MERN Stack Interview Questions & Answers (2024)

Are you looking for a job as a MERN Stack developer? Do you feel worried about your next interview? Are you looking for questions and answers that can help you with your interview? You are not alone. Many job seekers feel the same way. We want to help you get a job as a MERN Stack developer. 

This article has common MERN Stack interview questions and their answers. This article will help you get ready for your next interview.

Foundational MERN Stack Interview Questions and Answers

These are the common questions that interviewers use to check your understanding of the MERN Stack as a whole. Knowing these answers shows you have a good understanding of how the different parts work together.

1) What is the MERN stack?

The MERN stack is a collection of technologies used for building modern web applications. It stands for:

• MongoDB: A NoSQL database that stores data in a flexible, JSON-like format. It makes data management easy for developers.
• Express.js: A web application framework for Node.js. It provides tools for building APIs and handling routing.
• React.js: A JavaScript library for building user interfaces. It helps create interactive and reusable UI components.
• Node.js: A JavaScript runtime that lets you run JavaScript code outside of a web browser. This makes it possible to build server-side applications with JavaScript.

These components work together seamlessly. MongoDB stores the data, Express.js handles server-side logic and APIs, React.js builds the front-end, and Node.js provides the runtime environment.

The MERN stack offers several benefits:

• Full-stack JavaScript: You use one language (JavaScript) for both front-end and back-end development, simplifying the development process.
• Flexibility: You can easily customise and adapt the stack to meet your specific needs.
• Scalability: The MERN stack is designed to handle large amounts of data and traffic, making it suitable for growing applications.
• Community Support: A large and active community of developers contribute to the MERN stack, providing support, resources, and tools.

2) Why is the MERN stack popular?

The MERN stack has gained popularity for several reasons:

• Full-stack JavaScript: Using one language throughout the stack makes development faster and more efficient.
• Flexibility: The stack’s components are modular and can be easily replaced or extended. This lets developers choose the tools that best suit their project.
• Scalability: The MERN stack is designed to handle high traffic and large amounts of data, making it suitable for applications of all sizes.
• Community Support: A large and active community of developers provides support, libraries, and tutorials, making it easier to learn and use the MERN stack.
• Modern Development: The MERN stack incorporates modern development practices and tools, leading to better code quality and maintainability.

3) Key differences between MEAN and MERN stacks.

Both the MEAN and MERN stacks share many similarities, but they differ in one key component:

• MEAN: Uses Angular.js as the front-end framework. Angular.js is a full-fledged framework with a strong emphasis on structure and conventions. It is well-suited for large-scale applications and teams who prefer a more opinionated approach.
• MERN: Uses React.js as the front-end library. React.js is a more lightweight and flexible library that gives developers more control over their code. It is a good choice for smaller projects and teams who prefer a more hands-on approach.

The choice between MEAN and MERN often comes down to personal preference and project requirements. Both stacks are capable of building powerful web applications.

4) What are the typical responsibilities of a MERN stack developer?

A MERN stack developer is a full-stack developer who works with all layers of the application. Their responsibilities include:

• Building APIs: Designing and developing RESTful APIs using Express.js and Node.js to expose application data and functionality.
• Front-End Development: Building user interfaces with React.js, creating reusable components, and managing state.
• Database Management: Designing database schemas, writing queries, and optimising database performance with MongoDB.
• Deployment: Deploying the application to a production environment, ensuring it is scalable and secure.
• Testing: Writing unit and integration tests to ensure code quality and functionality.

MERN stack developers must have a strong understanding of JavaScript, web development principles, and database concepts. They must also be able to work collaboratively with designers and other developers.

Now that you’ve got the basics down, let’s move to React. It’s a popular JavaScript library for building user interfaces, so you’ll want to be ready for these questions.

React Interview Questions and Answers

React is a big part of the MERN Stack. These questions focus on React’s core concepts, components, and how it manages data. This is a key part of the interview process.

1) How does React work?

React is a JavaScript library for building user interfaces. It makes it easy to create reusable pieces of code called components.  React doesn’t directly change the webpage elements when something updates. Instead, it uses a virtual DOM (Document Object Model).

Virtual DOM: This is a copy of the real DOM, but it’s faster to work with.  When you change something in React, it updates the virtual DOM first. Then, React compares the virtual DOM to the real DOM and only changes what’s different. This saves time and makes your app more efficient.

Components: Components are the building blocks of a React app. They are like small functions that take data (called props) and return what should be shown on the screen (JSX).

JSX: JSX stands for JavaScript XML. It looks like HTML but it lets you write JavaScript code inside your components. It helps you build your user interface easily.

Component Lifecycle: React components have a lifecycle with phases like mounting (when they are created), updating (when their data changes), and unmounting (when they are removed).

