50 MERN Stack Interview Questions to Help You Land the Job in 2026

Over the past few months, I have taken several JavaScript and MERN Stack interviews, and one thing candidates always wonder is: where do interview questions actually come from? Honestly, most interviewers, including me, search the internet, shortlist popular and underrated questions, add a few fundamentals, and keep reusing them.

In this article, I decided to share my top 50 MERN Stack interview questions for 2026. Go through them carefully, don’t just memorise but understand each concept and its related topics. Good luck!

BTW, I have already shared my list of 50 JavaScript interview questions, if you haven’t read it yet.

General MERN Stack Interview Questions

This first section covers the big picture. We will look at what the MERN stack is, how the parts work together, and why it is so popular.

1. What is the MERN Stack?

The MERN stack is a popular collection of technologies used to build complete web applications. It is a full-stack framework, meaning it covers both the frontend (what you see) and the backend (what powers it). The name “MERN” comes from its four key components:

MongoDB: A NoSQL database that stores the application data.
Express.js: A backend web framework that runs on Node.js.
React.js: A JavaScript library for building the user interface.
Node.js: The JavaScript runtime environment that lets us run JavaScript on the server.

2. How do the MERN stack components work together?

We can think of the MERN stack components as a team working together.

React.js (Frontend): This is the user interface that the user interacts with. When it needs data, like a list of users, it sends a request to the backend.
Express.js (Backend): This is the backend server, running on Node.js. It receives the request from React. It understands what data is needed and makes a query to the database.
MongoDB (Database): This is where all the data is stored. It receives the query from Express, finds the correct data, and sends it back to Express.
Express.js (Backend): Express receives the data from MongoDB, formats it as JSON, and sends it back to React.
React.js (Frontend): React receives the JSON data and uses it to update the webpage for the user.

3. Why choose the MERN stack?

The biggest advantage of the MERN stack is that it uses JavaScript for the entire application. The frontend (React), the backend (Node.js and Express.js), and even the database (MongoDB uses JSON-like documents) are all based on JavaScript. This makes it easier for developers, as we only need to be experts in one language. It also makes development faster. The MERN stack is also highly scalable, has great performance, and has a very large, supportive community.

4. What is the difference between MERN and MEAN?

The MERN and MEAN stacks are very similar. They both use MongoDB, Express.js, and Node.js. The only difference is the frontend technology.

MERN uses React.js.
MEAN uses Angular.js.

Generally, React is known for being more flexible and having a faster learning curve, which is why many developers prefer it.

5. What is a RESTful API?

A RESTful API is an architectural style for building web services. It is a set of rules for how applications communicate over the internet. It uses standard HTTP methods, which are a set of commands like:

GET: To read data.
POST: To create new data.
PUT: To update existing data.
DELETE: To remove data. In our MERN stack, Express.js builds the RESTful API that React.js uses to get and send data.

6. What is the MVC architecture?

MVC stands for Model-View-Controller. It is a popular way to organize code into three separate parts to keep it clean and easy to manage.

Model: This part manages the data and business logic. In MERN, our Mongoose schema and models in MongoDB represent the Model.
View: This is the user interface. In MERN, our React.js components are the View.
Controller: This part takes user input and connects the Model and the View. In MERN, our Express.js routes and API endpoints act as the Controller.

7. How do you pass data from the frontend (React) to the backend (Express)?

We pass data from React to Express by making an HTTP request, usually with the fetch API or a library like axios. If we are creating new data, we use a POST request. We attach the data (like a username and password) to the body of the request. We usually format this data as a JSON string.

8. How do you deploy a MERN stack application?

Deploying a MERN stack application involves deploying its parts separately, as they are distinct applications.

Frontend (React): We build the React app into a set of static files. We then host these files on a static hosting service like Netlify or Vercel.
Backend (Express/Node.js): We host the backend server on a platform like Heroku or AWS.
Database (MongoDB): We use a cloud database service. The most popular one is MongoDB Atlas. We then connect all three services by setting environment variables (like the database connection string) in our backend application.

MongoDB and Mongoose Interview Questions

These MERN stack interview questions focus on the “M” in MERN. We will cover MongoDB, our database, and Mongoose, the library we use to work with it.

9. What is MongoDB?

MongoDB is a NoSQL database. This means it does not use the traditional tables and rows that SQL databases use. Instead, it stores data in flexible, JSON-like documents. These documents are organized into “collections,” which are similar to tables.

10. How is MongoDB different from SQL databases?

The biggest difference is the schema.

SQL databases (like MySQL) are relational and have a rigid schema. This means you must define your tables and columns before you can add data, and all rows must match that structure.
MongoDB is non-relational and has a flexible schema. This means you can save documents without a predefined structure, and documents in the same collection can have different fields.

