Categories: Reactjs

15 React.js Interview Questions

How does it feel to know exactly what React.js interview questions the hiring panel will be asking you in your next interview?

It’s true I can not read the minds of your interview panel but this article will help you understand 15 React.js interview questions that are always frequently asked.

If you have been dreaming of that front end developer job and you will be having an interview soon then this article is suited for you.

Kindly consider the following list as among 15 best React.js interview questions.

1. What is React?

  • React is a front-end JavaScript library that was developed by Facebook in the year 2011.
  • Normally it takes the component-based approach which is useful in building reusable UI components.
  • React is also very essential because it is used in developing complex and interactive web and mobile UI.
  • Despite being open-source, it has one of the largest communities supporting it.

2. State features of React

Some of the major features of React are

  • It uses Virtual DOM, which enables it to build scalable and fast applications.
  • It uses server-side rendering.
  • One way data binding.
  • It supports JSX

3. Can you explain to me what Flux means?

By definition, Flux is an architectural pattern that is usually used by Facebook internally while using React.

The main importance of Flux is controlling derived data to enable multiple components to mingle with data without any form of risk pollution.

The main components of Flux are;

  • Stores which are containers for application state and logic which have callbacks that are registered to the Dispatcher.
  • Controller views.
  • Dispatcher which facilitates the receiving of actions and broadcasts.
  • Actions that enable the passing of data to the Dispatcher.

4. What is the difference between Real DOM and Virtual DOM

Real DOMVirtual  DOM
1. Its updates are very slow.1. Its updates are faster.
2. Real DOM supports direct update HTML.2. Virtual DOM can’t directly update HTML.
3. Real DOM usually creates a new DOM if element updates.3. Virtual DOM updates the JSX if element updates.
4. To manipulate Real DOM is very difficult and expensive.4. To manipulate Virtual DOM is very easy.
5. With Real DOM there is too much memory wastage.5. With Virtual DOM there is no memory wastage.

5. What are the common methods of styling a React application?

There are several methods of styling React applications that one can consider. Deciding on which styling method to use is never that easy as it might seem. There are several factors that should be considered, that include, user preferences, relevance to your project, and above all, the architectural goals of the project you working on.

Currently, there are about eight different React.Js styling methods that you can work with;

  • Normal CSS
  • Inline CSS
  • Sass and SCSS
  • CSS in JS
  • Stylable
  • Styled Components
  • Less
  • CSS Modules

It important to determine the need for dependencies, level of difficulty, scalability, reusability among other factors before deciding on which styling method to settle for.

6. What are some of the major benefits of React?

The major advantages of using React are;

  • It increases the application’s performance.
  • supports faster rendering.
  • Can be easily and conveniently be used on the client as well as server-side
  • Since it has JSX, the code’s readability increases by far.
  • React is compatible with other frameworks like Meteor, Angular.
  • Using React for writing UI test cases becomes extremely easy than usual.
  • React gives a very stable code.
  • React has a version known as React Native that is used for mobile app development.

7. Briefly explain how automated tooling be used to improve the accessibility of a React Application?

Web accessibility is also known as a11y. Accessibility of React application involves designing and creating websites that can easily be accessed and used by anyone. Accessibility support is very important since it helps assistive technology in the interpretation of web pages. Generally, there are two main categories of automated tools that are handy in identifying accessibility issues:

Browser tools

Browser accessibility tools such as aXe and Google Lighthouse are handy in performing automated accessibility at the app level. This is important since it is used in discovering more real-world issues because a browser is used to improve the manner in which a user uses a website. These tools are capable of running in a continuous integration environment, for example, Jenkins or Travis. Because these tools take much time to execute, many developers prefer running these tools just within their local browser in most cases, for example when reaching project milestones.

Static Analysis Tools

For static analysis, there are several Linting tools, for example, ESLint which is compatible with plugins such as eslint-plugin-jsx-a11y to help to analyze React projects at the component level. Static analysis tools are faster, therefore, they are associated with great benefits at very affordable costs.

8. What is JSX?

JSX is an acronym for the word JavaScript XML. Basically, JSX is an HTML like template syntax that is used by React to help in extending ECMAScript so that HTML can co-exist with React code.

