ReactJS: Top 15 Interview Questions and Answers for 2023

Juan SalasJuan Salas

Juan Salas

Top 15 ReactJS Interview Questions and Answers for 2023
Top 15 ReactJS Interview Questions and Answers for 2023.

TDLR: This article lists the top fifteen ReactJS interview questions and answers for 2023. It is intended to guide those preparing for interviews related to ReactJS. The questions and answers are meant to be comprehensive, covering components, state, props, hooks, and more. Additionally, tips for success in an interview related to React JS are included.

1. What is ReactJS?

React JS is an open-source JavaScript library that facilitates the development of user interfaces. Facebook and a community of individual developers and companies maintain it.

React allows developers to create large web applications that use data that can change over time without reloading the page. It aims to provide a highly performant and simple way to create interactive UIs.

2. What are components in React?

Components are the building blocks of any React application. A typical React app will have many components, each responsible for rendering a small, reusable piece of HTML code.

Components can be defined using either classes or functions. They accept arbitrary inputs known as props and return elements describing what should appear on the screen.

3. What are props in React?

Props (short for properties) pass data from one component to another, similar to parameters in a function call.

Props are read-only within the component, so they should not be modified directly; instead, changes should be made using setState().

They let developers make reusable and configurable components, allowing for better code reuse and flexibility when building applications with React.

4. What is state in ReactJS?

State is an object that stores information about the current state of an application, such as whether a user logs in or out, what page they’re currently viewing, etc.

Also, State is managed within components by calling setState(), which triggers a re-render of the component with updated values from the state object. This object can also be passed down from parent components to child components through props for better organization and code reuse.

5. What distinguishes states from props in ReactJS?

States are mutable, while props are immutable in ReactJS. This means states can be changed, updated, or modified, while props cannot be changed once they are passed to a component.

Developers use states to store local or dynamic data that can change over time, such as user input or server responses.

Props pass data from one component to another, such as from parent components down to child components. Props cannot be changed by the component receiving them – they must remain the same throughout their lifetime.

The main difference between states and props is that states are managed within the component itself, while props must be passed down from parent components.

6. What are hooks in React?

Hooks are functions that let you “hook into” ReactJS features from functional components instead of writing class-based components.

Hooks allow developers to add stateful logic into functional components without needing to convert them into classes; this makes it easier to manage complex applications while keeping code concise and maintainable simultaneously.

Some popular hooks include useState(), useEffect(), useReducer(), etc., all of which provide helpful ways to interact with data in an application without needing classes or lifecycle methods like componentDidMount().

7. What does JSX mean?

JSX stands for JavaScript XML and is a syntax extension to the JavaScript language used for creating user interfaces. As an XML-like syntax, it allows developers to write HTML-like code in JavaScript.

This function improves the efficiency of the program, ensuring that the generated HTML file is legible.

8. Do browsers have the capability to interpret a JSX file?

Web browsers do not natively support it, so it must be compiled into regular JavaScript before running in the browser.

There are several tools available that can compile JSX into standard JavaScript, such as Babel and Webpack. Once the code has been compiled, the browser can interpret it just like any other JavaScript code.

9. What is the Flux concept In ReactJS?

The Flux concept is an application architecture for React applications. It is a unidirectional data flow architecture that provides better control over the data and user interface components in a React application.

The main idea behind Flux is that the application's state should be kept in a single store, and each application component should have access to this store. This allows for better control over how data flows throughout the application and provides a single source of truth for all components.

Flux utilizes "actions" dispatched from components within the application that activate "reducers" to modify the state of the store. The updated state is then passed back into each component, allowing them to update their view accordingly. This helps ensure that all components remain up-to-date with any changes made within the application and helps keep everything running smoothly.

10. What is Redux in ReactJS?

Redux is a state management library for JavaScript applications, most commonly used with the ReactJS library. It allows developers to store and manage application state in one centralized location, providing a predictable and maintainable way to manage data within an application.

Redux helps developers create more performant applications, easier to debug and test. It stores all of the application's state in an object called the store. The store can be accessed by any application component, enabling components to share data without passing it through props or other methods.

