Preparing for a React JS interview as a fresher can be daunting, but with the right knowledge and practice, you can ace it. This guide covers a comprehensive set of React JS interview questions that are commonly asked, ranging from basic concepts to advanced topics.
Whether you’re new to React or have some experience under your belt, these questions will help you assess your understanding, identify areas for improvement, and boost your confidence for the upcoming interview. We’ve organized the questions into three categories – basic, intermediate, and advanced – to cater to different levels of expertise.
By thoroughly understanding the fundamentals of React, its core features, and the latest advancements, you’ll be well-equipped to demonstrate your skills and potential to potential employers. Remember, the key to success lies in continuous learning, hands-on practice, and the ability to explain concepts clearly.
So, dive in, explore the questions, and let this guide be your companion on your journey to becoming a React JS pro. Good luck with your interview!
Basic Questions
What is React?
React is an open-source JavaScript library used for building user interfaces, particularly for single-page applications. It allows developers to create reusable UI components.
What are the main features of React?
Key features include:
- Component-based architecture
- Virtual DOM for efficient rendering
- Unidirectional data flow
- JSX syntax for templating
Explain JSX.
JSX (JavaScript XML) is a syntax extension that allows writing HTML-like code within JavaScript. It is not directly readable by browsers and needs to be transpiled into regular JavaScript using tools like Babel.
What is the Virtual DOM?
The Virtual DOM is a lightweight representation of the real DOM. React maintains a virtual copy of the DOM in memory, allowing it to efficiently update only the parts of the DOM that have changed, rather than re-rendering the entire UI.
How does React differ from other frameworks like Angular?
React focuses on the view layer and uses a component-based architecture, while Angular is a full-fledged MVC framework. React utilizes a virtual DOM for performance, whereas Angular manipulates the real DOM directly
What is the difference between a functional component and a class component?
Functional components are simpler and are defined as JavaScript functions. They can use hooks for state and lifecycle methods. Class components are ES6 classes that extend React.Component and have access to lifecycle methods and state management.
What is the purpose of key prop in React?
The key prop is a unique identifier for elements in a list. It helps React identify which items have changed, are added, or are removed, improving performance during re-renders.
What is the purpose of ReactDOM.render()?
ReactDOM.render() is the main method used to render a React element into the DOM in the root of the HTML page.
What is the difference between state and props?
Props are passed from parent to child components, while state is managed within the component itself. Props are read-only, while state can be modified.
What are the different phases of the component lifecycle?
The component lifecycle consists of three main phases: mounting (when an instance of a component is being created and inserted into the DOM), updating (when a component is being re-rendered due to changes in props or state), and unmounting (when a component is being removed from the DOM).
What is the purpose of super(props) in a class component?
super(props) is used in the constructor of a class component to call the constructor of the base class (React.Component). This ensures that the component is properly initialized.
What is the difference between setState() and replaceState()?
setState() is used to update the state of a component by merging the new state with the previous state. replaceState() is used to replace the current state with a new state.
What is the purpose of shouldComponentUpdate()?
shouldComponentUpdate() is a lifecycle method that allows you to control when a component should update. It takes nextProps and nextState as arguments and returns a boolean value indicating whether the component should update or not.
What is the purpose of componentDidCatch()?
componentDidCatch() is a lifecycle method introduced in React 16 that allows you to catch errors in their child components. It takes two arguments: error and info, and can be used to log errors or display fallback UI.
What is the purpose of forwardRef()?
forwardRef() is a higher-order component that allows you to pass a ref to a child component. It is useful when you want to access the underlying DOM element of a child component.
What is the purpose of React.lazy()?
React.lazy() is a feature that allows you to dynamically import components when they are needed, rather than loading them all upfront. It is used for code splitting to improve performance.
What is the purpose of Suspense?
Suspense is a component that allows you to display a fallback UI while waiting for a lazy-loaded component to finish loading. It is used in conjunction with React.lazy() for code splitting.
Intermediate Questions
What are Props and State in React?
Props (short for properties) are read-only attributes passed from parent to child components, while state is a mutable object that holds data specific to a component and can change over time.
What is Prop Drilling?
Prop drilling refers to the process of passing data through many layers of components, which can lead to complications and inefficiencies in managing state across the application.
Explain the lifecycle methods of a React component.
Lifecycle methods are hooks that allow you to run code at specific points in a component’s life, such as componentDidMount, componentDidUpdate, and componentWillUnmount, which manage side effects and resource cleanup
What are controlled and uncontrolled components?
Controlled components are form elements whose value is controlled by React state. Uncontrolled components manage their own state internally and can be accessed using refs.
What is Context API in React?
