Junior Mobile App Developer Interview Questions & Answers
40 questions for Junior Mobile App Developer interviews — 24 easy, 16 medium.
About the Junior Mobile App Developer role
A junior mobile app developer builds screens, connects them to APIs and fixes bugs in Flutter or React Native apps under guidance from senior developers. They convert designs into responsive UI, handle forms and validation, and test features on Android and iOS devices. Freshers start here at startups, agencies and IT services firms, and published portfolio apps greatly improve their chances in interviews.
Flutter or React Native basics UI from designs REST API integration Git Device testing
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01
What are over-the-air updates and what are their limits?
Over-the-air updates push new JavaScript bundles and assets to React Native apps without a store release, using services like EAS Update. They fix bugs quickly but can only change JavaScript and assets, not native code, permissions or SDK versions, which need a new build. Store policies allow them only if the app's core purpose does not change. Use runtime versions to ensure updates match compatible native builds, and roll out gradually.
02
What is the difference between ListView and ListView.builder?
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.
03
What are runtime permissions and how do they differ between platforms?
Both platforms require runtime approval for sensitive features like camera, location, contacts and notifications. Android requires declarations in AndroidManifest.xml and runtime requests for dangerous permissions; iOS requires usage description strings in Info.plist, and the app crashes if a description is missing when requesting. iOS allows asking only once; later the user must change settings. Plugins like permission_handler or react-native-permissions unify the API.
04
What is a WebView and when is it appropriate to use one?
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.
Hermes is a JavaScript engine optimised for React Native. It compiles JavaScript to bytecode at build time, so the app does not parse and compile JavaScript on startup, which reduces time to interactive, memory use and app size. It is the default engine on Android and iOS. It supports debugging with Chrome DevTools-based tools and sampling profiling to find slow JavaScript.
06
What is the role of a mobile developer in an Agile team?
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.
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.
08
When would you choose a cross-platform framework over native development?
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.
09
What is the difference between native, cross-platform and hybrid mobile apps?
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.
BuildContext is a handle to a widget's location in the element tree. It is used to look up inherited data like Theme.of(context), MediaQuery and Navigator, and providers. Each widget's build method gets its own context. Common mistakes are using a context from above a widget that provides data, or using a context after an async gap when the widget may be unmounted; check context.mounted before using it after await.
11
A React Native app shows a red screen error in development: Text strings must be rendered within a Text component. What does it mean?
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.
12
How do you create reusable custom widgets in Flutter?
Extract repeated UI into a StatelessWidget class with clear constructor parameters, rather than helper methods that return widgets, because classes get their own element, can be const and rebuild independently. Accept callbacks for events, use the theme for colours and text styles, and provide sensible defaults. Group them into a shared widgets folder or design system package and document them with widget previews or a showcase screen.
13
Write a Flutter screen that fetches a list from an API using FutureBuilder.
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.
15
What is the difference between a debug and release build in mobile apps?
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.
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.
17
How does Flutter render UI differently from React Native?
Flutter draws every pixel itself using its own rendering engine, Impeller, onto a canvas, so widgets look identical across platforms and do not depend on native UI components. React Native maps components like View and Text to real native views, so the UI uses platform controls. Flutter gives pixel consistency and predictable performance; React Native gives native look and easier reuse of native view libraries, plus web developer familiarity with JavaScript.
18
Tell me about a time you had to learn a new framework under a deadline.
Situation: my team decided to build a client app in Flutter, and I had only React Native experience. Task: deliver the login and dashboard screens in three weeks. Action: I completed the official codelabs in the first week, mapped concepts I knew like components and hooks to widgets and state, and asked for early code reviews. Result: the screens shipped on time, and I later wrote a short guide for other React Native developers moving to Flutter.
19
Write a simple counter using StatefulWidget and setState.
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.
21
Tell me about a time you worked with a backend developer to fix an integration issue.
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.