Thus making the HTML file easy to understand. JSX usually makes applications robust and also helps in boosting their performance. Below is an example of JSX:

9. Why are browsers not compatible with JSX?

All current browsers available can only read JavaScript objects but JSX is not a regular JavaScript object.

Therefore, to enable the compatibility of browsers reading JSX, first, there is a need to transform JSX file into a JavaScript object using JSX transformers like Babel and then pass it to the browser.

10. Imagine a scenario that you are working on a React application that is rendering a page very slowly, how would you identify the problem and solve the issue?

With technology, problems are bound to happen, and sluggish performance is one of the main problems. ln case of performance issues such as slow rendering is seen within a React app, the first step is to use the Profiler tool which is inbuilt within the React Developer Tools browser plugin, and is available in most popular browsers like Google Chrome and Mozilla Firefox.

It is very simple because the profiler tool helps developers to identify components that take a long time to render or those that render more frequent than expected.

The most popular issue in React applications is the unnecessary rendering of components. Thus, React has two tools that help to solve these issues;

  • React.memo(): This normally helps in preventing the unnecessary re-rendering of function components
  • PureComponent: This solves the problem of unnecessary re-rendering of class components

All these tools depend on a shallow comparison of the props passed into the component and if there is no change in props, then the component will not re-render. Despite these tools being very handy, the shallow comparison comes with several performance penalties.

Therefore, it is worth noting that if these tools are used wrongly they can result in negative performance. When using the React Profiler, is easy to measure performance before and after using these tools to make sure that performance is boosted by making relevant changes.

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

Every component of React must have a render () compulsorily. It helps in returning the React element which is a design of the native DOM component. In a situation where several HTML elements have to be rendered, they must be categorized together inside one enclosing tags such as <form>, <group>,<div> and several others. This function should not be altered to enable it to provide the same result each time it is used.

13. At a high level, what is the virtual DOM (VDOM) and how does React use it to render to the DOM?

Virtual DOM is a programming concept, providing a critical part of the React architecture. Apart from interacting directly with the DOM, changes are instead first rendered to the VDOM. VDOM is a lightweight representation of the target state of the DOM.

The changes made to the VDOM are integrated together to prevent unnecessary repetitive changes to the DOM. Every time these integrated are persued to the DOM, React gives a diff between both current and previous representation in the DOM, then applies the diff to the DOM.

14. What is the arrow function in React and how are they used?

The Arrow function in React is very important and should be well used because if it is wrongly used it causes serious performance problems. On the other hand, the Arrow function makes code writing easier and simpler.

Developers normally use function components and the inline arrow functions because of the following reasons;

  • Utilize the lifecycle hook.
  • Enable the use of PureComponets.
  • To block child props references.

15. Briefly explain the different phases of the React component’s lifecycle?

React components lifecycle is broadly categorized in four main categories namely;

  • Initialization: In this phase, the components start the journey by setting up the state and the props. This stage usually takes place inside the constructor method.
  • Mounting: As the name goes, the mounting stage is the phase where the React Components mounts to the DOM. This is the second phase after initialization and it’s in this phase when the components render for the first time. Lastly, there are two methods available at this phase namely; ComponentWillMount() and ComponetDidMount().
  • Updating phase: This is the third stage in which the components go through. At this phase the state of the components changes and thus, the rendering process takes place. Furthermore, it is in this stage that the state and the props updates as a result of user activities like typing and clicking. The methods available in this stage are; ShouldComponentUpdate(), ComponentWillUpdate() ,and ComponentDidUpdate().
  • Unmounting phase: This is the last stage of the component’s life cycle. As the name clearly states, it is in this stage when the components get unmounted from the DOM. The method that is available in this phase is called ComponentWillUnmount().

Conclusion

As you are preparing for the interview, I wish you all the best. But it is worth remembering that there is more to interviewing a professional for a job than just these technical questions.

Answering all these questions does not mean that you will be hired, employers will also consider other factors. Feel free to drop a comment and you are free to share with a friend to help in their next job interview.

David Dimba

Published by
David Dimba

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