11. What is Mongoose?

This is a common point of confusion. Mongoose is not the database. Mongoose is an Object Data Modeling (ODM) library for Node.js. It is a tool that we use in our Express application to connect to our MongoDB database. It helps us manage data by providing features like schemas and data validation.

12. What is a schema in Mongoose?

Even though MongoDB has a flexible schema, our application usually needs some structure. A Mongoose schema defines the structure for our documents at the application level. We can set what fields each document should have (like name or email), what data type they should be (like String or Number), and add validation rules (like required: true or unique: true).

13. What is indexing in MongoDB?

Indexing is a way to make our database searches run much faster. An index stores the values of a specific field (or fields) in a special data structure that is easy to search. It is like the index at the back of a textbook. Instead of searching the whole book, you can use the index to find the page you need quickly.

14. What is an aggregation pipeline in MongoDB?

An aggregation pipeline is a powerful tool for data processing. It takes a stream of documents and passes them through a series of “stages.” Each stage transforms the data. Common stages include:

$match: This filters the documents, like a find() query.
$group: This groups documents together to perform calculations, like finding a sum or an average.
$sort: This sorts the documents.

15. What is Replication in MongoDB?

Replication is the process of creating and maintaining multiple copies of our data on different servers. These servers form a “replica set.” The main purpose of replication is to provide high availability. If the main server (the “primary”) fails, one of the copy servers (a “secondary”) will automatically be promoted to become the new primary, so our application can keep running.

16. What is Sharding in MongoDB?

Sharding is a method for distributing data across multiple servers, or “shards”. We use sharding when our dataset is too large to fit on a single server, or when we need to handle a very high number of requests. While replication copies the entire database for safety, sharding splits the database for size and performance.

17. What is the difference between embedding and referencing in MongoDB?

These are the two ways we handle relationships between data.

Embedding is when we store related data inside the main document. For example, we might store an array of comments directly inside a blog post document.
Referencing is when we store a link (an _id) to a document in another collection. For example, a blog post document might have a userId field that “refers” to a document in the users collection.

18. How do you update a document in MongoDB?

We use the db.collection.updateOne() method. This method takes two main arguments. The first is a filter to find the document we want to update. The second is an update operator, like $set, which tells MongoDB what fields to change. For example: db.users.updateOne({ name: "Alice" }, { $set: { age: 30 } }).

Express.js Interview Questions

Now we move to the “E” in MERN. Express.js is our backend framework, and these MERN stack interview questions cover its most important concepts, like middleware and routing.

19. How does Express.js work in the MERN Stack?

Express.js is a lightweight framework for Node.js that builds our backend. In the MERN stack, its main job is to create a RESTful API. It defines routes (URLs) that our React frontend can send requests to. It handles these incoming HTTP requests, processes logic (like talking to MongoDB), and sends back a response.

20. What is middleware in Express.js?

Middleware functions are the heart of Express.js. They are functions that run “in the middle” of a request and its response. A middleware function has access to the request object (req), the response object (res), and a special function called next. We use middleware for many tasks, such as:

Parsing the body of a request.
Checking if a user is authenticated.
Logging information about a request.

21. What is the purpose of next() in Express middleware?

The next() function is very important. When a middleware function finishes its job, it must call next() to pass control to the next middleware function in the line. If we forget to call next(), the request will get “stuck” in our middleware, and the user will never get a response.

22. How do you define a route in Express.js?

We define a route by using a method on our app object that matches an HTTP method, like app.get(), app.post(), app.put(), or app.delete(). This method takes two arguments: the URL path and a callback function that runs when that path is requested. For example: app.get('/users', (req, res) => { res.send('Hello users'); });.

23. How do you handle routing in Express.js?

For a very small application, we can define all our routes in the main server.js file. For a larger application, this becomes messy. The best practice is to use express.Router(). This allows us to create separate files for our routes (like userRoutes.js or postRoutes.js) and then import them into our main server.js file. This keeps our code clean and organized.

24. How do you handle errors in Express.js?

We handle errors in Express by creating a special error-handling middleware. This middleware is special because it must have four arguments: (err, req, res, next). We place this middleware at the very end of our middleware stack. If any route or middleware calls next() with an error, Express will skip all other middleware and go straight to this error-handling function.

25. What is CORS and how do you enable it in Express.js?

CORS stands for Cross-Origin Resource Sharing. It is a security feature built into web browsers that blocks HTTP requests between different domains. For example, the browser will block a request from my-react-app.com (our frontend) to my-api.com (our backend) by default. We fix this in Express by using the cors middleware package. We simply install it and add app.use(cors()) to our Express app, which tells the browser that our backend allows requests from other domains.

