They join standups, demos and retrospectives, review peers' code and keep builds releasable. They also give input on platform constraints like store review times and OS limits, which affect release planning more than in web development.

What is Dart and why does Flutter use it?

Show answer

Dart is a typed, object-oriented language created for client development. Flutter uses it because Dart supports JIT compilation for fast hot reload in development and AOT compilation to native machine code for fast release apps. Dart has sound null safety, async-await, isolates for concurrency, and a garbage collector optimised for many short-lived objects, which suits Flutter's frequent widget rebuilding.

Explain sound null safety in Dart.

Show answer

Since Dart 3, all code is null safe. Types are non-nullable by default, so String cannot hold null while String? can. The compiler forces you to handle nulls with checks, the ?. and ?? operators, or the ! operator which throws if null. The late keyword declares a non-null variable initialised later. Sound means the compiler can trust non-null types completely, enabling optimisations and preventing many runtime null errors.

What is the difference between final and const in Dart?

Show answer

final means a variable is assigned once at runtime and cannot be reassigned. const means the value is a compile-time constant and deeply immutable. In Flutter, const constructors like const Text("Hello") let the framework reuse the same widget instance across rebuilds, skipping unnecessary work. Use const wherever possible for widgets with fixed values; the linter rule prefer_const_constructors helps enforce it.

What is the difference between StatelessWidget and StatefulWidget?

Show answer

A StatelessWidget has no mutable state; its build output depends only on constructor parameters and inherited data. A StatefulWidget creates a State object that persists across rebuilds and holds mutable data; calling setState marks it dirty so build runs again. Use stateful widgets for local UI state like animations, text controllers or toggles, and lift shared or business state into a state management solution.

How do Future, async and await work in Dart?

Show answer

A Future represents a value that will be available later, like an HTTP response. Marking a function async lets it use await, which pauses that function until the Future completes without blocking the event loop. Errors are handled with try-catch. Dart runs on a single-threaded event loop per isolate, so awaiting network calls keeps the UI responsive. Use Future.wait to run independent futures concurrently.

Write a simple counter using StatefulWidget and setState.

Show answer

class Counter extends StatefulWidget { const Counter({super.key}); @override State<Counter> createState() => _CounterState(); } class _CounterState extends State<Counter> { int _count = 0; @override Widget build(BuildContext context) => Column( mainAxisSize: MainAxisSize.min, children: [ Text('Count: $_count'), FilledButton( onPressed: () => setState(() => _count++), child: const Text('Add'), ), ], ); }

Explain Row, Column, Expanded and Flexible in Flutter layouts.

Show answer

Row lays out children horizontally and Column vertically, with mainAxisAlignment and crossAxisAlignment controlling placement. Expanded forces a child to fill remaining space along the main axis, and Flexible lets a child take up to its share but be smaller. Using flex values divides space proportionally. A common error is an unbounded width or height, such as a ListView inside a Column without Expanded, which throws a layout exception.

What is the difference between ListView and ListView.builder?

Show answer

The default ListView constructor builds all children at once, which is fine for a few items. ListView.builder builds items lazily as they scroll into view, using itemBuilder and itemCount, so it handles long or infinite lists efficiently. For mixed scrolling content like headers, grids and lists together, use CustomScrollView with slivers. For large lists, also set item extent or use prototypeItem to improve scroll performance.

A Flutter screen shows a yellow and black overflow stripe on small phones. How do you fix it?

Show answer

The overflow stripe means content exceeds the available space, often a Row with long text or a Column taller than the screen. Wrap text in Expanded or Flexible with overflow ellipsis, make the screen scrollable with SingleChildScrollView or a ListView, use LayoutBuilder to adapt for narrow widths, and avoid fixed heights. Also handle the keyboard by allowing the content to scroll when it opens.

Write a Dart model class with fromJson and toJson.

Show answer

class Product { const Product({required this.id, required this.name, required this.price}); final String id; final String name; final double price; factory Product.fromJson(Map<String, dynamic> json) => Product( id: json['id'] as String, name: json['name'] as String, price: (json['price'] as num).toDouble(), ); Map<String, dynamic> toJson() => {'id': id, 'name': name, 'price': price}; } // For larger apps use json_serializable or freezed to generate this.

Describe a time you helped a teammate who was stuck on a problem.

Show answer

Situation: a new teammate spent two days on a Flutter layout that kept overflowing. Task: help without taking over. Action: I sat with them, showed how to use the widget inspector to see constraints, and explained the constraints go down, sizes go up rule with their own screen. Result: they fixed it in an hour and later solved similar issues alone. I also added the explanation to our team wiki.

Why do Flutter apps need a state management solution beyond setState?