2) What are props and states in React?

Props: Props are like arguments you pass to a function. They let you send data from a parent component to a child component. They are read-only, so you cannot change them directly in the child component.

State: State is data that a component manages internally. It can change over time, and when it does, React re-renders the component to show the updates. This helps you build dynamic and interactive user interfaces.

3) What are React hooks?

React hooks are functions that let you “hook into” React features from functional components.

• useState: This hook lets you add state to a functional component. You can change state values to trigger re-renders.
• useEffect: This hook lets you run side effects (like fetching data or subscribing to events) after a component renders or when its dependencies change.
• Other Common Hooks: React provides other hooks like useContext, useReducer, and useCallback. These help you manage complex state logic, optimise performance, and work with React’s context API.

React Component Lifecycle Methods

• componentDidMount: This method runs after the component is first rendered to the real DOM. It’s a good place to fetch data or set up timers.
• shouldComponentUpdate: This method is called before a component re-renders. You can use it to optimise performance by preventing unnecessary re-renders if the data hasn’t changed.
• componentDidUpdate: This method runs after a component updates. You can use it to interact with the DOM after changes have been made.
• componentWillUnmount: This method runs just before a component is removed from the DOM. You can use it to clean up resources like timers or event listeners.

4) What is the difference between functional and class components?

Class Components: These were the older way to write components. They used the class keyword and had methods like render, componentDidMount, etc.

Functional Components: These are simpler and the preferred way to write components now. They are just JavaScript functions. You can use hooks like useState and useEffect to add state and handle side effects in functional components.

5) How do you handle routing in React applications?

React Router is a library that helps you manage navigation in your React apps. It lets you:

• Define routes: You specify which component should be shown for each URL in your app.
• Navigate between views: You can create links that change the URL and update the content shown on the screen.
• Handle nested routes: You can create layouts with nested components for complex page structures.

With React covered, let’s dive into Node.js and Express.js, the backend technologies that power MERN Stack applications.

Node.js and Express.js Interview Questions and Answers

Node.js and Express.js handle the server-side of MERN Stack applications. These questions check your knowledge of how they work, including how they handle requests and responses, and how they connect to the database.

1) What is Node.js and its advantages?

Node.js is a runtime environment that executes JavaScript code outside of a web browser. It uses the V8 JavaScript engine from Google Chrome. Node.js is open-source and cross-platform.

Advantages of Node.js:

• Fast: Node.js uses an event-driven, non-blocking I/O model. This makes it fast and efficient for building scalable network applications.
• Scalable: Node.js can handle many concurrent connections with high throughput. This makes it a good choice for building applications that need to scale.
• Large and Active Community: Node.js has a large and active community of developers. This means that there are many resources available to help you get started and solve problems.

Event-driven, Non-blocking I/O:

In traditional I/O models, each request blocks the thread until it completes. This can lead to performance issues when there are many concurrent requests. Node.js uses an event-driven, non-blocking I/O model. This means that requests do not block the thread. Instead, they are handled by an event loop. The event loop can handle many requests concurrently without blocking the thread.

Use Cases of Node.js:

Node.js is a good choice for building the following types of applications:

• Real-time applications: Node.js can handle many concurrent connections with low latency. This makes it a good choice for building real-time applications like chat applications and online games.
• Network applications: Node.js is a good choice for building network applications like web servers and APIs.
• Data streaming applications: Node.js is a good choice for building data streaming applications because it can handle large amounts of data efficiently.

2) What is Express.js and its role in the MERN stack?

Express.js is a web application framework for Node.js. It provides a set of features for building web applications and APIs. Express.js is the E in the MERN stack, which is a popular technology stack for building full-stack JavaScript applications.

Role in the MERN Stack:

Express.js provides the backend for the MERN stack. It handles requests from the frontend and sends responses back to the frontend. Express.js also provides a set of features for building RESTful APIs.

Building RESTful APIs and Web Applications:

Express.js simplifies the process of building RESTful APIs and web applications. It provides a set of features like routing, middleware, and templating. These features make it easy to build complex applications quickly.

3) How does the event loop work in Node.js?

The event loop is the heart of Node.js. It is responsible for handling asynchronous operations. The event loop is a single-threaded loop that continuously checks for events. When an event occurs, the event loop calls the callback function associated with that event.

Handling Asynchronous Operations:

Node.js uses an event-driven, non-blocking I/O model. This means that asynchronous operations are not handled in the main thread. Instead, they are handled by the event loop. The event loop uses callbacks to handle asynchronous operations. When an asynchronous operation completes, the callback function associated with that operation is called.

4) What is middleware in Express.js? Give examples.

