Top 25 React JS Interview Questions and Answers
Are you preparing for a React JS interview? Whether you’re a beginner or an experienced developer, mastering React’s core concepts is essential to ace your interview.
In this post, we’ve compiled the top 25 React JS interview questions and answers to help you sharpen your skills and boost your confidence. From the basics of components, JSX, and props, to advanced concepts like hooks, Redux, and optimization techniques, these questions will cover everything you need to know to excel in your React JS interview.
1. What are components in React?
Components are the building blocks of a React application. They are reusable and encapsulate a piece of UI, including its logic and styling. Components can be either functional or class-based. Functional components are simple JavaScript functions that return JSX, while class-based components are ES6 classes that extend React.Component.
Example:
// Functional Component
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}
// Class-based Component
class Greeting extends React.Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}
2. How do you handle events in React?
In React, events are handled using camelCase event handler attributes such as onClick, onChange, etc. These attributes are provided as props to React elements and accept callback functions that will be invoked when the event occurs. Inside the callback function, you can access event properties like target.value for input elements or target.checked for checkboxes.
Example:
class Button extends React.Component {
handleClick() {
console.log(‘Button clicked!’);
}
render() {
return <button onClick={this.handleClick}>Click me</button>;
}
}
3. What are the lifecycle methods in React? Explain a few of them.
React components have several lifecycle methods that allow you to hook into different points in a component’s lifecycle. Some commonly used lifecycle methods include:
- componentDidMount: Invoked immediately after a component is mounted to the DOM. It is commonly used for initialization tasks such as fetching data from an API.
- componentDidUpdate: Invoked immediately after updating occurs. It is useful for performing side effects such as updating the DOM in response to prop or state changes.
- componentWillUnmount: Invoked immediately before a component is unmounted from the DOM. It is used for cleanup tasks such as removing event listeners or canceling network requests.
4. What is the purpose of keys in React lists?
Keys in React lists are used to identify unique items in the list. They help React identify which items have changed, been added, or removed. Keys should be stable, unique, and consistent across renders. React uses keys to optimize the re-rendering process by minimizing DOM manipulations.
Example:
function ListComponent() {
const items = [‘Apple’, ‘Banana’, ‘Orange’];
return (
<ul>
{items.map((item, index) => (
<li key={index}>{item}</li>
))}
</ul>
);
}
In this example, each list item is assigned a unique key using the index of the item in the array. While using array indices as keys is common, it’s important to note that keys should ideally be stable, unique identifiers associated with the data being rendered. If the order or contents of the list may change, it’s better to use stable keys that uniquely identify each item.
5. Explain the concept of lifting state up in React.
Lifting state up in React involves moving the state from a child component to its parent component. This is useful when multiple components need access to the same state or when a parent component needs to control the state of its children. By lifting state up, you ensure that the state remains in sync across all components that rely on it.
import React, { useState } from ‘react’;
function Calculator() {
const [count, setCount] = useState(0);
const increment = () => {
setCount(count + 1);
};
const decrement = () => {
setCount(count – 1);
};
return (
<div>
<h2>Counter: {count}</h2>
<Controls onIncrement={increment} onDecrement={decrement} />
</div>
);
}
function Controls({ onIncrement, onDecrement }) {
return (
<div>
<button onClick={onIncrement}>Increment</button>
<button onClick={onDecrement}>Decrement</button>
</div>
);
}
export default Calculator;
6. How do you pass data between components in React?
Data can be passed between components in React using props. Props allow you to pass data from parent to child components. If components need to communicate with each other, you can lift the state up to a common ancestor and pass down callback functions as props to update the state.
Additionally, you can use context or libraries like Redux for managing global state or passing data between deeply nested components.
7. What are the limitations of React?
The few limitations of React are as given below:
- React is not a full-blown framework as it is only a library.
- The components of React are numerous and will take time to fully grasp the benefits of all.
- It might be difficult for beginner programmers to understand React.
- Coding might become complex as it will make use of inline templating and JSX.
