React Interview Questions and Answers: (Updated)

Are you preparing to interview for your dream job as a React developer? Or are you considering a career change to become a React developer? Wouldn’t it be great if you knew what questions a potential employer will ask you in a React interview? This article will share top ReactJS interview questions and answers, including ReactJS, React Native, Redux, and so on.

For your convenience, we have covered ReactJS interview questions into the following categories:

• General ReactJS Interview Questions
• React Components Interview Questions
• Redux Interview Questions
• React Native Interview Questions

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General ReactJS Interview Questions and Answers

Q1: What is React?

Ans: React is an open source JavaScript library used to develop complex and interactive web and mobile user interfaces. It is used to create web browser applications, build dynamic libraries, and create user interfaces that are well equipped to display large data sets. React follows a component-based approach to create complex and reusable user interfaces from small, isolated pieces of code called components.

Q2: What are the features of React?

Ans: React features are:

• High performance: Instead of updating all the components at once, React only updates the components that have changed. This makes web applications much faster.
• One-way data binding: One-way data binding in React keeps things fast and modular. Due to the one-way nature of data flow, integrating child components into the parent component is a common practice when building React applications.
• JSX: JSX is a JavaScript syntax extension. It is used with React to specify how the user interface should look. We can write HTML structures in the same JavaScript code file using JSX.
• Components: Every React application is built from components, and an application usually consists of multiple components. It divides the user interface into separate, reusable and independently managed components.

Q3. What are the advantages of React?

Ans. The advantages of React are:

• Better performance: React uses a virtual DOM, which speeds up inline applications. Like traditional web applications, the virtual DOM compares its current state to its previous state and updates only those parts of the real DOM whose state has changed.
• Easy to develop dynamic applications: React offers more capacity with less writing, whereas JavaScript applications tend to have quite complex code very quickly. Therefore, React makes it easy to create dynamic online applications.
• Reusable components: Any React application is built from components, and an application usually consists of many components. Application development time is significantly shortened thanks to these components, each of which has its own logic and controls and can be reused throughout the application.
• Dedicated tools for easy debugging: We can use the Chrome extension provided by Facebook to debug React applications. This makes the process of debugging React for web applications faster and simpler.

Q. 4 Explain the process of creating a React app?

Ans. The following steps will help you create a React app:

1. First of all, you need to have NodeJS installed on your computer because the Node package manager (npm) is required to install the React library.
2. Npm contains several JavaScript libraries, including React.
3. Install the ‘create-react-app’ package via command prompt/terminal.
4. Install any suitable text editor.

Q5. What are the disadvantages of React?

Ans. The disadvantages of React are:

Ans. The differences between the Real DOM and the Virtual DOM are:

Q9. Explain Virtual DOM.

Ans. DOM stands for “Document Object Model”. Virtual DOM (VDOM) is a virtual representation or copy of the real DOM. It is a programming concept in which an ideal or virtual representation of a user interface is kept in memory and synchronized with the actual DOM. So whenever the application state changes, the virtual DOM will be updated instead of the real DOM.

Q10. What is React.createClass?

Ans: Since JavaScript has no classes, React has its own class system. React.createClass allows you to create component classes. So the component class uses the class system implemented by React. With ES6, React allows you to implement component classes using ES6 JavaScript classes. Although the implementation is different, the end result is the same.

Q11. Explain the lifecycle methods of ReactJS?

Ans: ReactJS lifestyle methods are:

• componentWillMount: This method is used for application level configuration in the root component. It is executed before rendering
• componentDidMount: This method is executed only after the first rendering on the client side. In this context, all AJAX requests, DOM or state updates, etc. are all stored.
• componentWillReceiveProps is called when props are updated and before the next render is called.
• ShouldComponentUpdate: It will return true or false and will determine if the component is updated or not. This is set to true by default. If you’re sure that a component doesn’t need to be rendered after updating state or props, you can return false.
• componentWillUpdate: It is called just before displaying the component.
• componentDidUpdate: It is called immediately after rendering and is used to update the DOM in response to changes in props or state.
• componentWillUnmount: It is called after the component is unmounted from the DOM.

Q12. What do the keys mean in React?

Ans: Keys are used in React to identify unique VDOM elements with corresponding data that drive the user interface and help React identify elements that have been changed, added, or removed. The key must be a unique number or string.

Q13. What are refs in React?