Show answer

setState works for local UI state inside one widget, but shared state like the logged-in user, cart or settings must be read and changed by many screens. Passing it through constructors becomes messy, and rebuilding large trees hurts performance. State management libraries separate business logic from UI, make state testable, rebuild only widgets that depend on changed data, and give a clear, predictable flow of updates.

How do you handle navigation in Flutter with go_router?

Show answer

go_router is a declarative, URL-based routing package maintained by the Flutter team. You define GoRoute entries with paths like /product/:id, navigate with context.go or context.push, and read parameters from state.pathParameters. It supports nested navigation with ShellRoute for bottom tabs, redirects for authentication guards, and deep links on mobile and web. It wraps the Router API, which is complex to use directly.

How do you handle app-wide theming and dark mode in Flutter?

Show answer

Define ThemeData for light and dark modes in MaterialApp, using ColorScheme.fromSeed to generate consistent Material 3 colours, and set themeMode from a settings state, saved in shared preferences. Widgets read Theme.of(context).colorScheme and textTheme instead of hardcoded colours. Add ThemeExtension classes for brand-specific tokens. This keeps the design consistent and makes switching themes a single state change.

How do you explain your architecture choices to a new team member?

Show answer

I start with the problem each layer solves, using one real feature from our app to trace a user tap through widget, state notifier, repository and API, and back to the UI. I share a short diagram and the team's architecture document, then give them a small task touching every layer. I review their first pull request closely and invite questions so they understand the reasons, not only the rules.

What is an Expo project and how does it differ from a bare React Native project?

Show answer

Expo is a framework and toolset for React Native that provides prebuilt native modules, file-based routing with Expo Router, EAS cloud builds and over-the-air updates. With Continuous Native Generation, Expo creates the android and ios folders from app config and config plugins, so you rarely edit native code. A bare project manages native folders directly. Expo is now the officially recommended way to start React Native apps.

What is React Native and how does it work?

Show answer

React Native lets you build mobile apps with React and JavaScript or TypeScript. You write components like View, Text and Image, and React Native renders them as real native views on Android and iOS. JavaScript runs on the Hermes engine, and with the New Architecture it communicates with native code directly through JSI rather than an asynchronous JSON bridge. Business logic and most UI code are shared between platforms.

Explain the core React Native components.

Show answer

View is the basic container, like a div, supporting flexbox layout and styles. Text displays text, and all strings must be inside Text. Image shows images from network or local sources. TextInput handles keyboard input. Pressable handles touches with press states. ScrollView scrolls a small amount of content, while FlatList and SectionList render long lists virtually. SafeAreaView or the safe area context library keeps content away from notches.

How does styling and layout work in React Native?

Show answer

Styles are JavaScript objects created with StyleSheet.create, using camelCase properties like backgroundColor. Layout uses flexbox through the Yoga engine, with flexDirection defaulting to column, unlike the web. Units are density-independent pixels without px. There is no CSS cascade, so styles are applied per component and composed with arrays. Libraries like NativeWind bring Tailwind-style classes, and themes are usually provided through context.

What is the difference between ScrollView and FlatList?

Show answer

ScrollView renders all its children at once, which is fine for short content but uses lots of memory and slows rendering for long lists. FlatList virtualises, rendering only items near the viewport and recycling as you scroll, with props like keyExtractor, initialNumToRender and windowSize. For very large or complex lists, FlashList offers better recycling and performance. Never nest a FlatList inside a ScrollView of the same direction.

Explain useState and useEffect in React Native.

Show answer

useState stores local state and returns the value and a setter; calling the setter re-renders the component. useEffect runs side effects after render, like fetching data or subscribing to events, with a dependency array controlling when it re-runs, and a returned cleanup function to unsubscribe. Missing dependencies cause stale values, while unstable dependencies cause loops. For data fetching, libraries like TanStack Query are usually better than raw useEffect.

Why is TypeScript recommended for React Native projects?

Show answer

TypeScript adds static types to JavaScript, catching mistakes like wrong prop names, missing fields in API responses and undefined values at compile time. It improves editor autocomplete and makes refactoring safer in large codebases. New React Native and Expo templates use TypeScript by default, and Codegen for TurboModules and Fabric components uses typed specs. Combined with strict mode, it significantly reduces runtime crashes.

How do you handle platform-specific code in React Native?

Show answer

Use Platform.OS checks or Platform.select({ ios: ..., android: ... }) for small differences in styles or behaviour. For larger differences, create files with platform extensions like DatePicker.ios.tsx and DatePicker.android.tsx, and the bundler picks the right one automatically. Keep platform branches inside small components so screens stay shared. Native platform differences, such as permissions setup, are configured in native files or Expo config plugins.

A React Native app shows a red screen error in development: Text strings must be rendered within a Text component. What does it mean?