Redux also provides several tools, such as reducers and middleware, that help developers write code that is easier to read and understand.

11. What is a HOC in React?

A Higher-Order Component (HOC) is a function that takes a component as an argument and returns a new component.

HOCs are used to extend the functionality of existing components, such as adding additional props or states or altering the output of the component in some way.

HOCs can also create reusable logic for components, such as authentication, data fetching, and more.

By abstracting this logic into a single HOC, multiple components can use the same code with minimal effort. HOCs are a powerful tool for making React applications more efficient and maintainable.

12. Explain Virtual DOM and how does it work?

Virtual DOM (VDOM) is an in-memory representation of a web page's Document Object Model (DOM). It is a lightweight, tree-structured copy of the actual DOM that can quickly update the UI without having to re-render the entire page.

VDOM works by keeping track of all changes made to the DOM and then updating the actual DOM with those changes when necessary. This process helps ensure that web pages render quickly and efficiently.

When a user interacts with a web page, VDOM will:

  • Detect any changes made and create a new virtual version of the DOM.
  • Compare to the previous version stored in memory with the new. If there are any differences between them, only those changes will be applied to the actual DOM instead of re-rendering the entire page.
  • Ensure that any changes made are synchronized across all users viewing the same page as long as they have access to the same VDOM instance.

The main benefit of using VDOM is its ability to optimize performance since it eliminates unnecessary re-rendering and keeps data up-to-date across all users viewing the same page. It can also help reduce code complexity by allowing developers to manually manipulate data without updating HTML elements.

13. What are Controlled and Uncontrolled Components in ReactJS?

Controlled components are React components that control their own state. They are written as a class component and use the setState() method to update their state.

Uncontrolled components are React components that do not control their own state. They are usually written as a function component and rely on external sources, such as DOM nodes or props, for their state.

14. What are the React component lifecycle methods?

ReactJS component lifecycle methods are methods that get called at various points in a component’s life cycle. These methods allow developers to control how components behave when they are created, updated, and destroyed.

The three main React component lifecycle methods include:

  1. componentDidMount(): This method is called after a component is mounted (inserted into the tree). This is where developers can initiate network requests, set up subscriptions, and generally prepare for the component to be used.
  2. shouldComponentUpdate(): This method allows developers to decide if a re-rendering of the component is necessary based on comparing the current props and state to the next.
  3. componentWillUnmount(): This method is called before a component is unmounted (removed from the tree). Developers can use this method to clean up any subscriptions or tasks created in the componentDidMount() method.

In addition to these three main methods, there are several other lifecycle methods available in React, such as getDerivedStateFromProps(), getSnapshotBeforeUpdate(), and others. These methods give developers control over how their components behave throughout their life cycle and help them create better user experiences.

15. What are the ReactJS component’s lifecycle phases?

React components have several lifecycle phases, each providing a different point in the development. The phases are:

  • Initialization: This is when the component is first created and initialized with default props and state. This is also when any initial data fetching or setup occurs.
  • Mounting: After initialization, the component is mounted onto the DOM (Document Object Model) tree and rendered to the screen. This is where any additional data fetching or setup that needs to occur after mounting can be done.
  • Updating: When props or state values change, this phase occurs, and the component updates its internal state accordingly. This can include re-rendering itself if necessary.
  • Unmounting: When a component is removed from the DOM tree, this phase occurs, and any cleanup tasks, such as unsubscribing from event handlers or clearing timers, can be done here.
  • Error Handling: If an error occurs while rendering, updating, or unmounting a component, this phase will execute and handle any errors that may have happened during those processes.

In Conclusion

ReactJs is one of the most used front-end Frameworks in the world. As a developer, you should know and handle the basic principles of ReactJS and Javascript very well. Knowing how to use this tool and its concepts will help you get job opportunities in 2023.

As a recruiter, it is essential to have a proper interview process in place. Build a bank of questions and answers that suits the project. This will ensure that the developer hired is the right one for the level of complexity.

If you feel that recruiting ReactJS developers is overwhelming and you need to establish a good recruiting and hiring strategy, feel free to give goAlto a try.