Refs is a function in React used to reference a DOM element or class element from the parent component. It allows you to read and modify the value of a child component without using any props. References can also be helpful when using third-party libraries in animations.

Q14. What is a higher order component (HOC)?

Ans: Higher order components are an advanced way to reuse component logic. This is a pattern that stems from React’s compositional nature and is not considered part of the React API. HOCs are functions that take an element and return a new element. HOCs are pure components because they can accept any dynamically provided child components, however, they will not copy or modify any of the behavior of their input components.

Q 15. Explain an event in React?

An event is a user- or system-initiated activity, such as a mouse click or key press. Instead of using lower case in HTML, React event names are written in CamelCase. Unlike HTML which uses strings as event handlers, JSX uses functions.

Q16. What are React hooks?

Ans: Hooks allow you to use state and other React features without writing classes. These built-in functions allow developers to use state and lifecycle methods inside functional components. They don’t work in the classroom. By using Hooks, you can completely avoid using lifecycle methods, such as componentDidMount, componentDidUpdate, and componentWillUnmount.

React Components Interview Questions 

Q17. What is Props?

Ans. Short for properties, props are arguments passed to React components. In JavaScript, they act as function arguments. Props accept arbitrary input and return React elements detailing what will be displayed on the screen. The data is kept in a one-way flow by props and only passed from parent to child parts throughout the application. A child component can never return a prop to the parent component.
They are immutable.

Q18. What is state in React?

Ans: States are the source of data. These are objects that determine the rendering and behavior of components. While props are immutable, states are mutable and can change over time. States create dynamic, interactive components and are accessed using this state(). When using the State component, you’ll set the component’s default state, navigate to the current state, and update the state.

Q19. What is the purpose of render() in React?

Ans: The render() function in React renders the specified HTML code inside the specified HTML element.
Every React component must have render(). This function takes two arguments, HTML code and an HTML element, and returns a single element that represents the root DOM element. If multiple HTML elements need to be displayed, they must be grouped in an accompanying tag, such as , , and .

Q20. What are pure components in React?

Ans: Pure components are components that produce the same results for the same condition and the same accessories. These are the simplest and fastest components that can be written. They can replace any component that only has render(). Therefore, a function is said to be pure if:

• Its return value is determined only by its input values ​​
• Its return value is always the same for the same input values ​​

Q21. What are events in React?

Ans: React is action based on user events. An event can be triggered by a user action or by a system-generated event. React has HTML-like events, such as clicks, edits, and mouseovers. React events are written using CamelCase syntax and are passed as functions instead of strings. Event arguments have a specific set of properties and behaviors.

Q22. Explain the different phases of the ReactJS component lifecycle? 

Ans. There are four different phases in the ReactJS component lifecycle: Initialize , Mount, Update, and Unmount.

1. Initialize: This is the first step in the ReactJS lifecycle. In this phase, the component is constructed with the provided properties and a default state.
2. Assembly: This is the phase that involves injecting elements into the DOM. It includes the ComponentWillMount and ComponentDidMount lifecycle methods.
3. Updated: This step updates a component’s step and repaints the application. This phase includes the ShouldComponentUpdate and ComponentWillUpdate lifecycle methods.
4. Unmount: This is the final step where the component is removed from the page.

Q23. How to create an event in React?

Ans. An event can be created in React using the following:

class Display extends React.Component({

show(msgEvent) {

},

render() {

return (

<div onClick={this.show}>Click Me</div>

);

}

});

Q24. Explain synthetic events in React. 

Ans. Composite events are cross-browser wrappers around native browser events. It combines the behavior of various native browser events into a single API, including stopPropagation() and PreventDefault(). The browser can recognize different types of events such as MouseEvent, KeyboardEvent, etc. and there will be different levels of functionality for each of those event types. However, in React there is only a SyntheticEvent object, with common functionality that creates cross-browser consistency.

e is a Synthetic event in the below example: 

function ActionLink() {

function handleClick(e) {

e.preventDefault();

console.log(‘You had clicked a Link.’);

}

return (

<a href=”#” onClick={handleClick}>

Click_Me

</a>

);

}

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Q25. What is the arrow function in React? 

Ans. Arrow functions are a type of function syntax in JavaScript. Arrow functions do not redefine the value of “this” in their function body compared to another method that uses binding calls inside the constructor. The scope of “this” is global and not limited to any calling function. This makes it easier to predict their behavior when passed as a callback, and avoids errors caused by using “this” in a callback.