26. What is the difference between app.use() and app.get()?

app.get() is used for routing. It specifically matches only GET requests to a particular URL path.
app.use() is used for middleware. It is more general. It will match any HTTP method (GET, POST, etc.) for a given path. We use it to apply middleware to all requests or all requests under a certain path.

27. How do you handle file uploads in MERN?

To handle file uploads, our React frontend sends the data as multipart/form-data. On our Express backend, we cannot use our normal body-parser for this. We must use a special middleware called multer. multer is designed to parse this multipart/form-data, extract the file, and make it available in our route handler (for example, as req.file) so we can save it to our server or a cloud storage service.

React.js Interview Questions

This is the largest section, covering the “R” in MERN. React is the user interface, and these MERN stack interview questions cover its most important ideas, from components to state management.

28. What is React?

React is a free, open-source JavaScript library used for building fast and interactive user interfaces. It is important to call it a library, not a framework. It focuses only on the “View” part of an application. Its main idea is building the UI out of reusable pieces called “components”.

29. What is JSX in React?

JSX stands for JavaScript XML. It is a special syntax that lets us write code that looks like HTML directly inside our JavaScript files. It is not real HTML, but it makes our component code much easier to read and write. A tool called Babel compiles our JSX into regular JavaScript code that the browser can understand.

30. What is the virtual DOM in React?

The virtual DOM is a core concept that makes React fast. The “real DOM” is the browser’s representation of our webpage. Changing the real DOM is very slow. The virtual DOM is a lightweight copy of the real DOM, kept in memory. When our component’s state changes, React first updates this lightweight virtual DOM. Then, it compares the new virtual DOM with the old one to find the exact differences. Finally, it only updates those specific, changed parts of the real DOM. This process is called “reconciliation”.

31. What distinguishes a class component from a functional component?

These are the two ways we create components in React.

Class Components are the older way. They are JavaScript classes that extend React.Component and use lifecycle methods (like componentDidMount) and manage state with this.state.
Functional Components are the modern way. They are simple JavaScript functions. Before, they were “stateless,” but with the introduction of Hooks (like useState), we can now manage state and side effects in functional components. They are cleaner and easier to read.

32. What is the difference between props and state in React?

This is one of the most fundamental questions in React.

Props (short for properties) are data passed down from a parent component to a child component. Props are read-only, meaning a child component can never change the props it receives.
State is data that is managed inside a component. State is private to that component and can be changed. When state is changed (using useState), the component re-renders itself.

33. What are hooks in React?

Hooks are special functions that “hook into” React’s features. They were introduced in React 16.8 and let us use state and other React features in functional components. The two most important hooks are useState and useEffect.

34. What is the purpose of the useState hook?

The useState hook is how we add local state to a functional component. When we call useState, we pass the initial value for the state. It returns an array with two items: the current state value and a function to update that value.

35. What is the useEffect hook used for in React?

The useEffect hook is used for handling “side effects”. A side effect is anything in our component that is not part of the main render. The most common examples of side effects are:

Fetching data from an API after the component renders.
Setting up timers or subscriptions.
Manually changing the DOM.

36. What is prop drilling?

Prop drilling is a common problem in React applications. It is the situation where we need to pass data from a high-level parent component to a deeply nested child component. To do this, we have to pass the prop through all the “middle” components, even if those components do not need the data at all. This makes the code messy and hard to maintain.

37. How does the Context API work in React?

The Context API is React’s built-in solution to the “prop drilling” problem. It allows us to create a “context” (a global state) at a high level. We create a “Provider” component that holds the data. Then, any component in the tree below, no matter how deeply nested, can “consume” that data directly, without needing it to be passed down as props.

38. What is Redux and when do you use it?

Redux is another solution for state management, but it is a separate, external library. Like Context, it solves prop drilling. Redux is more powerful and complex. It keeps the entire state of the application in one single, global “store.” We use Redux for very large and complex applications where many different components need to share and update a lot of state.

39. What is the difference between useState and useReducer in React?

Both are hooks for managing state.

We use useState for simple state, like a number, a string, a boolean, or a simple object.
We use useReducer for more complex state logic. It is a good choice when the next state value depends on the previous one, or when the state logic is complicated and we want to manage it all in one place.

40. What are higher-order components (HOCs) in React?

A Higher-Order Component (HOC) is an advanced React pattern. It is not a component, but a function that takes a component as an argument and returns a new component. The new component “wraps” the original one and gives it new props or logic. A common use case is an HOC that checks if a user is authenticated before rendering a component.

