ReactJS Training in Gandhinagar

In React, keys are essential when rendering dynamic lists using .map(). They uniquely identify each list item and help React track which items have changed, been added, or removed. This improves rendering performance by allowing React to only update the affected items, rather than re-rendering the entire list. Without unique keys, React cannot accurately optimize updates, which could lead to UI bugs and performance drops.

• Controlled Components: The input form elements are controlled by React state. Their value is bound to a state variable, and changes are handled via onChange events. This approach ensures the form data is always in sync with the component.
• Uncontrolled Components: These manage their own state internally using the DOM. Input values are accessed using refs (useRef) when needed. They are simpler to use for basic forms but offer less flexibility and validation control.

Both hooks allow side effects in functional components, but they differ in execution timing:

• useEffect runs after the component has rendered and the DOM has been updated, making it suitable for tasks like API calls or event subscriptions.
• useLayoutEffect runs synchronously after all DOM mutations but before the browser paints the screen, making it ideal for layout-related changes like measuring DOM elements or setting scroll positions. Using useLayoutEffect may cause blocking if misused, so it's typically reserved for performance-critical tasks.

Portals in React allow you to render children into a different DOM node outside of the parent component’s hierarchy. This is useful for scenarios like modals, tooltips, and dropdowns that need to break out of container boundaries (e.g., z-index or overflow). React provides the ReactDOM.createPortal(child, container) API to achieve this, ensuring event bubbling still works as expected even though the component is rendered elsewhere in the DOM.

Jest is a JavaScript testing framework developed by Facebook that is widely used for testing React applications. It allows developers to write unit tests, integration tests, and snapshot tests to ensure components and logic behave correctly. Jest supports features like mocking, code coverage reports, and an intuitive syntax. It works well with Babel and offers a zero-config setup, making it ideal for React developers to test components, state management, and APIs.

Enzyme is a JavaScript testing utility for React, developed by Airbnb, that allows for detailed examination of component output and behavior. It supports different rendering modes:

• Shallow rendering (only the current component),
• Full DOM rendering (with child components), and
• Static rendering (render to static HTML).
  Enzyme makes it easy to traverse, manipulate, and simulate events on components, making it a powerful tool for unit and integration testing.

Snapshot testing is a technique used to ensure that the UI does not change unexpectedly. When a component is rendered, a snapshot of its output is saved in a file. During future test runs, Jest compares the current output to the stored snapshot. If differences are found, it alerts the developer. This helps catch unintended changes in the UI and is commonly used in conjunction with Jest for React 

Best practices for structuring React projects include:

• Organizing components into feature-based folders.
• Separating UI components and logic (hooks, services).
• Using reusable and atomic components (Button, Input).
• Maintaining a consistent naming convention (PascalCase for components).
• Placing test files near the components being tested.
• Keeping styles modular (CSS Modules or styled-components).
  These practices make the project scalable, maintainable, and easier to collaborate on in teams.

Performance issues in React often arise from:

• Excessive re-renders caused by unnecessary state or prop changes.
• Large component trees with deeply nested children.
• Improper usage of hooks, especially useEffect, causing repeated operations.
• Unoptimized state management, like lifting state too high.
• Large initial bundle sizes, which increase load time.
  Performance can be improved using techniques like memoization (React.memo, useMemo), code splitting, and lazy loading.

Event delegation in React involves attaching a single event listener to a parent element (usually the root) and handling events for all child elements through this listener. React uses this approach internally to improve performance and simplify event management across the virtual DOM.

Default events in form elements include automatic behaviors such as form submission, checkbox selection, or input changes. React allows developers to override or prevent these behaviors using event.preventDefault() in event handlers to manage form actions manually.

Performance optimization ensures that a React application remains fast, responsive, and scalable. It helps reduce loading times, improves user experience, and ensures smooth rendering of UI elements. Techniques like memoization, lazy loading, and virtual DOM diffing play crucial roles in achieving optimal performance.

Key performance optimization techniques include:

• Using React.memo and useMemo for memoization
• Implementing code-splitting with React.lazy()
• Preventing unnecessary re-renders using shouldComponentUpdate or PureComponent
• Optimizing list rendering with keys
• Lazy loading images and components
• Efficiently managing large datasets using virtualized lists

In large applications, managing state with just React’s useState and useContext can become complex. Libraries like Redux, MobX, Recoil, or Zustand provide centralized and predictable state containers. They offer better debugging, time-travel capabilities, and structure for handling application-wide state.

Testing plays a crucial role in ensuring reliability, preventing bugs, and maintaining code quality in React apps. It allows developers to validate that individual components and overall application logic work as expected. Tools like Jest, React Testing Library, and Enzyme are commonly used for unit, integration, and snapshot testing.

Snapshot testing involves taking a serialized snapshot of a component’s rendered output and comparing it against future renders. If the output changes, the test fails, indicating that the component may have changed unexpectedly. It is useful for tracking UI changes and preventing regression.

Custom hooks are developer-defined functions that use one or more of React’s built-in hooks to encapsulate reusable logic. They promote code reuse, separation of concerns, and cleaner component structures. For example, a custom hook could handle form input logic, authentication, or data fetching.

 React JS offers several benefits, including:

• Component-based architecture for reusability and scalability
• Virtual DOM for efficient rendering
• Strong community and ecosystem with many libraries and tools
• JSX syntax for writing HTML-like code in JavaScript
• Support for mobile development via React Native
• Excellent performance with support for optimization techniques

 These advantages make React a preferred choice for building modern, interactive web applications.