8. What are Custom Hooks?
A Custom Hook is a function in Javascript whose name begins with ‘use’ and which calls other hooks. It is a part of React v16.8 hook update and permits you for reusing the stateful logic without any need for component hierarchy restructuring.
In almost all of the cases, custom hooks are considered to be sufficient for replacing render props and HoCs (Higher-Order components) and reducing the amount of nesting required. Custom Hooks will allow you to avoid multiple layers of abstraction or wrapper hell that might come along with Render Props and HOCs.
The disadvantage of Custom Hooks is it cannot be used inside of the classes.
9. What is a key in React?
A key is a special string attribute you need to include when creating lists of elements in React. Keys are used in React to identify which items in the list are changed, updated, or deleted. In other words, we can say that keys are used to give an identity to the elements in the lists.
10. What is conditional rendering in React?
Conditional rendering in React involves selectively rendering components based on specified conditions. By evaluating these conditions, developers can control which components are displayed, allowing for dynamic and responsive user interfaces in React applications.
Let us look at this sample code to understand conditional rendering.
{isLoggedIn == false ? <DisplayLoggedOut /> : <DisplayLoggedIn />}
Here if the boolean isLoggedIn is false then the DisplayLoggedOut component will be rendered otherwise the DisplayLoggedIn component will be rendered.
11. What is the difference between controlled and uncontrolled components in React?
- Controlled components: The component’s state is managed by React, and the state value is bound to an input element.
- Uncontrolled components: The state is managed by the DOM itself, and React does not control the input element’s value.
12. What are React Fragments?
React Fragments allow you to group multiple elements without adding extra nodes to the DOM. It is a way to return multiple elements from a component without needing a wrapper element.
13. What is useMemo and useCallback?
- useMemo: A hook that memorizes the result of a computation to avoid unnecessary re-calculations on each render.
- useCallback: A hook that returns a memoized version of a callback function, ensuring that the function reference stays the same between renders.
14. What are render props in React?
Render props is a pattern where a component uses a function (a render prop) to know how to render its output. This allows for more flexible code reuse and customisation of component behaviour.
15. What is code splitting in React?
Code splitting is a technique to split the codebase into smaller bundles, which are then loaded on-demand. This improves the performance of your React application by reducing the initial load time.
16. What are React Refs?
Refs are used to reference DOM elements or class components directly in React. They allow you to interact with DOM nodes or class methods without needing to re-render the component.
17. How does React optimize performance?
React optimizes performance through features like:
- Virtual DOM and efficient re-rendering.
- React Fiber (asynchronous rendering).
- shouldComponentUpdate lifecycle method and React.memo for memoizing components.
- Lazy loading and code splitting to load components only when needed.
18. What is the context API in React?
The context API allows you to pass data through the component tree without having to manually pass props down at every level. It is useful for managing global state like themes, authentication status, etc.
19. What is the useState hook?
The useState hook is used to declare state variables in functional components. It returns an array where the first element is the current state value, and the second is a function to update the state.
20. What is the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It runs after every render and can be used to fetch data, update the DOM, or subscribe to events. It can also clean up resources when a component unmounts or dependencies change.
21. What is Redux?
Redux is a state management library for JavaScript applications. It provides a centralized store for all the components in an app and ensures that the state is consistent across the app by using actions and reducers.
22. What are React keys, and why are they important?
Keys are unique identifiers used by React to track elements in a list. They help React identify which items have changed, are added, or are removed, optimizing the re-rendering process.
23. What is React Router?
React Router is a library for managing navigation and rendering different components based on the URL. It enables single-page applications (SPAs) to have multiple views, mimicking traditional web navigation.
24. What is a state in React?
State is an object that represents the data or properties of a component. When the state changes, React re-renders the component to reflect the new state.
25. What are props in React?
Props (short for properties) are inputs to a React component, passed from a parent to a child. Props are immutable, meaning they cannot be modified by the child component.
These top 25 React JS interview questions and answers are designed to help you master the essentials and advanced concepts of React. By reviewing these key topics and understanding their applications, you’ll be well-equipped to impress your interviewers and take your React skills to the next level. Stay confident, keep practising, and get ready to land your next React JS role with ease!