20 Common React JS Interview Questions

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React JS, developed by Facebook, has emerged as a leading JavaScript library for building user interfaces. Its popularity is due to its component-based architecture and the ability to create interactive and dynamic web applications.

As the demand for responsive and efficient user experiences continues to rise, React JS has become a go-to choice for developers. Its virtual DOM and efficient rendering make it an ideal framework for building scalable applications.

Here are 20 most common react.js interview questions from beginner to intermediate to advanced.

Explain JSX in React.

Answer

JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React. It allows you to write HTML elements and components in a JavaScript file, making it easier to visualize and work with the structure of the UI.

How Do You Optimize Performance in a React Application?

Answer

Performance optimization involves using tools like React.memo, avoiding unnecessary re-renders, and optimizing the use of the virtual DOM.

What is the virtual DOM?

Answer

The virtual DOM is a lightweight copy of the actual DOM (Document Object Model). React uses it to improve performance by updating only the parts of the actual DOM that have changed, rather than re-rendering the entire DOM.

What is the significance of state?

Answer

State is a JavaScript object that stores component-specific data that may change over time. When the state of a component changes, React re-renders the component to reflect the updated state.

Explain the difference between props and state.

Answer

Props (short for properties) are used to pass data from a parent component to a child component. State, on the other hand, is managed within a component and is used to handle changes in data over time.

What are React components?

Answer

React components are reusable, self-contained modules that represent a part of the user interface. They can be either functional or class-based, and they manage their own state.

What is the purpose of the useEffect hook?

Answer

The useEffect hook is used in functional components to perform side effects, such as data fetching, subscriptions, or manually changing the DOM, after the component has rendered.

Explain the concept of lifting state up in.

Answer

Lifting state up refers to the practice of moving the state from a child component to its parent component. This is done to share state between components that need access to the same data.

What is the role of Redux in a React application?

Answer

Redux is a state management library for JavaScript applications, often used with React. It provides a predictable state container, making it easier to manage the state of an entire application in a single store.

What is the purpose of keys in React lists?

Answer

Keys give React a hint about the identity of elements in a list. They help React identify which items have changed, been added, or been removed, improving the efficiency of updates.

Explain the concept of controlled components.

Answer

Controlled components are components whose behavior is controlled by React. Form elements, for example, are controlled components when their state is handled by React, allowing dynamic updates and validation.

What is the significance of refs?

Answer

Refs access and interact with the DOM directly in React. They are often in use to reference a specific DOM element or to get a reference to a React component.

What are hooks?

Answer

Hooks are functions that allow functional components to use state and lifecycle features that were previously only available in class components. Examples include useState, useEffect, and useContext.

What is the purpose of the useContext hook?

Answer

The useContext hook is used to consume values from context, allowing components to access global data without prop drilling (passing the data through multiple components).

How does conditional rendering work?

Answer

Conditional rendering is achievable by using JavaScript expressions or conditional operators to decide what content to render based on the state or props of a component.

What is the significance of the shouldComponentUpdate method in React?

Answer

The shouldComponentUpdate method is used in class components to optimize performance by preventing unnecessary re-renders. It allows developers to specify conditions under which a component should or should not update.

What is the purpose of the withRouter higher-order component?

Answer

withRouter is a higher-order component provided by React Router that allows non-route components to access the match, location, and history props associated with the nearest <Route> ancestor.

Explain the concept of lazy loading.

Answer

Lazy loading is a technique that defer the loading of certain parts of an application until they are actually in need. This can improve the initial loading time of the application.

What are fragments?

Answer

Fragments are a lightweight syntax for grouping multiple elements without introducing an additional DOM node. They allow developers to return multiple elements from a component without wrapping them in a div or other container.

How can you optimize performance in a React application?

Answer

Performance optimization is achievable by using techniques such as code splitting, memoization, virtualization, optimizing the rendering process, and using tools like React Profiler to identify performance bottlenecks.

Best Practices in Interview Responses

Providing Clear Explanations

When responding to interview questions, clarity is key. Explain your thought process and reasoning in a concise and understandable manner.

Demonstrating Problem-Solving Skills

Employers value candidates who can approach challenges methodically. Share your problem-solving process when tackling coding questions.

Emphasizing Code Readability and Maintainability

In the world of professional development, clean and readable code is of high value. Discuss how you prioritize code readability and maintainability in your projects.

Tips for Preparing for a React JS Interview

Reviewing React Documentation

Stay familiar with the official React documentation to understand best practices, updates, and new features.

Practicing Coding Exercises

Regularly engage in coding exercises and challenges to sharpen your problem-solving skills and coding proficiency.

Staying Updated with React Trends and Updates

To stay well-prepared for interviews in the rapidly evolving tech industry, actively keeping informed of the latest trends is essential.

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