React Junior Developer Interview

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This is a React interview for a junior developer position (Question Part).

React Interview Questions and Answers

What is React?

React is an open-source JavaScript library for building user interfaces, focusing on component-based architecture and efficient updates to the UI.

What is JSX?

JSX is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript files, making it easier to create and visualize React components.

What is the virtual DOM?

The virtual DOM is a lightweight in-memory representation of the actual DOM. React uses it to optimize updates, ensuring they are faster and more efficient.

  • Advantages: Improves performance by reducing direct DOM manipulation, allows efficient updates, and enhances user experience.
  • Disadvantages: Adds overhead for simple applications, introduces complexity, and has a learning curve.

What is the difference between controlled and uncontrolled inputs?

Controlled inputs are managed by React state, providing a single source of truth, while uncontrolled inputs manage their own state internally and are accessed using refs.

Common hooks in React

Common hooks include useState, useEffect, useContext, useReducer, useCallback, useMemo, and useRef. Proper use of these hooks ensures efficient state and side-effect management.

What is useState and how does it work?

useState is a hook that lets you add state to functional components. It returns a state variable and a function to update that state, triggering a re-render when the state changes.

What is useMemo and how does it work?

useMemo is a hook that memoizes a computed value, recomputing it only when one of its dependencies changes. This optimization helps avoid unnecessary recalculations on every render.

What is useCallback and how does it work?

useCallback is a hook that returns a memoized version of a callback function, which only changes if one of its dependencies has changed. This helps prevent unnecessary re-renders of child components.

What is useRef and how does it work?

useRef provides a way to persist values across renders without causing a re-render. It is often used to access DOM nodes or store mutable values.

How does it differ from useState?

useRef does not trigger a re-render when its value changes, while useState triggers a re-render whenever the state is updated.

What is Context and how does it work?

React Context provides a way to pass data through the component tree without having to pass props down manually at every level. It helps avoid prop drilling by sharing values between components.

What is the problem with prop drilling?

Prop drilling can lead to increased complexity and boilerplate code, making the code harder to maintain and understand.

What is state management and when is it useful?

State management involves implementing patterns to handle complex state logic that spans multiple components. It is useful for managing global state and ensuring consistency across the application.

What are some examples of state management libraries?

Examples of state management libraries include Redux, MobX, Zustand, and the Context API.

It is recommended to structure code by feature or functionality, modularizing components to improve maintainability and scalability. Here is an example of structuring by file types and features:

└── src/
    ├── assets/
    ├── api/
    ├── configs/
    ├── components/
    │   ├── auth/
    │   │   └── SignUpForm.tsx
    │   ├── payment/
    │   │   └── PaymentForm.tsx
    │   ├── common/
    │   │   └── Button.tsx
    │   └── employees/
    │       ├── EmployeeList.tsx
    │       └── EmployeeSummary.tsx
    ├── hooks/
    │   ├── auth/
    │   │   └── useAuth.ts
    │   ├── payment/
    │   │   └── usePayment.ts
    │   └── employees/
    │       ├── useEmployees.ts
    │       └── useUpdateEmployee.ts
    ├── lib/
    ├── services/
    ├── states/
    └── utils/

This approach helps in maintaining a clean and organized codebase, making it easier to manage and scale.

What are best practices for writing React Code?

  1. Component Structure:

    • Break down the UI into reusable components.
    • Keep components small and focused on a single responsibility.

  2. State Management:

    • Use state wisely, keeping it minimal and lifting it up when needed.
    • Utilize Context API or state management libraries like Redux for complex state needs.

  3. Hooks:

    • Prefer functional components with hooks over class components.
    • Use custom hooks to encapsulate reusable logic.

  4. Styling:

    • Use CSS-in-JS solutions like styled-components or emotion for component-scoped styles.
    • Maintain a consistent styling approach across the application.

  5. Code Quality:

    • Follow coding standards and use linters like ESLint.
    • Write unit and integration tests for your components.
    • Document components and their props with tools like PropTypes or TypeScript.

  6. Performance Optimization:

    • Use React.memo and useMemo/useCallback hooks to avoid unnecessary re-renders.
    • Optimize rendering with key props and React.lazy for code splitting.

  7. Error Handling:

    • Implement error boundaries to catch JavaScript errors anywhere in the component tree.
    • Handle asynchronous operations with proper error handling mechanisms.

What are the React DevTools and what can you use them for?

React DevTools is a browser extension for inspecting the React component hierarchy in real-time. It allows developers to:

  1. Inspect Component Hierarchy:

    • View the component tree, including parent-child relationships.
    • Check the current state and props of each component.

  2. Debug Performance:

    • Analyze component render times and identify performance bottlenecks.
    • Use the Profiler tab to record performance information and pinpoint slow components.

  3. Edit State and Props:

    • Directly modify the state and props of components to test behavior.
    • Trigger re-renders and observe changes without modifying the codebase.

  4. Inspect Hooks:

    • View and modify the state of hooks used within functional components.
    • Debug custom hooks and their state.

  5. Trace Updates:

    • Identify components that re-render frequently and optimize them.

What is a good way to test your React application?

  1. Popular Testing Framework:

    • Jasmine
    • Mocha ( Need Other tools )
    • Jest (Recommended)

  2. Unit Testing:

    • Test a unit of application without the external resources (eg db)
    • Use Jest for writing unit tests for individual components.
    • Employ testing libraries like React Testing Library to interact with components as a user would.

  3. Integration Testing:

    • Test the application with the external resources.
    • Test how different components work together.
    • Simulate user interactions and verify the application behaves as expected.

  4. End-to-End (E2E) Testing:

    • Test the application through its UI.
    • Use tools like Cypress or Selenium to automate tests that run through the entire application flow.
    • Ensure critical paths (like user registration or checkout processes) function correctly from start to finish.

  5. Static Code Analysis:

    • Implement ESLint and Prettier for maintaining code quality and style consistency.
    • Use TypeScript for static type checking and to catch potential errors early in the development process.

  6. Continuous Integration (CI):

    • Set up CI pipelines (using tools like GitHub Actions, Travis CI, or Jenkins) to run tests automatically on code commits.
    • Ensure that all tests pass before merging new code into the main branch.

  7. Visual Regression Testing:

    • Utilize tools like Storybook and Chromatic to catch visual changes and regressions.
    • Verify that UI components render correctly across different states and screen sizes.