Middleware is a function that is called before a request is handled by the route handler. Middleware can be used to perform a variety of tasks, such as:

• Authentication: Middleware can be used to authenticate users before they can access certain routes.
• Logging: Middleware can be used to log requests and responses.
• Error handling: Middleware can be used to handle errors that occur during the request-response cycle.

Common Middleware Examples:

• body-parser: This middleware is used to parse the body of a request.
• cors: This middleware is used to enable cross-origin resource sharing (CORS).
• helmet: This middleware is used to set security headers.

5) How do you handle errors in Express.js applications?

There are a few different ways to handle errors in Express.js applications.

• Error Handling Middleware: You can use error handling middleware to handle errors that occur during the request-response cycle. Error handling middleware is a special type of middleware that takes four arguments: err, req, res, and next.
• Centralised Error Handling: You can use a centralised error handler to handle all errors that occur in your application. This can be done by using a middleware function that is called at the end of your middleware stack.
• Async/Await and Try/Catch: You can use async/await and try/catch to handle errors in asynchronous code.

Best Practices:

• Use error handling middleware: This will help you to keep your code clean and organised.
• Centralised error handling: This will make it easier to manage errors in your application.
• Use async/await and try/catch: This will help you to handle errors in asynchronous code.
• Log errors: This will help you to troubleshoot errors in your application.

Next up, we’ll cover MongoDB, the database that stores data for MERN Stack applications. Knowing how it works is essential.

MongoDB Interview Questions and Answers

MongoDB is the database for MERN Stack applications. These questions focus on how it stores and organises data, how it queries information, and best practices for using it.

1) What is MongoDB and its features?

MongoDB is a NoSQL database. It does not use tables and rows like traditional relational databases. It uses a document-oriented data model. This means it stores data in documents that are similar to JSON (JavaScript Object Notation) objects. These documents can have different structures within the same collection, making it flexible for various types of data.

MongoDB is also known for its scalability. It can easily handle large amounts of data and high traffic. It can be scaled horizontally by adding more servers to a cluster. This allows it to grow with the needs of an application. MongoDB is flexible and can adapt to changing requirements easily. It can handle unstructured or semi-structured data, making it a good choice for modern applications with diverse data needs.

2) What are the advantages of using MongoDB with the MERN stack?

The MERN stack (MongoDB, Express.js, React, Node.js) is a popular choice for building web applications. MongoDB fits well into this stack because it uses JSON for data storage. This means that data can flow seamlessly between the front-end (React), back-end (Express.js and Node.js), and the database (MongoDB) without needing to convert it between different formats. This makes development faster and easier.

Explain the different data types in MongoDB:

MongoDB supports a variety of data types:

• Strings: Used for text data (e.g., names, descriptions).
• Numbers: Used for numerical values (e.g., ages, prices).
• Arrays: Used for storing lists of values (e.g., tags, categories).
• Objects: Used for storing more complex data structures (e.g., addresses, product details).
• Dates: Used for storing timestamps (e.g., creation dates, modification dates).
• Booleans: Used for storing true/false values.
• Null: Used to represent the absence of a value.
• ObjectId: A unique identifier automatically generated for each document.

3) What is Mongoose and why is it used with MongoDB?

Mongoose is an Object Data Modeling (ODM) library for Node.js and MongoDB. It provides a way to structure and validate data before it’s saved to the database. This is important because MongoDB is schemaless, meaning you can store documents with different structures in the same collection.

Mongoose adds a layer of organisation by defining schemas. A schema is like a blueprint for your documents. It defines the fields a document should have, their data types, and any validation rules. This helps ensure data consistency and makes it easier to work with data in your application.

4) How do you perform CRUD operations in MongoDB?

CRUD stands for Create, Read, Update, and Delete. These are the basic operations you can perform on data in a database.

Create: You use the insertOne() or insertMany() methods to create new documents in a collection.

JavaScript

db.collection.insertOne({ name: “John”, age: 30 });

Read: You use the find() or findOne() methods to retrieve documents from a collection.

JavaScript

db.collection.find({ age: { $gt: 25 } }); // Find documents where age is greater than 25

Update: You use the updateOne() or updateMany() methods to modify existing documents.

JavaScript

db.collection.updateOne({ name: “John” }, { $set: { age: 31 } }); // Update John’s age to 31

Delete: You use the deleteOne() or deleteMany() methods to remove documents from a collection.

JavaScript

db.collection.deleteOne({ name: “John” }); // Delete the document where name is John

You’ve made it to the end! Now let’s tackle some more challenging MERN Stack questions that might come up in interviews for more senior roles.

Advanced MERN Stack Interview Questions and Answers

These questions go deeper into MERN Stack concepts. They are more likely to come up in interviews for experienced positions, but they’re good to know even if you’re starting out.

1) How do you optimise performance in MERN stack applications?

