15 Best React Interview Questions and Answers (2023)
React interviews
React is a popular JavaScript library, which works well for building high-performing applications. These developer interview questions will help you identify the right developer for your team. We created them to help you find developers without “gotcha” questions or brainteasers. The best developers tend to be able to give an answer that’s based on real-life experience and that holds up to scrutiny, which is what you should be looking for in the answers you get.
And if you want to pair your React interviews with coding tests, take a look at our React coding challenges.
Beginner React interview questions
These basic React interview questions work well as introductory questions for more junior developers. Remember that there usually is no textbook answer- instead, listen to how candidates answer these questions.
1. What is React?
Answer:
This question might sound simple, but it’s designed to test developers' understanding of React.
React is an open-source, front-end JavaScript library, developed by Facebook. React is used to build reusable UI components and it’s useful for developing complex and interactive UI for both web and mobile.
2. How is React different from other JavaScript frameworks? Have you noticed this in your own projects?
Answer:
The goal of this question is to gauge how well candidates understand the JavaScript ecosystem.
React is a small library for building UI components and works seamlessly with all technology stacks, which makes it different from other JavaScript frameworks. It’s incredibly flexible.
Other frameworks, such as Angular, are less flexible. Angular in particular is based on extending HTML markup and injecting constructs at runtime. As a result, it only works for certain tech stacks.
The downside of React is that the developer has to choose/build the architecture that fits them best.
(Note that you can ask follow-up questions here, such as why a candidate prefers React to a specific alternative, like Vue, in which situations they prefer React to its alternatives, or when they’ve used React in a project and seen its benefits first-hand).
3. What does the React component life cycle look like?
Answer:
React component life cycles are one of the most important parts of the library. That’s why candidates, who build maintainable applications, should know how components work over time.
React component life cycles have three events. Components define events for managing properties, rendered output, and state. That’s why candidates should understand each event. The events are:
1) Initialization
2) State/property updates
3) Destruction
4. How does React use virtual DOM? What is virtual DOM?
Answer:
Again, a question with the aim to understand a candidate’s knowledge of React.
React uses a virtual DOM, which means that a virtual representation of the real DOM is kept inside the memory. It’s synced with the real DOM through a library.
Virtual DOM is a lightweight JavaScript Object which originally is a copy of the Real DOM. Virtual DOM is a node tree that lists elements, their attributes and contents as Objects and their properties.
The render function of React creates this node tree out of the React components. React then updates the tree in response to actions done by the system or the user. When any underlying data changes, the entire UI is re-rendered in the Virtual DOM representation.
Then the differences between the last DOM representation and the new one are calculated. When the calculations are done, the real DOM will be updated with the things that have changed.
5. Is React a full-blown framework?
Answer:
A skilled React developer understands the importance of this question. They need to have a good knowledge of third-party React libraries.
You see, React is just a library, not a full-blown framework.
This is again a reason why React is different from other JavaScript frameworks. For example, Angular is bootstrapped with nearly everything you need for a complete, large-scale app.
That’s not the case with React. To build applications with React, you need to choose these packages and tools. And React developers must understand what their choices are.
Advanced React interview questions
Here are some more advanced React interview questions. These questions are less about defining what React is and isn’t and more about what React can do.
6. Does React use JSX? What is it?
Answer:
React embraces the fact that rendering logic is inherently coupled with other UI logic: how events are handled, how the state changes over time, and how the data is prepared for display.
Instead of artificially separating technologies by putting markup and logic in separate files, React separates concerns with loosely coupled units called “components” that contain both.
7. Is everything in React a component? If so, why?
Answer:
This question is at the core of why React is so flexible. In React, everything is a component- even standard HTML tags.
Components are the building blocks of the UI in an application built using React and this is one of the main strengths of React as a library/framework. These components split the entire UI into small pieces that are independent of each other and can be reused.
So while a ‘normal’ web page might consist of a menu bar, content area, and footer, you create these areas as components in React. This gives room for a lot of flexibility and saves on code duplication.
8. Does every React component need render()? If so, why?
Answer:
Yes, every React component needs render(). render() returns a single React element, which is the representation of the native DOM component.
9. What is an event in React?
Answer:
This question digs a bit deeper into React events. There are several ways to answer this question, but here are the basics.
React can perform actions based on user events, just like HTML. Events are triggered based on different, specific actions such as mouse click, keypress, web page loading, and so forth.
React has its own handling system for events, Synthetic Events. This is similar to handling events on DOM elements.
10. How do you create an event in React?
Answer:
When using React, you generally don’t need to call addEventListener to add listeners to a DOM element after it is created. Instead, just provide a listener when the element is initially rendered.
When you define a component using an ES6 class, a common pattern is for an event handler to be a method on the class.
11. What are Higher Order Components?
Answer:
A HOC is an advanced technology, so asking about it in interviews can give you some insight into a candidate’s deeper knowledge of React.
HOC or Higher-Order Components is a more advanced way of reusing the component logic. It is a pattern that is derived from React’s compositional nature, so it’s not directly part of the React API.
HOCs are custom components that wrap another component within them. They’re especially useful in larger apps where similar patterns will happen over and over. Higher-order components let you define the logic in a single place and share it across many components.
12. What is Flux?
Answer:
An answer to this question is likely based on a developer’s own experience. Look out for their answer in terms of why they prefer Flux or why they use alternatives, such as Redux or MobX, instead.
Flux is an architectural pattern, which enforces the unidirectional data flow. It controls derived data and enables the communication between multiple components using a central Store that retains authority for all data.
Any update in the data only needs to occur here. The benefits of Flux are that it provides stability to the application and reduces run-time errors.
13. What is the significance of keys in React?
Answer:
The importance of Keys is that they help React identify which items have changed, are added, or are removed. They should be given to elements within the array to give elements a stable identity.
More specifically, Keys in React are used to identify unique Virtual DOM Elements with their corresponding data driving the UI (and virtual DOMs help React perform so effectively because they are much faster to update than real DOM elements). The Key prop helps React optimize rendering by recycling already existing DOM elements.
14. What are some common approaches for styling a React application?
Answer:
There are a number of ways to style React components and there’s no perfect way to do this. That’s why this question is more about understanding how a candidate works and why they would choose a certain approach over another. Do they take use cases and architectural goals into consideration?
Some examples are:
- Stylus
- Less
- Sass
- Inline CSS
- CSS Classes
- CSS-in-JS Modules, such as Emotion, Styled-JSX, and Styled Components
15. What are examples of automated tools that support React development?
Answer:
Examples are Browser Tools, Build Tools (Webpack), and Static Analysis Tools.
Browser Tools such as Google Lighthouse or aXe perform automated accessibility at the app level. This improves the chance of finding an increased amount of real-world issues. A browser is used to simulate the way that a user interacts with the website.
Static Analysis Tools, on the other hand, can be used to analyze React projects at the component level. These tools run quickly, so they can bring benefits at a low cost.
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