👨🏾‍💻🚀Top 35 Advanced ReactJS Interview Questions
Tags: javascript, react, interview, career
Published: 2023-10-23 06:41:26

##1. In which situation would you use refs in React?
Refs in React are a powerful tool for interacting with the DOM directly. They come in handy when you need to access or modify a DOM element outside of the typical React data flow. For instance, you might use refs to focus an input field, integrate with third-party libraries that rely on direct DOM manipulation or perform imperative actions like animations. Refs provide a way to bridge the gap between the declarative world of React and the imperative nature of the DOM.

##2. How would you use to handle events in React?
In React, event handling is vital for building interactive user interfaces. You can use event handlers like onClick, onChange, and more to respond to user interactions. These functions allow you to define actions or behavior when events occur. For example, you can use the onClick event to trigger a function when a button is clicked. Event handling in React is essential for creating dynamic and responsive web applications.

##3. Why would you use super constructors with props arguments?
When creating class components in React, it's essential to understand the relationship between the component and its parent. Using super(props) in the constructor ensures that the parent class's constructor is called, passing the props argument correctly. This is crucial because it sets up the component's initial state and allows you to access this.props within the child component, enabling you to work with the data passed from the parent component effectively.

##4. How would you use validation on props?
You can use PropTypes or TypeScript to validate props, ensuring that the right data types are passed to components and catching potential errors early. PropTypes allow you to specify the expected data types for each prop, and React will issue warnings in development mode if the prop types don't match. TypeScript, on the other hand, provides static type checking, ensuring that your props are correctly defined and used throughout your components. These validation methods are valuable for maintaining code quality and reducing runtime errors.

##5. Which method would you use to add attributes to components conditionally?
In React, you often need to conditionally render elements or add attributes based on certain conditions. You can achieve this using standard JavaScript constructs such as conditional statements (if-else), ternary operators, or logical && operators. For instance, you can conditionally render a CSS class or an attribute based on some data or user interaction. This flexibility allows you to build dynamic and responsive components that adapt to different scenarios, improving the user experience.

##6. What methods would you use to check and improve slow app rendering in React?
Optimizing app rendering performance in React is crucial for delivering a smooth user experience. Techniques like shouldComponentUpdate, React.memo, and React Profiler can help optimize rendering performance in React.

##7. In which situation would you use useMemo() in React?
useMemo is used to memoize expensive calculations in functional components, preventing unnecessary recomputations.

##8. How would you avoid binding in React?
Use arrow functions for event handlers or bind methods in the constructor to avoid the need for explicit binding in class components.

##9. Explain what a higher-order component is.
A higher-order component (HOC) is a function that takes a component and returns a new component with added functionality. HOCs enhance reusability and modularity.

##10. Explain what a mounted component is.
A mounted component is one that has been initialized, added to the DOM, and is visible to the user.

##11. Explain what useState is.
useState is a Hook in React used to add state to functional components, enabling them to manage and display dynamic data.

##12. Explain what an event is in React.
In React, an event is a user interaction with a component, like clicking a button or typing in an input field, leading to actions and updates.

##13. What is a class component?
A class component is a traditional way of creating components in React using ES6 classes and lifecycle methods.

##14. What is a component?
A component is a reusable, self-contained building block in React, representing a UI element with its behavior and rendering logic.

##15. What is the difference between class and functional components?
Class components use ES6 classes and have lifecycle methods, while functional components are JavaScript functions and use Hooks for state and side effects.

##16. What is a state object?
The state object holds data that can change over time and affects the rendering of a component. It's managed by React.

##17. What is a props object?
The props object contains data passed from a parent component to a child component, enabling the customization of child components.

##18. How are state objects different from props objects?
State is managed within a component and can change, while props are received from a parent component and are immutable within the child component. State is controlled by the component itself, while props are provided by external components.

##19. Explain what MVC architecture is.
The Model-View-Controller (MVC) architecture is a design pattern used to organize the structure of software applications. In this pattern, the "Model" represents the application's data and business logic, the "View" deals with the user interface and presentation, and the "Controller" acts as an intermediary between the Model and View, handling user interactions and controlling the flow of data. MVC promotes separation of concerns, making code easier to maintain and extend. While React itself doesn't strictly follow MVC, understanding this architecture is beneficial for designing scalable and well-structured applications.

##20. Name an architectural difference between React and Angular.
One notable architectural difference between React and Angular is the level of flexibility and scalability they offer. React is a JavaScript library primarily focused on building UI components. It gives developers more flexibility in choosing other libraries and tools to complement their projects, leading to a less opinionated and more modular architecture. On the other hand, Angular is a comprehensive framework that offers a full-fledged solution, including features like routing, forms, and state management, with a more opinionated architecture. Understanding these architectural differences helps in making informed decisions when selecting a technology for your projects.

##21. Explain what a controlled component is.
In React, a controlled component is a form element, like an input field or textarea, whose value is controlled by the application's state. You set the value of the form element and handle changes through event handlers. This approach allows you to have complete control over the input's value, making it easier to validate, manipulate, or synchronize with other components. Controlled components are a best practice for handling form data in React applications.