Example: 

//General way

render() {

return(

<MyInput onChange={this.handleChange.bind(this) } />

);

}

//With Arrow Function

render() {

return(

<MyInput onChange={ (e) => this.handleOnChange(e) } />

);

}

Q26. Describe the component lifecycle methods.

• getInitialState(): This is done before the component is created.
• componentDidMount(): is executed when the component is rendered and added to the DOM.
• ShouldComponentUpdate(): is called whenever a component detects changes in the DOM and, depending on the specific case, returns “true” or “false”.
• componentDidUpdate(): Called immediately after rendering.
• componentWillUnmount(): called immediately before a component is permanently unmounted and destroyed.

React Redux Interview Questions and Answers

Q27. What is Redux?

Ans. Redux is a predictable state container for JavaScript applications. It is a state management tool that allows developers to:

• write applications that behave consistently
• run in a variety of environments and
• can be tested easily

It is commonly used Applicable with React, but also works with other JavaScript libraries and frameworks.

Q28. What are the three fundamental principles of Redux?

Ans. The three fundamental principles of Redux are:

1. Single source of truth. A single store that stores your application state as a tree of objects. The single state tree will make editing and testing the application easier.
2. Read-only status. This means the only way to change the state is to trigger an action.
3. A pure function is used to make changes. You need pure functions to specify how actions transform the state tree

Q29. What are Redux components?

Ans: Redux has 4 components, which are:

• Action: It is an object that tells what happened. Actions send data from the app to the store and are the single source of information for the store.
• Reducer : Reducer specifies how the application state should change in response to actions sent to the store. While actions determine what happened, reducers determine how the state will change in response to an action.
• Store: This is where the state tree or objects of the entire application are stored. Store combines Actions and Reducers
• Views: It displays the data provided by the Store.

Q30. What is the use of Reducer?

Ans. By triggering an action, the reducer describes how the application state changes. The reducer takes the previous state and action and returns the new state.

Simply put, if you repeat the same test with the same input data as arguments, you can still expect the same results.

Q31. How are actions defined in Redux?

Ans: Actions are simple JavaScript objects with a type field. There must be a type attribute on actions indicating the type of action. Defining them as string constants allows you to add more properties to them.

We can create actions using functions called Action Creators. We can create actions using functions called Action Creators.

Example: 

function addTodo(text) {

return {

type: ADD_TODO,

text

}

}

Q32. What is the difference between React and Flux?

Ans. The differences between React and Flux are:

Q33. What are the advantages of React Redux?

Ans. Some of the key benefits of React Redux are:

1. Redux helps predict state.
2. This makes the code easier to maintain.
3. Easily debug an application.
4. Redux performs many performance optimizations internally.
5. Easily test Redux applications.
6. Allow users to keep partial application state on local storage and restore it after refresh.
7. Server-side rendering is possible.

Q34. How can you access the Redux store outside of a component?

Ans: To access the Redux store outside of a component, we will need to export the Store from the module where it was created using the createStore() method while also ensuring that it will not pollute the overall window space .

Below is the code: 

store = createStore(myReducer)

export default store

React Native Interview Questions and Answers

Q35. What is the difference between Hot Reloading and Live Reloading in React Native? 

Ans. Live reload reloads or refreshes the entire application when a file is changed. Hot reloading refreshes only the changed files without losing the application state. It only shows the code changes without restarting the application.

Q36. What is the difference between ReactJS and React Native? 

Ans. The differences between React JS and React Native are:

Q37. Explain the use of Flexbox in React Native? 

Ans. Flexbox is a layout implementation in React Native. It provides an efficient way to create user interface and position control in React Native. It provides 3 properties flexDirection,justContent and alignItems to achieve the desired layout.

Q38. Explain the use of the ScrollView component.

Ans. The ScrollView component is a generic scrolling container. It can contain multiple components and views. However, scrollable elements do not have to be uniform. It allows you to scroll vertically and horizontally by setting the horizontal property.

Q39. What do you mean by Component Driven Development (CDD)?

Ans. Component-driven development (CDD) is a method of managing the development process around components. It is a bottom-up user interface creation process, starting at the component level and ending at the page or screen level. It’s a concept that applies to modern JavaScript frameworks, design systems, and template libraries.

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Q40. How to create a stackNavigator in React Native?

Ans. Below is the code to create a stackNavigator in React Native

const AppNavigator = createStackNavigator({
Home: {
Screen: HomeScreen,
},
});