Top 20+ ReactJS Interview Questions and Answers
Prepare for an interview with the top 20+ ReactJS Interview questions and answers. Gain insights and boost your confidence to excel in ReactJS job interviews.
1. What is React?
Answer: React is a JavaScript library for building user interfaces, used to create a single-page application. It allows developers to create reusable UI components and manage the view layer of web applications efficiently.
2. What are the features of React?
Answer:
- JSX (JavaScript XML): A syntax extension that allows mixing HTML with JavaScript.
- Components: The building blocks of a React application.
- Virtual DOM: A lightweight copy of the actual DOM that React uses to optimize rendering.
- One-way Data Binding: Data flows in a single direction, making debugging easier.
- State Management: React manages state in components, enabling dynamic and interactive UIs.
3. What is JSX?
Answer: JSX is a syntax extension of JavaScript that looks similar to HTML. It allows developers to write HTML-like code within JavaScript, making the code easier to understand and maintain. JSX gets transformed to regular JavaScript by tools like Babel.
Example: const element = <h1>Hello, world! </h1>;
4. What are the components in React?
Answer: Components are the building blocks of a React application. They are reusable pieces of UI that can be either class-based or functional. Components can accept inputs (called “props”) and return React elements that define the UI.
- Functional Components: Stateless components are defined as functions.
- Class Components: Stateful components defined as ES6 classes.
Example:
// Functional Component
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
// Class Component
class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
5. What is the difference between state and props?
Answer:
- Props (short for properties): Immutable data passed from a parent component to a child component. Props are used to pass data and event handlers to components.
- State: A component’s local data storage, which is mutable and managed within the component itself. The state is used to manage dynamic data and can be updated using setState() in class components or the useState hook in functional components.
- Example:
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
Import React, {Component} from ‘react’;
class Counter extends Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
render() {
return <h1>Count: {this.state.count}</h1>;
}
}
6. What is the Virtual DOM?
Answer: The Virtual DOM is a memory representation of the actual DOM that React maintains in memory. When the state of an object changes, the Virtual DOM is updated first, and then the difference between the previous and the new state (a process called “reconciliation”) is calculated. Only the changed elements are updated in the actual DOM, to improve the speed and performance of the application.
7. What are React Hooks?
Answer: React Hooks are predefined functions that give the power to achieve the functionality of class components in functional components. It’s Introduced in React 16.8, hooks like useState, useEffect, and useContext enable functional components to manage state, side effects, and context without needing to convert them into class components.
Example:
import React, { useState, useEffect } from ‘react’;
function Counter() {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = `Count: ${count}`;
});
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
8. Explain the useEffect hook.
Answer: The useEffect hook allows you to perform side effects in functional components. It runs after every render by default but can be controlled to run only when certain values change by passing dependencies as a second argument.
useEffect(() => {
// This code runs after every render
console.log(‘Component rendered’);
return () => {
// Cleanup code runs before the component is unmounted
console.log(‘Cleanup’);
};
}, []); // The empty array ensures it runs only on the mount and unmount
9. Why is there a need for using keys in Lists?
Answer: Keys are very important in lists for the following reasons:
- A key is a unique identifier and it is used to identify which items have changed, been updated, or deleted from the lists
- It also helps to determine which components need to be re-rendered instead of re-rendering all the components every time. Therefore, it increases performance, as only the updated components are re-rendered
10. What are forms in React?
Answer: React employs forms to enable users to interact with web applications.
- Using forms, users can interact with the application and enter the required information whenever needed. The form contains certain elements, such as text fields, buttons, checkboxes, radio buttons, etc
- Forms are used for many different tasks such as user authentication, searching, filtering, indexing, etc
11. What is an arrow function and how is it used in React?
- An arrow function is a short way of writing a function to React.
- It is unnecessary to bind ‘this’ inside the constructor when using an arrow function. This prevents bugs caused by the use of ‘this’ in React callbacks.
Example:
export const Greet= () => {
return <h1>Hello All!!!</h1>
}
const function App ()
{
Return <div>
<Greet/>
</div>
}
export default App;
Q12. What are the rules that must be followed while using React Hooks?
- React hooks must be called only at the top level.
- It’s not allowed to call them inside from any Java script function, loop, or condition.
- It’s allowed to call hooks from the react functional component.
Q13.What is react Router and why is it implemented?
React router is a standard library for routing in React. It provides the feature of navigation among the different components in the react application. It mounts the component based on the URL passed in the browser.
