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Cognizant · React Native
Practice React Native questions specifically asked in Cognizant interviews – ideal for online test preparation, technical rounds and final HR discussions.
Questions
21
Tagged for this company + subject
Company
Cognizant
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Subject
React Native
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Go through each question and its explanation. Use this page for targeted revision just before your Cognizant React Native round.
A parent can pass down a callback function as a prop. The child calls this function with the required data. This pattern allows controlled components and two-way communication while maintaining React’s one-way data flow.
Parent → <Child onValueChange={(val) => setValue(val)} />
Child → props.onValueChange(newValue);For complete preparation, combine this company + subject page with full company-wise practice and subject-wise practice. You can also explore other companies and topics from the links below.
The flexWrap property lets items wrap onto multiple lines when there isn’t enough space. By default, React Native uses 'nowrap', so you need to explicitly set flexWrap to 'wrap' to allow wrapping.
Common mistakes include confusing main and cross-axis directions, using row when column is intended, and combining alignItems and justifyContent incorrectly. Developers also sometimes forget that React Native defaults to flexDirection: 'column', unlike web Flexbox where 'row' is default.
React Navigation uses react-native-gesture-handler to handle swipe and tap gestures for screen transitions. It must be installed and linked correctly before using navigators like StackNavigator or DrawerNavigator.
Middleware acts as a layer between dispatching an action and reaching the reducer. It is used for logging, asynchronous operations (like Redux Thunk or Saga), and side-effects such as API calls. This keeps reducers pure and predictable.
Axios automatically converts JSON responses and provides request / response interceptors. Fetch requires manual response.json() calls and additional logic for interceptors.
Create an Axios instance using axios.create() with baseURL and common headers. Import this instance across files to ensure all requests share consistent configurations like interceptors and tokens.
const api = axios.create({ baseURL: 'https://api.app.com', headers: { 'Auth': token } });The bridge is the communication layer that sends messages between the JavaScript thread and native threads. This mechanism ensures that native features and UI elements can be controlled using JavaScript APIs.
useCallback memoizes function definitions, ensuring the same reference is used between renders unless dependencies change. This prevents child components wrapped with React.memo from re-rendering when their props contain unchanged callback functions.
A custom native module is useful when you need a feature not yet supported by React Native or third-party libraries. For example, integrating a specialized Bluetooth SDK, using advanced camera APIs, or controlling device audio routing directly on Android.
React.memo wraps a functional component and skips re-rendering if props are the same as before. It is useful for optimizing list items or child components that receive frequent updates.
export default React.memo(MyComponent);
Hermes is a small and efficient JavaScript engine designed to improve app startup time and reduce memory usage. It compiles JavaScript ahead of time into bytecode, optimizing performance for Android and iOS.
jest.mock() replaces real modules or dependencies with fake ones for isolated testing. This helps test components without performing actual API calls or file reads.
jest.mock('axios', () => ({ get: jest.fn() }));Jest focuses on unit and snapshot testing of individual components or functions. It runs entirely in JavaScript and doesn’t require a simulator. Detox performs end-to-end testing by running real user flows on actual devices or emulators. It validates the integration of components and APIs together.
Fabric is React Native’s new rendering layer that replaces the old UIManager. It makes rendering faster and more consistent by synchronizing React Fiber and native UI updates directly through JSI.
Fabric uses a new layout engine that works in sync with React Fiber. It reduces frame drops by performing layout calculations more efficiently and prioritizing visible UI updates first.
React Navigation supports nesting navigators inside one another to create complex navigation structures. For example, you can have a TabNavigator inside a StackNavigator or vice versa. Each nested navigator manages its own navigation state but still communicates with parent navigators for transitions and back actions.
State should only be changed using setState (for class components) or the setter from useState (for functional components). Direct modification won’t trigger a UI re-render and can lead to inconsistent data.
this.setState({ count: this.state.count + 1 });The Context API allows data to be shared globally without manually passing props through multiple component levels. It’s commonly used for global themes, authentication data, or language settings.
TurboModules use the JavaScript Interface (JSI) to directly call native code without serializing messages. This reduces latency and significantly improves communication speed between JS and native layers.
The React Native bridge enables communication between the JavaScript thread and native threads. It converts JS commands into native code (Objective-C, Swift, Java, or Kotlin), which is then executed by the platform.