Use React Native DevTools, the default debugger since 0.76, for console logs, breakpoints, React component inspection and profiling. Inspect network requests with the network panel or a proxy. For native crashes, check Logcat in Android Studio and the Xcode console. Use the performance monitor overlay for frame rates, and add a crash reporting tool like Sentry or Crashlytics for production errors with source maps uploaded.
23
Write a custom hook that fetches data with loading and error states.
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.
26
How do you set up multiple environments such as dev, staging and production in a mobile app?
Create build flavours on Android and schemes or configurations on iOS, each with its own app ID suffix, app name, icon and API base URL, so all can be installed side by side. In Flutter, use --flavor with --dart-define-from-file for config values; in React Native, use react-native-config or Expo app config with environment variables. Keep secrets out of the repo and set them in CI.
27
What are keys in Flutter and when do you need them?
Keys help Flutter match widgets to existing elements and state when the widget list changes. Without keys, Flutter matches by type and position, so reordering or removing items in a list of stateful widgets can attach state to the wrong item. Use ValueKey with a unique ID for list items, UniqueKey to force recreation, and GlobalKey rarely, for accessing state across the tree such as a Form.
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.
29
What is responsive design in mobile apps and how do you handle different screen sizes?
Responsive design means layouts adapt to screen width, orientation, foldables and tablets instead of using fixed sizes. Use flexible layouts, percentages or flex, and breakpoints for compact, medium and expanded widths. In Flutter use LayoutBuilder and MediaQuery.sizeOf; in React Native use flexbox and useWindowDimensions. Respect safe areas for notches and system bars, support text scaling for accessibility, and test on small budget phones.
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.
31
Why is TypeScript recommended for React Native projects?
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.
32
Why are you interested in this mobile developer role at our company?
Connect your skills to the role honestly. For example: your app serves millions of users in tier 2 and tier 3 cities, and I have focused on performance for budget phones and slow networks in my projects. I have shipped two cross-platform apps to both stores, and I want to work where release quality and user experience matter at scale. I also like that your team uses Flutter, which I enjoy and want to master.
33
Explain Row, Column, Expanded and Flexible in Flutter layouts.
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.
34
How does the keyboard affect layouts in React Native and how do you handle it?
When the keyboard opens it can cover inputs, especially at the bottom of the screen. Use KeyboardAvoidingView with behavior padding on iOS, and rely on Android's adjustResize window setting, or use react-native-keyboard-controller for consistent behaviour on both. Wrap forms in a ScrollView with keyboardShouldPersistTaps set to handled so buttons work while the keyboard is open, and dismiss the keyboard on submit.
35
What is the difference between StatelessWidget and StatefulWidget?
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.
36
How do you stay productive when working with both Android and iOS platforms?
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.
37
A Flutter screen shows a yellow and black overflow stripe on small phones. How do you fix it?
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.
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.
39
How do you handle platform-specific code in React Native?
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.
40
After an API call finishes, the app throws an error about using BuildContext across async gaps or setState after dispose. How do you fix it?
The user left the screen before the Future completed, so the widget was unmounted. Before using context or calling setState after await, check if (!context.mounted) return; or if (!mounted) return; in State. Better still, move async logic out of widgets into a state management layer that the UI listens to, and cancel subscriptions or timers in dispose so no callbacks run on a dead widget.
Frequently asked questions
What does a Junior Mobile App Developer do?
A junior mobile app developer builds screens, connects them to APIs and fixes bugs in Flutter or React Native apps under guidance from senior developers. They convert designs into responsive UI, handle forms and validation, and test features on Android and iOS devices. Freshers start here at startups, agencies and IT services firms, and published portfolio apps greatly improve their chances in interviews.
Which skills are tested in a Junior Mobile App Developer interview?
Expect questions on Flutter or React Native basics, UI from designs, REST API integration, Git, Device testing. Most rounds also ask you to explain a project and solve one practical problem live.
How do I prepare for a Junior Mobile App Developer interview?
Revise Mobile Development Fundamentals, Dart and Flutter Basics, React Native Core first using the topic sheets, practise the questions on this page aloud, then do one full mock interview to practise timing.