##22. Explain what an uncontrolled component is.
An uncontrolled component in React is a form element whose value is not directly controlled by React's state. Instead, the value is managed by the DOM itself. Uncontrolled components can be useful in scenarios where you need to integrate React with non-React code or work with third-party libraries that manage the input state independently.

##23. How are controlled and uncontrolled components different?
Controlled components have their values managed by React's state, allowing for greater control and synchronization with the application's data. Uncontrolled components, on the other hand, rely on the DOM to manage their values, which can be advantageous in scenarios where you need to integrate React with non-React code or work with third-party libraries that manage the input state independently.

##24. Explain what React Hooks are.
React Hooks are functions introduced in React 16.8 that allow functional components to manage state and side effects. They enable functional components to have state and lifecycle-like features, making it easier to reuse logic and manage complex component behavior. For example, the useState and useEffect Hooks allow you to add state and side effects to functional components, reducing the need for class components.

##25. Explain what three dots mean in React.
The three dots, also known as the spread (...) operator, are used in React for various purposes. They allow you to spread the properties of an object or array into another object or array, making it easy to create copies, merge properties, or pass props to components. The spread operator simplifies component composition and data sharing, providing a concise and effective way to work with data in React.

##26. What are package managers in React.js?
Package managers like npm (Node Package Manager) and Yarn are tools for managing dependencies and packages used in React applications. They simplify the process of installing, updating, and sharing JavaScript packages, making it easier to handle the complex web of dependencies in modern web applications. Package managers are essential for ensuring version compatibility and efficient integration of external code into your React project.

##27. Explain what prop drilling is.
Prop drilling, often referred to as "lifting state up," is a common issue in React where you pass data through multiple layers of nested components. While it's a common practice, it can lead to issues with maintainability and readability when components deep in the hierarchy need to access the passed props. To address prop drilling, you can use tools like React Context or Redux to provide a centralized store for your application's data, making it more accessible to components without having to pass props through many layers.

##28. Explain what StrictMode is.
React's StrictMode is a built-in component that helps identify potential problems in your application during development. When wrapped around your app's root component, StrictMode performs additional checks and warnings to highlight issues in your code. It helps catch and address common problems like unsafe lifecycle methods, redundant re-renders, and deprecation warnings. StrictMode is a valuable tool for maintaining code quality and preventing potential bugs in your React application.

##29. Explain what the Shadow DOM is.
The Shadow DOM is a web platform feature that encapsulates the styles and structure of a web component, isolating it from the rest of the page. It allows developers to create self-contained, reusable components with encapsulated styles and DOM elements. This isolation ensures that the component's styles do not interfere with the styles of the surrounding web page, making it easier to build web components that can be seamlessly integrated into different projects without style conflicts.

##30. Explain what the virtual DOM is.
The virtual DOM is a lightweight, in-memory representation of the actual DOM in a React application. React uses the virtual DOM to optimize rendering and updates. When there's a change in a component's state, React creates a new virtual DOM tree, compares it to the previous one, and calculates the most efficient way to update the actual DOM. This process minimizes the number of DOM operations, improving performance and making React applications faster.

##31. How is the Shadow DOM different from the virtual DOM?
The Shadow DOM and the virtual DOM serve different purposes. The Shadow DOM focuses on encapsulating the styles and structure of web components, making them self-contained and isolated from the rest of the page. In contrast, the virtual DOM focuses on reducing DOM manipulations and improving rendering performance by providing an efficient way to update the actual DOM. While the Shadow DOM is about encapsulation, the virtual DOM is about performance optimization.

##32. Explain what the React.js lifecycle methods are.
React.js lifecycle methods are special functions that are called at specific points in a component's lifecycle. They include methods like componentDidMount, componentDidUpdate, and componentWillUnmount. These methods allow you to perform actions such as setting up data, handling updates, and cleaning up resources during a component's lifespan. However, with the introduction of functional components and Hooks, some of these lifecycle methods have been replaced by equivalent Hook functions, making it easier to manage component behavior.

##33. Explain what a pure function is.
A pure function is a function where the output or result is solely determined by its input, and it has no side effects. Pure functions are predictable and easy to test, as they always produce the same output for the same input. In React and many other programming paradigms, using pure functions improves code reliability and maintainability.

##34. Explain what JSX is.
JSX, short for JavaScript XML, is an extension to JavaScript used in React to define the structure and appearance of UI components. JSX allows developers to write HTML-like code within JavaScript, making it easier to create and visualize the structure of user interfaces. React components render JSX elements, which are transpiled into JavaScript code by tools like Babel before being included in the final application bundle.

##35. Explain what Flux architecture is.
Flux is an architectural pattern used to manage the flow of data in React applications. It emphasizes unidirectional data flow, ensuring that data changes are handled in a predictable and controlled manner. In the Flux architecture, actions trigger updates to the central data store, and the changes are then propagated to the views. While not a strict framework like Redux, understanding Flux principles is beneficial for managing state in React applications.

##36. What are bundlers in React.js?
Bundlers in React.js, such as Webpack or Parcel, are tools that help package and optimize various assets, including JavaScript files, CSS, and images, into a single bundle that can be efficiently loaded by web browsers. Bundlers handle tasks like code splitting, tree shaking, and minification, making it easier to deliver optimized applications to users. They are an essential part of modern web development, helping to manage complex dependency trees and improve application performance.

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