Top 10 Most Asked React JS Interview Questions 2023
An updated list of questions and the answers that'll help you ace your next React Interview!
What is React JS, and what are its main features?
React JS is an open-source JavaScript library used to build user interfaces. It was created by Facebook and has since become one of the most popular front-end development libraries. React JS works by using a virtual DOM, which allows for efficient rendering of components and updating of the UI.
The main features of React JS include:
Declarative: React makes it easy to describe how your UI should look at any given moment, based on the current state of your application.
Component-based: React allows you to build your UI using reusable components, which can be combined and composed to create more complex interfaces.
Virtual DOM: React uses a virtual DOM to render changes to the UI more efficiently, improving the overall performance of your application.
JSX: React allows you to write HTML-like syntax directly in your JavaScript code, making it easier to create and manage UI elements.
One-way data flow: React follows a unidirectional data flow model, which makes it easier to debug and reason about your application.
What is the difference between a presentational component and a container component in React JS?
Presentational components are focused on rendering UI elements and typically have little to no logic. They are responsible for rendering the UI based on the props that they receive. Examples of presentational components include buttons, input fields, and other UI elements.
Container components, on the other hand, are responsible for managing the state of the application and passing down data to presentational components. They are typically more complex and can include business logic and data fetching. Examples of container components include screens, pages, and other higher-level components.
What are the advantages of using React JS?
Some advantages of using React JS include:
Improved performance: React's virtual DOM allows for efficient updates to the UI, making it faster and more responsive than traditional UI frameworks.
Reusable components: React's component-based architecture makes it easy to create reusable UI components, reducing development time and increasing code reusability.
Easy to learn: React's simple API and clear documentation make it easy for new developers to get started with the library.
Large community: React has a large and active community, which provides helpful resources, tools, and support for developers.
Flexibility: React can be used with a variety of other libraries and frameworks, making it easy to integrate with existing code bases.
What is virtual DOM, and how does it work in React JS?
The virtual DOM is a lightweight representation of the actual DOM in a JavaScript object tree. When changes are made to the state of a component, React re-renders the virtual DOM instead of the actual DOM. It then compares the new virtual DOM with the previous version and updates only the parts of the actual DOM that have changed.
This approach is more efficient than re-rendering the entire DOM each time a change is made, as it avoids unnecessary updates and improves the performance of the application.
You can checkout my detailed blog on Virtual DOM here.
What are the lifecycle methods of React JS components?
React components have several lifecycle methods that are called at different stages of the component's lifecycle. Some common lifecycle methods include:
componentDidMount: called after the component is mounted to the DOM.
componentWillUnmount: called before the component is unmounted from the DOM.
componentDidUpdate: called after the component has updated and the changes have been rendered to the DOM.
These methods allow developers to perform actions at different stages of the component's lifecycle, such as fetching data from an API, updating the UI based on changes in the component's state, or performing cleanup tasks.
What is JSX, and how is it used in React JS?
JSX is a syntax extension for JavaScript that allows you to write HTML-like syntax directly in your JavaScript code. It is used in React to define and create UI components in a more expressive way.
For example, instead of creating a React component with plain JavaScript like this:
const Button = (props) => {
return React.createElement('button', { onClick: props.onClick }, props.children);
};
You can use JSX to write the same component in a more readable and intuitive way:
const Button = (props) => {
return <button onClick={props.onClick}>{props.children}</button>;
};
JSX is not mandatory, but it is recommended as it makes it easier to create and manage UI elements in React.
What is the role of Redux in React JS?
Redux is a state management library that is commonly used in React applications. It provides a centralized store for managing application state, which can be accessed and modified by different components in the application.
The role of Redux in React is to help manage complex application state and to make it easier to reason about how data is flowing through the application. By centralizing the application state in a single store, it becomes easier to debug, test, and manage the state of the application.
Redux works by defining actions and reducers that modify the application state in response to user interactions or other events. These changes to the state are then propagated to the relevant components in the application.
What are higher-order components in React JS, and how do they work?
