Top 20 Reactjs Interview Questions in 2020

September 11,2020 - 15 min read
Top reactjs interview questions 2020

Let us have a look at an overview of technology and where React fits in the whole landscape before we discuss the top 20 Reactjs interview questions.

Technology is taking giant steps day by day in the direction of progress and development. What was considered cool a decade ago is now redundant. Newer technologies are emerging and companies are constantly looking for technocrats with skills that are relevant.

React is one such addition in the stack of latest technologies that made a difference. React is developed by Facebook and maintained also by Facebook but along with a community of dedicated developers and companies as it is open sourced. Front end developers across the globe are switching to React to create reusable UI components.

If you are reading the article, you are probably well aware of the basic functionalities and features of React but for those who wandered their way here, there's is a basic definition before we move on to Reactjs interview questions. It is important to note that React is JavaScript library and not a framework. It is a User Interface library that helps in creating rich internet applications.

Coming to Reactjs interview questions, let’s start from the very basic and then proceed to more complex nuances of React. The questions are framed in a formal structure but the answers are more in a language that is explanatory hence try to understand the concept and then construct your own responses for the interview rather than mugging everything up.

Also, while we are discussing Reactjs interview questions, our main focus would be on React but here and there you may also find some useful interview tips!

Basic Reactjs Interview Questions

Q1. What is React?

React, as mentioned earlier, is a Javascript library developed by Facebook in 2011. It is used to create reusable UI components. The approach used by React is component based. The approach helps in creating complex and dynamic web and mobile user interface. It is also backed by one of the largest community of developers supporting it. Some of the distinct features of Javascript are usage of Virtual DOM, fast and efficient application performance and ease of making UI test cases.

Now, the answer may become extremely lengthy if you plan to blurt out everything you know in this question itself. However, the points mentioned above form a decent response.

This question makes for one of the most asked Reactjs interview questions. It might be the ice breaker in some cases, whatever the situation be, make sure you answer with confidence as this would one be one of the easiest questions you would face.

Q2. How does React work?

React works on the principal of declarative code. This declarative code is used to create components. Components are the building blocks of React. The UI design is split into separate, usable and independent pieces. These components (individual blocks) interact with each other using props to create a complete structure (the UI interface).

This makes up for quite a technical answer and the panel will be pretty much impressed provided that you answer other Reactjs interview questions with the same accuracy. We have used the word declarative code and render function (), be prepared to explain them in detail.

Q3. What do you mean by declarative code?

In a declarative code each line of code declares which element of the app it designs. It actually describes the ‘How’ but not the ‘what’. Declarative code is therefore lightweight, fast and easy to debug because it only focuses on the end result and doesn’t dwell in the details. As mentioned earlier, React uses the approach of a declarative code.

Q4. Explain the function render () in detail.

The render function significantly deals with React components. The function returns the representation of a React element which is a part of the original DOM. ReactDOM.render () is used to render an element into the root DOM node.

Q5. What is the difference between React Native and React?

React is a Javascript library that is used to build the UI interface as front-end web and being run on the server. React native is a mobile framework allows us to create native mobile applications i.e iOS, Android and windows.

Q6. React follows uni-directional data flow binding. Explain

In general, uni-directional flow of data means that there is only one way to be transferred to other parts. In React, the data coming from parent component to child component is called props. Props are shorthand for properties in React. They are immutable and are always passed from parent component to child component and never the other way round. This is the unidirectional data flow. It means that actions are triggered by the view and they can update the state. The state changed is passed to the view and to child components.

Uni-directional data flow binding has a handful of advantages. They are less prone to errors and otherwise easier to debug also as we are clear about the source and destination of the data and have more control on it. Overall, this makes the approach more efficient.

Q7. What is Flux?

Flux is an architecture that has a uni-directional data flow. It was developed by Facebook itself to use as an architecture internally when working with React. Flux is made up of individual components namely - Action, Dispatcher, Stores and Controller Views.

It enables communication between multiple components using a store in the centre which has authority for all data. Flux makes an application stable and minimises run time error.

Q8. Explain the term ‘state’ in React.