41. How do you optimize React applications for performance?

There are two main ways we optimize React performance.

Memoization: We prevent unnecessary re-renders. We can use React.memo to wrap components so they only re-render if their props change. We can use the useMemo and useCallback hooks to memoize values and functions.
Lazy Loading: We split our code. We use React.lazy and Suspense to load components only when they are needed (for example, when a user clicks a button to open a modal), instead of loading all the code at once.

42. What is server-side rendering (SSR) in React?

Normally, React runs in the browser (this is client-side rendering). The server sends a blank HTML file and a large JavaScript file, and React builds the page in the browser. With Server-Side Rendering (SSR), the server runs the React components itself and sends a fully rendered HTML page to the browser. This makes the initial page load much faster and is much better for SEO, because search engines can read the content immediately.

43. What are React fragments?

A React component must return a single element. This means if we want to return two paragraphs, we have to wrap them in a div. This adds an extra, unnecessary div to the DOM. Fragments solve this. They let us group a list of children elements without adding any extra node to the DOM. We can write <React.Fragment> or use the short syntax: <>... </>.

Node.js Interview Questions

Finally, we cover the “N” in MERN. Node.js is the JavaScript runtime that powers our entire backend. These MERN stack interview questions focus on Node.js core and its asynchronous nature.

44. What is Node.js?

Node.js is not a language or a framework. It is a JavaScript runtime environment. It is built on Chrome’s V8 JavaScript engine. Before Node.js, JavaScript could only run in web browsers. Node.js is a tool that allows us to take JavaScript and run it on the server to build backend applications.

45. Explain the event loop in Node.js.

The event loop is the absolute core of Node.js. It is what allows Node.js to be asynchronous and “non-blocking”. Node.js has only one main thread. The event loop is a process that constantly runs. When we ask Node.js to do a slow task (like read a file or make a network request), it does not wait. It hands that task off to the system and continues running the event loop. It keeps a “callback queue” of tasks that are finished. The event loop’s job is to constantly check this queue and run the callback functions for completed tasks, one at a time. This is why Node.js can handle thousands of connections at once.

46. What is a callback function in Node.js?

A callback function is a function that we pass as an argument to another function. In Node.js, we use callbacks to handle asynchronous operations. We tell a function (like fs.readFile) to “read this file, and when you are done, run this callback function with the data.”

47. What is the difference between synchronous and asynchronous methods in Node.js?

Synchronous methods (like fs.readFileSync) block the event loop. When we call one, our entire application freezes and waits for it to finish. We should almost never use synchronous methods in Node.js.
Asynchronous methods (like fs.readFile) are non-blocking. They use a callback or a promise. When we call one, Node.js starts the task and immediately moves on, keeping the event loop free to do other work.

48. How do you export and import modules in Node.js?

Node.js has two main module systems.

CommonJS (CJS): This is the traditional system. We use module.exports = myFunction to export a function, and const myFunction = require('./myFile') to import it.
ES Modules (ESM): This is the modern, standard JavaScript system. We use export default myFunction to export, and import myFunction from './myFile' to import. Modern Node.js supports both systems.

49. What is the purpose of the package.json file?

The package.json file is the “manifest” or “metadata” file for our Node.js project. It holds important information. Its two most important jobs are:

Listing all the dependencies and devDependencies (the libraries our project needs to run).
Defining scripts (like npm start or npm test) that we can run from the command line.

50. What are streams in Node.js?

Streams are a powerful concept in Node.js for handling data. They are a way to read or write data in chunks, one piece at a time, instead of loading the entire thing into memory. This is extremely efficient for large files. For example, if we want to read a 5GB video file, we can “stream” it instead of trying to load all 5GB into RAM, which would crash our server.

Conclusion

We did it. We have covered 51 of the most important questions. I hope this guide to MERN stack interview questions helps you feel much more prepared and confident. Remember that every developer has been in this exact same spot. You have studied and you are ready. Good luck, and go crack that interview.

References

To create this MERN Stack interview question list, I went through multiple real-world discussions and community experiences shared by developers. Some of the threads I analysed include:

A full-stack MERN interview surprise shared by a developer:
https://www.reddit.com/r/developersIndia/comments/1lfdf1p/got_surprised_in_my_full_stack_interview_mern/

Discussion on what companies look for when hiring a MERN/React developer:
https://www.reddit.com/r/javascript/comments/ve8mab/askjs_if_you_were_hiring_a_mernjsreact_dev_what/

Reddit thread on MERN Stack fresher interview experiences:
https://www.reddit.com/r/react/comments/1h72q2n/preparing_for_a_mern_stack_interview_as_a_fresher/