React JS Interview Questions (2021)

After a telephonic interview round, you will have to face a walk-in interview with typical questions about your qualifications and skills. Apart from the general preparations of open-ended questions, you should also know how to answer technical questions.

Even if you answer the questions with confidence and charisma, you must be accurate while answering technical questions. We have compiled a few technical questions for you to prepare before you appear for the interview.

Consider these as examples of the type of questions you can expect during a React JS interview.

Q.1) Explain the difference between Real DOM and Virtual DOM?

Ans: Virtual DOM updates faster, but it cannot directly update HTML. It can update JSX if elements are updated. It is an excellent option if you do not want to waste memory, and it is also easy to manipulate. 

On the other hand, Real DOM updates are slower, but they can directly update HTML. However, they are not cost-effective because created DOMs from element updates are expensive to manipulate. In addition to that, Real DOM also requires a lot of storage space. 

Q.2) How would you describe ‘React’?

Ans: Reach is developed by Facebook, which follows a component-based approach that helps in forming reusable UI components. It is a front-end JavaScrip library used for creating complex mobile UI and interactive web. React was open-sourced in 2015, and today it has extensive community support.

Q.3) Can you tell us the major advantages of using React? 

Ans: React increases the efficiency of an application’s performance, plus you can use it on both client and server sides. Codes written in React are readable due to JSX, and writing UI tests becomes hassle-free. One can integrate React with other frameworks like Angular and Meteor with ease. 

Q.4) What are the three main features of React?

Ans: React uses Virtual DOM instead of real DOM. It follows data binding or uni-directional flow while using server-side rendering, 

Q.5) Do you think there are any limitations to React?

Ans: Yes, I think it is difficult for a beginner-level programmer to grasp, and the coding can get extremely difficult due to JSX and inline templating. Also, being an extensive framework, it takes time to get familiar with React.

Q.6) Describe VIrtual DOM and its working. 

Ans: Virtual DOM is a copy of the real DOM, and it is a lightweight JavaScript object. It is a node tree that lists all the attributes, contents, and elements as objects with their properties. The render function in React creates a node tree from its components. When a user or system performs multiple actions, it keeps updating the tree to respond to the data model’s mutation. 

The Virtual DOM’s working follows three steps:

  1. Firstly, the entire UI gets rendered in Virtual DOM representation even when there are slight changes to the data.
  2. This creates a difference between the representation of new and previous DOM, which is calculated. 
  3. The real DOM only gets updated with things that were changed after the calculations are finished. 

Q.7) Tell us what you know about JSX?

Ans: JSX is a file that React uses along with HTML-like template syntax to utilize the expressiveness of javascript. It is a shorthand for JavaScript SML that makes any HTML file easy to grasp. JSX is used to increase the efficiency and performance of applications. 

Q.8) Explain the difference between React and Angular.

Ans: There are a few major differences between React and Angular, such as the architecture where the former only gets the view of MVC while the latter gets a view of complete MVC. When it comes to rendering, Angular has client-side rendering, and React goes with server-side rendering. React uses virtual DOM and one-way data binding, Angular uses real DOM with Two-way data binding. The debugging in Angular is runtime debugging, while React has compile-time debugging. Lastly, Google created Angular, whereas the credit for building React goes to Facebook. 

Q.9) What do we mean by Props?

Ans: Props are read-only components that should be kept immutable or pure, and it is the shorthand for ‘Properties in React’. In an application, they are sent from the parent to the child component, which helps to maintain the unidirectional flow of data. Props also help in rendering the data that’s dynamically generated.

Q.10) What are states in React, and how do you use them?

Ans: States are the soul of React components, they must be simplified because they are the data source. One can say that states are objects that determine components’ behavior and rendering. Changes can be made to state because they are mutable and efficient at creating interactive and dynamic components. 

Q.11) What are the major differences between states and props?

Ans: The parent component in props can change the value while it cannot happen in a state. However, there are changes inside the components of the state, but this is not the case with props. Similar to the parent component, the child component also does not have any changes, but they can be observed in the props. 

Q.12) What are the similarities of state and props?

Ans: There are mainly three similarities between the state and props.

  1. They both receive initial value from the parent component. 
  2. In both cases, you can set default values inside components. 
  3. In the child component, you can set the initial value with ease.

Q.13) What are stateful components? 

Ans: The state change in memory of components is stored by stateful component, they also have the authority to change state and be aware of all the changes present in a state. Stateless components send down the props after notifying them about the needed state change.

Q.14) What do we mean by stateless components?

Ans: The internal state of components is calculated by stateless components, and they do not have the authority to change state. They lack the knowledge of the present, past, and future state changes. Lastly, the stateful components send them props which they treat as callback functions. 

Q.15) Describe synthetic events. 

Ans: The objects which behave as a cross-browser wrapper around the native event of the browser are known as synthetic events. Their key function is to create one API by combining the behavior of multiple browsers. This ensures consistent properties in events across different browsers. 

Q.16) What are refs, and when should you use them?

Ans: Ref means References in React, and it is an attribute that stores a reference of a particular react component or element. The render configuration function in components returns these references. They help add methods to significant components and get DOM measurements. We can also use references to manage focus, select media playback or text. In addition, they also help in integrating with third-party DOM libraries and triggering imperative animations. 

Q.17) What are controlled components? 

Ans: Control components cannot maintain their state, and the parent component controls the data. Also, they acquire current values with the help of props and use callbacks to notify about changes.

Q.18) What do you know about uncontrolled components?

Ans: Uncontrolled components maintain their state. Here, the DOM controls the data, unlike controlled components. They also use refs to derive their current values. 

Q.19) State the advantages of Redux.

Ans: Redux offers the following advantages –

  • Organization: Teamwork becomes easier on Redux because it is particular about the organization of the code, making it more consistent. 
  • Easy to test: The code is independent and testable because it has isolated, small, and pure functions.
  • Community: Redux has the backup of a large and talented community that continues to improve the library while building multiple applications. 
  • Developer tools: It is easy for developers to track everything in an application, from state changes to action. 
  • Server-side rendering: In redux, you only need to send the store to the client-side, making it a better user experience because the application’s performance elevates. 
  • Maintainability: The code has a strict structure and a predictable outcome, making it easier to maintain. 
  • Predictability in outcomes: There is no confusion about syncing the current state action with parts of the application because there is only one store source. 

Conclusion

Learning React Js is not complicated if you are persistent and dedicated. Once you know your programming language inside out, you can crack any interview with ease. If you are just exploring the language now or want to deepen your knowledge, check out our Master of Science in Computer Science from Liverpool John Moore University. This 19-month course includes some of the top trending tools, including React, Java, JavaScript, Spring, Hyperledger, and Ethereum, to name a few. 

After completing this course, you will gain a deep understanding of React. Not just that, you will be eligible for bagging high-paying roles like Software Developer/Engineer, Full Stack Developer, Blockchain Developer, Cloud Expert/Architect, Cyber Security Engineer, Data Engineer, and DevOps Engineer.

What are the benefits of learning React?

There is a huge demand for React JS developers in the market, and it also provides good pay. In addition to that, React JS helps save time and money because developers work in teams and use fewer resources.

Is React challenging to learn?

React is relatively easy to learn, but you must know JS before learning this library. However, only basic knowledge of JavaScript is required to start learning React.

What are some of the essential skills for React JS developers?

React JS developers must possess a good understanding of HTML and CSS, JavaScript fundamentals, JSX, Git, Redux, Node, and NPM.

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