Show answer

React Native requires every string to be inside a Text component. The error usually appears when a string or a number is placed directly inside a View, or when a conditional like {count && <Badge />} renders 0 as text when count is zero. Wrap text in Text and use explicit conditions like {count > 0 ? <Badge /> : null} so falsy values are not rendered as raw strings.

How do you handle feedback that your code is not good enough?

React Native Navigation, State and Native ModulesEasy

Show answer

I treat it as information, not a personal attack. I ask for specific examples and what better looks like, fix the issues, and look for patterns so I do not repeat them. Early in my career, a reviewer pointed out that my components were too large; I learned to split them and write small hooks, and my review comments dropped steadily after that. I also thank reviewers for their time.

How does navigation work in React Native with React Navigation?

Show answer

React Navigation provides navigators: native stack for push and pop screens using platform transitions, bottom tabs, drawer and material top tabs. You nest navigators, for example tabs inside a root stack with modal screens. Screens receive navigation and route props or use hooks like useNavigation and useRoute. Version 7 adds a static configuration API for better TypeScript inference and automatic deep link setup.

How do you pass parameters between screens and type them safely?

React Native Navigation, State and Native ModulesEasy

Show answer

Pass small, serialisable params like IDs with navigation.navigate('Details', { id }), and read them with route.params. Define a param list type, such as type RootStackParamList = { Details: { id: string } }, and use it with the navigator and hooks for type checking. Avoid passing functions or large objects, which break state persistence and deep linking; fetch full data on the destination screen.

How do you stay productive when working with both Android and iOS platforms?

React Native Navigation, State and Native ModulesEasy

Show answer

I test every feature on one Android device and one iPhone or simulator before raising a pull request, keep a checklist of common platform differences like permissions, keyboard and safe areas, and use CI builds for both platforms on each merge. I learn basic Kotlin and Swift so I can read native errors and logs. This avoids late surprises when QA tests the second platform.

What backend options are common for mobile apps?

Show answer

Teams build custom REST or GraphQL APIs with Node.js, Java Spring Boot, Python or Go, hosted on cloud platforms. Backend-as-a-Service options like Firebase and Supabase provide authentication, databases, storage, functions and push notifications without managing servers, which suits startups and MVPs. Larger apps often combine a custom backend for business logic with managed services for auth, analytics and messaging.

What is Firebase and which services do mobile apps commonly use?

Show answer

Firebase is a mobile and web development platform. Commonly used services are Authentication for email, phone OTP and social logins; Cloud Firestore and Realtime Database for synced data; Cloud Storage for files; Cloud Functions for server code; Cloud Messaging for push notifications; Crashlytics for crash reports; Analytics; Remote Config for feature flags; and App Distribution for testers. FlutterFire and React Native Firebase provide SDKs.

What local storage options exist for Flutter and React Native apps?

Show answer

For simple key-value settings, Flutter uses shared_preferences and React Native uses AsyncStorage or MMKV. For sensitive data, flutter_secure_storage or expo-secure-store use Keychain and Keystore. For structured data and queries, use SQLite via sqflite or drift in Flutter and expo-sqlite or WatermelonDB in React Native. Object stores like Isar, Hive or Realm also exist, but check maintenance status before choosing.

Where should you keep API keys and secrets in a mobile app?

Show answer

Anything bundled in a mobile app can be extracted by decompiling, so never ship true secrets like payment secret keys or admin credentials. Keep them on your backend and let the app call your server. Public client keys, such as maps or Firebase config, are acceptable but should be restricted by app package name, bundle ID and certificate fingerprint. Use environment configuration in CI rather than committing keys to the repository.

How do you handle API errors gracefully in the UI?

Show answer

Map errors into categories in the data layer: no internet, timeout, server error, unauthorised and validation errors with field messages. The UI shows a friendly message and a retry action for network issues, inline messages for validation, and redirects to login for unauthorised. Keep cached data visible when refresh fails. Log unexpected errors to crash reporting with request context but never sensitive data.

Tell me about a time you worked with a backend developer to fix an integration issue.

Show answer

Situation: our app showed wrong prices after a discount launch. Task: find whether the app or API was at fault. Action: I captured the failing request and response with a proxy, shared it with the backend developer, and we found the API returned paise in one endpoint and rupees in another. We agreed on one format, updated the contract document and added a test. Result: the bug was fixed the same day.

What is the difference between a BaaS and a custom backend for a mobile startup?

Show answer

A Backend-as-a-Service gives ready authentication, database, storage and functions, so a small team launches faster without managing servers, and costs stay low at small scale. A custom backend gives full control over business logic, data model, integrations and cost at large scale, but needs more engineering effort. Many startups begin with BaaS for the MVP and gradually move complex or expensive parts to custom services.

What types of tests do you write in Flutter?