Q14. What is the difference between controlled and uncontrolled components?
Answer:
- Controlled Components: These are form elements in which React controls the value via state. The component’s state dictates the input’s value, and updates to the input call a function to update the state.
function ControlledInput() {
const [value, setValue] = useState(”);
return (
<input value={value} onChange={(e) => setValue(e.target.value)} />
);
}
- Uncontrolled Components: These components rely on the DOM to manage their state. Instead of managing the input’s value with state, you use refs to get the current value.
function UncontrolledInput() {
const inputRef = useRef(null);
return (
<input ref={inputRef} />
);
}
15. What is useMemo and why is it used?
Answer:
useMemo is a hook that memoizes the result of a function, ensuring that it only recomputes when its dependencies change. This is useful for optimizing performance, particularly when the function is computationally expensive and the component re-renders frequently.
const expensiveCalculation = useMemo(() => {
return computeExpensiveValue(a, b);
}, [a, b]);
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16. How do you handle errors in React components?
Answer:
React provides a way to handle errors in components using Error Boundaries. An error boundary is a component that catches JavaScript errors in its child component tree, logs them, and displays a fallback UI instead of crashing the entire application.
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
return { hasError: true };
}
componentDidCatch(error, errorInfo) {
// Log the error to an error reporting service
L ogErrorToService(error, errorInfo);
}
render() {
if (this.state.hasError) {
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}
Q17. What is Redux, and why would you use it in a React application?
Answer:
Redux is a state management library for JavaScript applications. It provides a centralized store for the application’s state, allowing for predictable state changes via actions and reducers. Redux is especially useful in large applications with complex state that needs to be accessed and modified across many components, reducing the need for “prop drilling” and making debugging easier.
Q18. What are actions in Redux, and how are they used?
Answer:
Actions in Redux are plain JavaScript objects that describe a change that should occur in the state. Each action has a type property that indicates the type of action, and may also contain a payload with additional data. Actions are dispatched to the Redux store, where they are handled by reducers to update the state.
const incrementAction = {
type: ‘INCREMENT’,
payload: { value: 1 }
};
Q19. What are React fragments, and why would you use them?
Answer:
React fragments (<React.Fragment> or the shorthand <> </>) allow you to group a list of children elements without adding extra nodes to the DOM. This is useful when you need to return multiple elements from a component, but don’t want to add unnecessary parent nodes like <div>.
function MyComponent() {
return (
<>
<h1>Title</h1>
<p>Some content</p>
</>
);
}
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Q20. What is the useRef hook, and how is it used?
Answer:
useRef returns a mutable object that persists for the lifetime of the component. It is commonly used to access and manipulate DOM elements directly or to store mutable variables that do not trigger a re-render when updated.
function FocusInput() {
const inputRef = useRef(null);
useEffect(() => {
inputRef.current.focus();
}, []);
return <input ref={inputRef} />;
}
Q21. What is useContext, and when would you use it?
Answer:
useContext is a hook that allows you to access the value of a context directly within a functional component. It eliminates the need for using Context.Consumer and is used when you need to access shared data (like theme, user information, etc.) across multiple components.
Example:
const ThemeContext = React.createContext(‘light’);
function ThemeButton() {
const theme = useContext(ThemeContext);
return <button className={theme}>Button</button>;
}
Q22. What is Prop Drilling, and how can it be avoided?
Answer:
Prop Drilling occurs when you pass props through several intermediate components to reach a deeply nested component. It can be avoided by using React Context to provide a global state that can be accessed by any component in the tree, or by using state management libraries like Redux.
Q23. What are Higher-Order Components (HOCs), and how are they used?
Answer:
Higher-order components are functions that take a component and return a new component with additional props or behaviors. HOCs are used for reusing component logic, such as authentication checks or data fetching.
function withLogging(WrappedComponent) {
return function(props) {
console.log(‘Rendering with props:’, props);
return <WrappedComponent {…props} />;
};
}const EnhancedComponent = withLogging(MyComponent);
Q24. What is a reducer in Redux?
Answer:
A reducer is a pure function that takes the current state and an action as arguments and returns a new state. Reducers specify how the state changes in response to an action. Because reducers are pure functions, they do not modify the original state but return a new state object instead.
function counterReducer(state = { count: 0 }, action) {
switch (action.type) {
case ‘INCREMENT’:
return { count: state.count + action.payload.value };
case ‘DECREMENT’:
return { count: state.count – action.payload.value };
default:
return state;}
}
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