Performance optimization is key to a good user experience. Here’s how you can optimise your MERN stack application:

• Caching: Use caching to store frequently accessed data. This reduces the need to fetch data repeatedly, leading to faster response times. You can implement server-side caching with tools like Redis or client-side caching with React Query or SWR.
• Code Splitting: Break your application into smaller chunks of code that load on demand. This way, users only download the code they need for the current page, speeding up initial load times.
• Lazy Loading: Load images or other resources only when they become visible in the viewport. This helps to reduce the initial page size and improves loading times.
• Server-Side Rendering (SSR): Render React components on the server-side and send the fully rendered HTML to the client. This makes your application faster for initial page loads, especially on slower connections or devices.

2) How do you secure MERN stack applications?

Security is a top priority for any web application. Here’s how to protect your MERN stack app:

• Authentication: Verify the identity of users before allowing them to access protected resources. Implement strong password policies, use multi-factor authentication (MFA), and consider using JWT (JSON Web Tokens) for managing authentication.
• Authorization: Control what actions different users can perform within your app. Use role-based access control (RBAC) or attribute-based access control (ABAC) to define permissions for various roles or attributes.
• Input Validation: Validate all user input on both the client-side and server-side to prevent malicious input from causing harm. Use libraries like express-validator on the backend and form validation libraries on the frontend.
• Protection against Common Attacks: Prevent common vulnerabilities like cross-site scripting (XSS), cross-site request forgery (CSRF), and SQL injection. Use security headers, input sanitization, and parameterized queries to mitigate these risks.

3) How do you deploy MERN stack applications?

Deploying MERN stack applications can be done using several platforms:

• Heroku: A cloud platform as a service (PaaS) that allows you to deploy, manage, and scale applications easily.
• Netlify: A popular choice for static site generation and hosting, well-suited for frontend-focused MERN apps.
• AWS: Amazon Web Services offers a wide range of services for deploying and managing applications, providing more control and flexibility than PaaS solutions.
• Docker Containers: Containerize your MERN app for consistent deployment across different environments, making it easier to manage dependencies and ensure portability.

4) What are some common challenges you’ve faced with the MERN stack, and how did you overcome them?

As with any technology stack, the MERN stack presents some challenges. Here are a few examples:

• State Management: In large React applications, managing state can become complex. Consider using tools like Redux or MobX to simplify state management and make your app more predictable.
• Database Modelling: Designing an efficient data model for MongoDB requires careful consideration. Use embedded documents and arrays to avoid unnecessary joins and improve performance.
• Performance Bottlenecks: Identify and address performance bottlenecks early on in development. Use profiling tools and optimise your code as needed.
• Error Handling: Implement robust error handling on both the frontend and backend. Use error boundaries in React and centralised error handling middleware in Express.js.

Sharing your experiences and solutions to these challenges demonstrates your problem-solving abilities and makes you a more attractive candidate.

Conclusion

You are now ready to face MERN Stack interviews. You know how to answer different types of questions. You also understand what interviewers expect. Remember, practice is key to success. Practice with mock interviews and coding challenges.

iScalePro is a great tool for practice. It has a large question bank and expert feedback. This helps you to improve your skills and increase your confidence. So start practising today.  With the right preparation, you can get your dream MERN Stack job.

MERN Stack Interview FAQs

1) How to Crack MERN Stack Interview?

To crack a MERN stack interview, you need to understand the technologies involved: MongoDB, Express.js, React.js, and Node.js.

• MongoDB: Learn about MongoDB’s data structures, queries, and how to use it with Mongoose.
• Express.js: Understand how to create web applications with Express.js, including routing and middleware.
• React.js: Learn React’s components, state management, and how to build user interfaces.
• Node.js: Understand how Node.js works, including asynchronous programming and event-driven architecture.

Practice coding challenges and build MERN stack projects. This will help you understand the concepts and show your skills to the interviewer.

2) Is MERN Stack Easy or Hard?

The difficulty of the MERN stack depends on your programming experience. If you have a good understanding of JavaScript, it will be easier to learn. However, learning the four technologies can take time and effort.

3) What is the MERN Stack Approach?

The MERN stack is a full-stack JavaScript solution for building web applications. It uses MongoDB for the database, Express.js for the server, React.js for the frontend, and Node.js as the runtime environment.

This approach offers several benefits, including:

• Consistent language: Using JavaScript throughout the stack makes development easier and more efficient.
• Large community: The MERN stack has a large and active community, providing resources and support.
• Scalability: The MERN stack can handle high traffic and scale easily.

4) Is MERN in High Demand?

Yes, MERN stack developers are in high demand. Many companies are looking for developers who can build full-stack web applications using this technology.

If you are looking for a career in web development, learning the MERN stack is a good option. It is a versatile and powerful technology that can help you build successful web applications.