A higher-order component (HOC) is a function that takes a component as an input and returns a new component with additional functionality. HOCs are a common pattern in React for reusing component logic across different components.
An HOC works by wrapping a component in a new component that adds new props or modifies the behavior of the original component. For example, an HOC could add a new onClick handler to a component, or it could pass down data from a Redux store to a component as props.
Here's an example of an HOC that adds a "loading" prop to a component:
const withLoading = (Component) => {
return class extends React.Component {
state = { loading: false };
componentDidMount() {
this.setState({ loading: true });
fetchData().then(() => {
this.setState({ loading: false });
});
}
render() {
return <Component {...this.props} loading={this.state.loading} />;
}
};
};
This HOC adds a "loading" prop to the wrapped component and manages the loading state of the component.
What is the difference between state and props in React JS?
In React, both state and props are used to represent data that a component needs to render. However, there are some key differences between the two.
State is an object that belongs to a component and can be modified by the component itself. It represents the internal state of the component, and is used to store data that can change over time, based on user actions or other events. When the state of a component changes, React will automatically re-render the component.
Props, on the other hand, are objects that are passed down to a component from its parent. They are read-only and cannot be modified by the component itself. Props are used to pass data and event handlers from the parent component to its children, so that the child components can access and use this data to render themselves.
Here's an example code snippet to illustrate the difference between state and props:
import React, { Component } from 'react';
class ParentComponent extends Component {
constructor(props) {
super(props);
this.state = {
counter: 0
};
}
handleButtonClick = () => {
this.setState({ counter: this.state.counter + 1 });
};
render() {
return (
<div>
<ChildComponent counter={this.state.counter} />
<button onClick={this.handleButtonClick}>Increment</button>
</div>
);
}
}
function ChildComponent(props) {
return (
<div>
<p>Counter: {props.counter}</p>
</div>
);
}
In this example, we have a ParentComponent that has a state object with a counter property that is initially set to 0. The component also has a handleButtonClick method that updates the state when the button is clicked.
The ParentComponent renders a ChildComponent and passes the state's counter property as a prop to the child. The ChildComponent then displays the counter value using the props it received.
In summary, state and props are both used in React to manage data and render components, but the key difference is that state is internal to a component and can be changed by the component itself, while props are passed down from a parent component and cannot be changed by the child component.
What is the significance of the key attribute in React JS, and how is it used?
In React, the key attribute is used to give a unique identity to elements in an array or a list of components. When React updates the components, it uses the key to determine whether a component needs to be added, removed, or updated.
The key should be unique among the siblings of a component and stable across re-renders, which means that it should not change if the component's position in the array or list changes. When a component is re-rendered, React compares the keys of the old and new components to determine whether it needs to be updated or replaced.
Here's an example code snippet to illustrate the use of the key attribute:
import React, { Component } from 'react';
class List extends Component {
constructor(props) {
super(props);
this.state = {
items: [
{ id: 1, text: 'Item 1' },
{ id: 2, text: 'Item 2' },
{ id: 3, text: 'Item 3' },
]
};
}
handleRemove = (id) => {
this.setState({
items: this.state.items.filter(item => item.id !== id)
});
};
render() {
return (
<ul>
{this.state.items.map(item =>
<li key={item.id}>
{item.text}
<button onClick={() => this.handleRemove(item.id)}>Remove</button>
</li>
)}
</ul>
);
}
}
In this example, we have a List component that renders an unordered list of items. Each item has an id and a text property. The key attribute is set to the id property, which is unique and stable across re-renders.
The List component also has a handleRemove method that removes an item from the state's items array when the Remove button is clicked. When an item is removed, React will re-render the list with the updated state, and it will use the key to determine which item needs to be removed from the DOM.
In summary, the key attribute is an important part of optimizing the performance of a React application, especially when rendering arrays or lists of components. By providing a stable and unique identity for each component, React can efficiently update the DOM without having to re-render the entire list.
These are the most asked React JS interview questions that will help you get a better understanding of some of the core components of React and help you answer in a better and more efficient way and ace your interviews.