States are object which decides component rendering and behaviour, they create dynamic and interactive components and are accessed via this.state(). The state of the component can be updated using this.setState().

Q9. What is a virtual DOM? Explain in context of React.

DOM stands for document object model. A virtual DOM is a lightweight object of Javascript. The virtual DOM is a node tree made out of React components. The function render () is used to create the tree.

Virtual DOM is that it allows mutations whenever there are changes by the user or the system. In Occurrence of a change, the entire UI is rendered in the virtual DOM and then the virtual and the real DOM is compared. Only the differences are updated in the real DOM and not every change.

Q10. What are the advantages of virtual DOM?

A virtual DOM updates the JSX elements faster and manipulation is easier and quicker than Real DOM. Another advantage of virtual DOM is that there is no memory wastage.

The answer is short yet enough. We have used the term JSX in our answer, there might be a counter question on that. Let’s prepare ourselves for that.

Q11. What is JSX?

JSX is the syntax extension for JavaScript XML. Any JavaScript expression can be embedded in JSX. It is not compulsory to use JSX with React but it is the recommended way of describing the UI in the application.

Q12. Can browsers read JSX?

No, browsers cannot read JSX. Browsers can only read Javascript objects and JSX doesn’t happen to be so. Before any JSX file gets to the web browser, the JSX complier has to translate it into regular Javascript object. The compilers use JSX transformers and then pass it to the browser.

Q13. Explain the React component’s lifecycle.

Initial Rendering, Updating and Unmounting are the three phases of React component’s lifecycle.

All three phases are self-explanatory as to what they do. The initial rendering phase is when the component starts its journey makes its way to the DOM. The updating phase incorporates in itself all the changes. Updating and rendering when prop or state changes occur happens only in this phase.

Q14. What is an event in React?

Events are triggered whenever there are actions like mouse hover, key press, clicking etc. In React, events are named using camel case instead of only uppercase or lowercase and are passed as functions instead of strings. Event type has its own properties and behaviour that are accessed by its specific event handler only.

Q15. What are the major differences in ES6 and ES5 syntax?

There are a few but significant changes in the syntax of both the versions. For example, in ES5 you used the function require () but in ES6 you use import for the same purpose. There is a change in export also. There are evident changes in the use of props, states and component and functions when it comes to ES6.

Q16. What is the arrow function in React?

The arrow function is used to write functions in a more concise syntax. They were introduced in ES6. They also allow you to bind the context of the components as binding is available by default in ES6. They are often used with higher order functions. The arrow function is also called the fat arrow function. The symbol of arrow functions is (=>).

Q17. Draw a contrast between React and Angular?

In React there is only server-side rendering while Angular supports client-side rendering. Another major difference is that React uses the Virtual DOM approach while Angular uses real DOM for its functioning. React has a single direction flow of data while Angular has a bi directional data binding. There is a difference of runtime and compile time debugging also in Angular and React respectively.

Lastly, both of them have different authors. React was invented by Facebook while Angular is a product of Google.

Q18. What is routing in React?

Whenever a user enters a url, a router defines multiple routes. The user is directed to the particular route in the router which matches the url the user has entered.

A router library is to be added to enable routing in our application which creates multiple routes that lead us to a unique view.

Q19. What are the advantages of routing in React?

The advantages of using Routing in react is that a router can be visualised as a single root component, we don’t need to manually set history value and the packages are split which makes it easy to switch over based on a similar coding style.

Q20. Is conventional routing different from routing in React?

Yes, they are different in several aspects. In React routing, only single HTML page is involved while in conventional routing each view is of a new file. Another difference is that we receive a HTML page only when a HTTP request is sent to a server but in react only the history attribute is changed.

Here we come to an end of our collection of top 20 Reactjs interview questions that are most expected to be asked. We hope you got a hang of this amazing product by Facebook called Reactjs.

These questions are compiled in a comprehensive manner which gives you a detailed understanding of React however in order to be a good React developer, you need to be trained by professionals. If you are an enthusiastic developer keen to develop web applications then a professional course in React is strongly recommended.

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