The Context API is a feature that allows you to share values (like state) between components without having to pass props down manually at every level. It is useful for managing global state.
How do you handle events in React?
Events in React are handled using camelCase syntax. For example, you can define an event handler like this: <button onClick={this.handleClick}>Click Me</button>.
What is the purpose of useEffect hook?
The useEffect hook allows you to perform side effects in functional components, such as fetching data, directly interacting with the DOM, or setting up subscriptions. It runs after the render and can be configured to run conditionally.
Advanced Questions
What are Hooks in React?
Hooks are functions that let you use state and other React features without writing a class. Common hooks include useState and useEffect, which manage state and side effects in functional components.
What is the difference between useRef and createRef?
useRef is a hook that maintains the same reference across re-renders, while createRef creates a new ref on each render. This distinction affects how refs persist in functional versus class components.
What is memoization in React?
Memoization is an optimization technique used to improve performance by caching the results of expensive function calls and returning the cached result when the same inputs occur again. In React, React.memo can be used to prevent unnecessary re-renders of functional components.
What are higher-order components (HOCs)?
HOCs are functions that take a component and return a new component, allowing you to reuse component logic. They are often used for cross-cutting concerns like authentication, logging, or data fetching.
Explain the concept of “lifting state up.”
Lifting state up refers to the practice of moving state from a child component to a common parent component to share data between siblings. This ensures that multiple components can access and modify the same state.
What is React Router?
React Router is a library for routing in React applications. It allows you to create single-page applications with navigation by defining routes that map components to specific paths.
What are fragments in React?
Fragments are a way to group multiple elements without adding extra nodes to the DOM. They can be used with <React.Fragment> or the shorthand syntax <>...</>.
What is the purpose of useReducer hook?
The useReducer hook is an alternative to useState for managing complex state logic in functional components. It is particularly useful for managing state that depends on previous state values.
How do you optimize performance in a React application?
Performance can be optimized by:
Using React.memo for functional components.
Implementing code-splitting with React.lazy and Suspense.
Avoiding unnecessary re-renders by using shouldComponentUpdate in class components or React.memo in functional components.
Utilizing the useCallback and useMemo hooks.
What is the purpose of React.StrictMode?
React.StrictMode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants, helping developers identify issues in their components.
What is the difference between useLayoutEffect and useEffect?
useLayoutEffect runs synchronously after all DOM mutations, making it suitable for reading layout from the DOM and synchronously re-rendering. useEffect, on the other hand, runs asynchronously after the paint, making it suitable for side effects that do not require immediate DOM updates.
What is a higher-order component in React?
A higher-order component (HOC) is an advanced pattern used to reuse component logic. It is a function that takes a component as an argument and returns a new component, allowing you to enhance the original component’s behavior.
What are synthetic events in React?
Synthetic events are a cross-browser wrapper around the native events provided by React. They normalize the behavior of events across different browsers, ensuring that the same interface is used regardless of the browser.
What is Redux Thunk used for?
Redux Thunk is a middleware that allows you to write action creators that return a function instead of an action. This enables you to handle asynchronous logic, such as API calls, within your Redux actions.
What is the Virtual DOM in React?
The Virtual DOM is a lightweight representation of the real DOM that React uses to optimize rendering. When changes occur, React updates the Virtual DOM first, compares it with the real DOM, and then only updates the parts of the real DOM that have changed.
What is prop drilling and how can you avoid it?
Prop drilling refers to the process of passing data through multiple layers of components. This can be avoided by using the Context API, which allows you to share data across components without passing props explicitly at every level.
What are custom hooks in React?
Custom hooks are JavaScript functions that start with “use” and can call other hooks. They allow you to encapsulate and reuse logic across multiple components while adhering to the rules of hooks.
How do you optimize performance in a React application?
Performance can be optimized by using techniques such as memoization with React.memo, lazy loading components with React.lazy, using the useMemo and useCallback hooks to prevent unnecessary re-renders, and minimizing the number of components rendered in the DOM.
What is the purpose of the useMemo hook?
The useMemo hook is used to memoize expensive calculations in functional components, preventing unnecessary recalculations on re-renders. It returns a memoized value that only changes when its dependencies change.
What is the difference between useRef and createRef?
useRef is a hook that returns a mutable ref object that persists for the full lifetime of the component, while createRef creates a new ref object on every render. useRef is typically used in functional components, while createRef is used in class components.
How do lifecycle methods affect performance in a React application?
Lifecycle methods allow developers to perform specific actions at different stages of a component’s life, such as fetching data or cleaning up resources. Proper use of these methods can help optimize performance by managing when and how components update and re-render.