React is a popular web development technology for creating user interfaces. Web developers use React for creating interactive web applications or web pages. Since many businesses depend on the React library for building seamless interfaces in their applications, employers look for React developers with strong conceptual knowledge and JavaScript skills. In this article, Pritish Kumar Halder looks at a few sample questions and answers to help you prepare for a technical interview.

Common React interview questions and sample answers

Here are some sample React interview questions and their answers:

1. What is React?

Interviewers usually start with this question to examine a candidate’s basic knowledge of React. Answer the question by stating the definition of React and then mentioning its core feature.

Example: “React is an open-source JavaScript library used for designing the front-end for single-page applications. It makes use of state-based components written using JavaScript instead of templates.”*

2. What do you understand by virtual DOM? Describe its function.

This question checks whether a candidate clearly understands React’s document object model (DOM). Begin your answer by explaining the virtual DOM and then briefly describe its function.

Example: “A virtual DOM is a lightweight JavaScript object with the same properties as the real DOM object, but resides as a copy in memory. Its primary function is to act as pre-update snapshots of the real DOM. When a UI element needs an update, React compares the current state with the virtual DOM snapshot and only updates the elements which have changed.”

3. How are real DOM and virtual DOM different from each other?

Usually, this is a follow-up question to virtual DOM. To answer this question, explain real DOM and virtual DOM first. Then you can mention their differences one by one.

Example: “Real DOM or HTML DOM is a concept of abstraction for representing HTML and XML pages. It acts as an interface so that other programs and scripting languages can modify the document’s structure, style or content. Virtual DOM is a reference to the real DOM. It resides in memory and acts as a diff tool for screen updates.

Real DOM is slow in performance because all nodes in the tree need to be traversed and refreshed. Virtual DOM is faster because it can update only the changed nodes and create a new snapshot in memory. Real DOM is used to update the HTML pages directly, whereas virtual DOM is only used to check which nodes have changed. Real DOM manipulation is computationally expensive while modifying the virtual Dom is less costly and memory-efficient.”

4. What are refs in React?

Questions related to refs often check if you know how to modify child components directly without using props in a parent-child workflow. Your answer should include a definition and a clear explanation of refs’ significance in React.

Example: “Refs are a feature in React which allows accessing a DOM and React elements outside of the re-render workflow with new props. It is an attribute that aids in storing a reference to a specific React component or element returned by the component’s render configuration function. They are useful for lightweight functions such as updating an element focus or selection and media playback.”*

5. Explain why React is a component-based framework.

Components are reusable blocks of JavaScript code in React. They use the concept of encapsulation to keep singular elements isolated from others. React uses states for components to simplify modifying their properties and methods. This knowledge can come in handy while answering this question.

Example: “Components are the building blocks of a React application’s UI. Components have a state object that stores their property values. React uses states to change the status of the components. Each component has a render() function similar to a getter method, in the sense that it always returns the same value with every call.

Components make handling the UI easier because React only needs to monitor changes in the state to call the render function and update it. Furthermore, you can create components as a class to take advantage of inheritance.”

6. Identify the main advantages of React.

You can use your knowledge of React to answer this question in a point-wise manner. It will demonstrate that while you possess operational experience of React, you are also aware of the reason it is useful in web development.

Example: “React provides a high degree of code reuse and consistent functionality because it is based on encapsulated components. It also improves the performance of a web application or a web page on the client device and server using virtual DOM techniques. Other frameworks, such as Angular and Meteor, are easy to incorporate because React acts as the interface in the model-view-controller (MVC) software design pattern.

Streamlining the process of writing UI test cases is faster with a set of native React tools because they act as development extensions for Chromium-based browsers. This development process is complemented by the benefits of object-oriented programming concepts, which reduce the amount of coding required. It is also easier to learn for developers who have experience in JavaScript.”

7. What are the disadvantages of React?

Understanding the limitations of React shows that you have good judgement and context when using it in your projects.

Example: “While React is a great UI development tool, we need to use a different, compatible library to program the back-end. This is because React is not a comprehensive framework but a JavaScript library. Coding with react is also complicated since it uses JSX and inline templating.

The syntax for React can be a little challenging for the new programmers to comprehend because JSX combines HTML and JavaScript. Furthermore, its library is large and complex, which takes time to understand and implement.”

8. Define JSX.

This question evaluates whether a candidate can define the term and provide an informed opinion based on personal experience. If you have used JSX to build React applications, you can give a brief description to enhance your answer.

Example: “JSX is an abbreviation for JavaScript XML. It is an optional feature of React, which combines both HTML and JavaScript. An example would be defining a constant type element and assigning a string value wrapped with header tags to it. The documentation describes it as a JavaScript syntax extension. JSX reduces the number of createElement calls in your application and provides more meaningful error messages and warnings. However, it does require a Babel for compiling.”

9. Can browsers read JSX code?

Such questions check if you know how JSX works in the context of React. Interviewers can also check if you have experimented using JSX with different browsers. Your answer should include an informed opinion about JSX.

Example: “Usually browsers only read JavaScript objects. We have to use a JavaScript compiler such as Babel to convert JSX into .js files before passing it to the browser. However, there is a way to use JSX directly in modern browsers by running the Babel standalone script. The only drawback is that it adds extra loading and compilation steps at runtime, which reduces the performance of your website.”

10. What do you understand by props?

Technical questions include in-depth information about components, props and their relationship in React. You can answer this question by explaining the abbreviation and providing a context for their use in a component.

Example: “Prop is an abbreviation for properties in React. Props are used as arguments for functions or to pass data as parameters to other components. It is important to know that props are read-only. If you try to change its value, the compiler outputs an error. Props are mostly used for passing dynamically generated data from one component to another. We can also pass data from a parent component to its child components using callback functions. However, a child component cannot transfer a prop back to the parent component, ensuring a unidirectional flow of data.”

11. Define an event in the context of React.

Most applications interact with their users through events. An event-based system helps reduce the polling time for user input. Most programming languages use events for this purpose, but the implementation and syntax can vary across different languages. When answering this question, try to describe an event in React and provide a short explanation of synthetic events to make your answer more impactful.

Example: “In React, events refer to triggered reactions to specific user actions such as a key press, mouse click or mouse hover. Dealing with these events is the same as dealing with DOM elements events. However, there are some syntactic differences. One is that the React events are named using the camel case rather than the lowercase. Another one is that the React events are passed as functions in curly brackets instead of string values.*

The event argument has several properties specific to each event. Every type has different behavior and properties, which can only be accessed through its event handler. Synthetic events are similar to native events in browsers. However, they provide cross-platform compatibility because they are designed as wrappers for a browser’s native event.”