Show answer

Unit tests check pure Dart logic like validators, repositories and state notifiers. Widget tests render a single widget or screen in a test environment with WidgetTester, simulate taps and text entry, and assert what appears, running fast without a device. Integration tests, using the integration_test package, run the full app on a device or emulator for end-to-end flows. Golden tests compare widget screenshots to catch visual regressions.

Write a Flutter widget test that taps a button and checks the counter text.

Show answer

testWidgets('increments counter on tap', (tester) async { await tester.pumpWidget(const MaterialApp(home: Scaffold(body: Counter()))); expect(find.text('Count: 0'), findsOneWidget); await tester.tap(find.text('Add')); await tester.pump(); expect(find.text('Count: 1'), findsOneWidget); });

How do you test React Native components?

Show answer

Use Jest as the test runner with React Native Testing Library, which renders components and queries them the way users do, by text, role or label. Fire events like fireEvent.press or userEvent, then assert on output. Mock native modules and network calls with jest mocks or MSW. For end-to-end tests on real builds, use Detox or Maestro, which drive the app on emulators and simulators.

Write a React Native Testing Library test for a login button showing a validation error.

Show answer

import { render, screen, fireEvent } from '@testing-library/react-native'; test('shows error when email is empty', () => { render(<LoginScreen onSubmit={jest.fn()} />); fireEvent.press(screen.getByRole('button', { name: 'Login' })); expect(screen.getByText('Email is required')).toBeTruthy(); });

How do you monitor crashes and performance in production?

Show answer

Integrate a crash reporting tool like Firebase Crashlytics or Sentry, upload debug symbols, dSYMs and source maps so stack traces are readable, and record non-fatal errors with context. Track crash-free users, ANR rate, app start time, slow frames and network latency per release. Set alerts on spikes and review store vitals. Tag events with app version, device and OS so problems can be isolated quickly.

How do you make sure your code works before handing it to QA?

Show answer

I test the happy path and edge cases myself on both Android and iOS, including no network, slow network, empty states, large text sizes and rotation. I write unit tests for logic and run the existing test suite. I review my own diff before raising the pull request and add clear testing notes for QA explaining what changed and what to check, which saves time for both of us.

What are the steps to publish an app on the Play Store?

Show answer

Create a Play Console developer account, create the app and fill in store listing details, screenshots, privacy policy, content rating, data safety and target audience forms. Build a signed release App Bundle with a unique application ID and increasing versionCode, and use Play App Signing. Upload to internal or closed testing first, then promote to production with a staged rollout. New personal accounts must complete a closed test with testers before production access.

What are the steps to publish an iOS app on the App Store?

Show answer

Enrol in the developer program, create an App ID and the app record in App Store Connect, and set up signing with certificates and provisioning profiles, often managed automatically by Xcode or EAS. Archive the release build with an incremented build number, upload it, and test through TestFlight. Complete metadata, screenshots, privacy nutrition labels and age rating, then submit for review. After approval, release manually, automatically or in phased release.

What are versionName, versionCode and build numbers?

Show answer

On Android, versionName is the user-visible version like 2.3.1 and versionCode is an integer that must increase with every upload. On iOS, CFBundleShortVersionString is the marketing version and CFBundleVersion is the build number, which must be unique per version upload. In Flutter, the pubspec version 2.3.1+45 sets both; in Expo, version and the platform build numbers are set in app config and can auto-increment with EAS.

What are TestFlight and Play testing tracks?

Show answer

TestFlight distributes iOS beta builds to internal testers on your team and external testers, up to 10,000, after a light beta review. Play Console offers internal testing for quick distribution to up to 100 testers, closed testing for selected groups, and open testing for anyone to join. Using these tracks catches device-specific bugs and gathers feedback before production, and staged rollouts then limit risk further.

Write a release checklist for a mobile app update.

Show answer

1. Bump version name and build number on both platforms. 2. Run full automated tests and smoke tests on release builds. 3. Verify signing, obfuscation and debug symbol upload. 4. Check new permissions and update store privacy declarations. 5. Update release notes and screenshots if UI changed. 6. Upload to internal track and TestFlight; QA sign-off. 7. Submit for review with reviewer notes and demo account. 8. Start staged rollout at 5 to 10 percent. 9. Monitor crashes, ANRs, reviews and key metrics. 10. Expand rollout or halt and hotfix.

Tell me about an app you published and what you learned from real users.

Show answer

Situation: I published a notes app with a friend and got about 2,000 installs. Task: improve ratings stuck at 3.6. Action: I read every review, found users disliked losing notes when switching phones, added cloud backup and a clear onboarding screen, and fixed two crashes from Crashlytics. Result: rating rose to 4.4 in two months. I learned to prioritise what users actually complain about over features I assumed they wanted.

Why are you interested in this mobile developer role at our company?