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. <\/p>\n\n\n\n
In this article, I decided to share my top 50 MERN Stack interview questions for 2026. Go through them carefully, don\u2019t just memorise but understand each concept and its related topics. Good luck!<\/p>\n\n\n\n
BTW, I have already shared my list of 50 JavaScript interview questions<\/em><\/a>, if you haven\u2019t read it yet.<\/p>\n\n\n\n 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.<\/p>\n\n\n\n The MERN stack is a popular collection of technologies used to build complete web applications.<\/sup> 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: <\/p>\n\n\n\n We can think of the MERN stack components as a team working together.<\/p>\n\n\n\n The biggest advantage of the MERN stack is that it uses JavaScript for the\u00a0entire\u00a0application.\u00a0The 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.\u00a0It also makes development faster. The MERN stack is also highly scalable, has great performance, and has a very large, supportive community.\u00a0\u00a0\u00a0<\/p>\n\n\n\n The MERN and MEAN stacks are very similar. They both use MongoDB, Express.js, and Node.js. The only difference is the frontend technology.<\/sup> <\/p>\n\n\n\n Generally, React is known for being more flexible and having a faster learning curve, which is why many developers prefer it.<\/sup> <\/p>\n\n\n\n A RESTful API is an architectural style for building web services. It is a set of rules for how applications communicate over the internet.<\/sup> It uses standard HTTP methods, which are a set of commands like: <\/p>\n\n\n\n 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.<\/sup> <\/p>\n\n\n\n We pass data from React to Express by making an HTTP request, usually with the Deploying a MERN stack application involves deploying its parts separately, as they are distinct applications.<\/sup> <\/p>\n\n\n\n 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.<\/p>\n\n\n\n MongoDB is a NoSQL database.<\/sup> 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.<\/sup> <\/p>\n\n\n\n The biggest difference is the schema.<\/p>\n\n\n\n This is a common point of confusion. Mongoose is\u00a0not\u00a0the database. Mongoose is an\u00a0Object Data Modeling (ODM)<\/strong>\u00a0library for Node.js.\u00a0It 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n Even though MongoDB has a flexible schema, our application usually needs some structure. A Mongoose schema defines the structure for our documents\u00a0at the application level.\u00a0We can set what fields each document should have (like\u00a0 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n An aggregation pipeline is a powerful tool for data processing.<\/sup> It takes a stream of documents and passes them through a series of “stages.” Each stage transforms the data. Common stages include: <\/p>\n\n\n\n Replication is the process of creating and maintaining multiple copies of our data on different servers.\u00a0These servers form a “replica set.” The main purpose of replication is to provide\u00a0high availability<\/strong>. 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n Sharding is a method for distributing data across multiple servers, or “shards”.\u00a0We 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\u00a0entire\u00a0database for safety, sharding\u00a0splits\u00a0the database for size and performance.\u00a0\u00a0\u00a0<\/p>\n\n\n\n These are the two ways we handle relationships between data.<\/p>\n\n\n\n We use the\u00a0 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.<\/p>\n\n\n\n Express.js is a lightweight framework for Node.js that builds our backend.\u00a0In 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n Middleware functions are the heart of Express.js. They are functions that run “in the middle” of a request and its response.\u00a0A middleware function has access to the request object ( The\u00a0 We define a route by using a method on our\u00a0 For a very small application, we can define all our routes in the main\u00a0 We handle errors in Express by creating a special\u00a0error-handling middleware. This middleware is special because it must have\u00a0four\u00a0arguments:\u00a0 CORS stands for\u00a0Cross-Origin Resource Sharing<\/strong>.\u00a0It is a security feature built into web browsers that blocks HTTP requests between different domains. For example, the browser will block a request from\u00a0 To handle file uploads, our React frontend sends the data as\u00a0 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.<\/p>\n\n\n\n React is a free, open-source JavaScript\u00a0library\u00a0used for building fast and interactive user interfaces.\u00a0It 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”.\u00a0\u00a0\u00a0<\/p>\n\n\n\n JSX stands for\u00a0JavaScript XML<\/strong>.\u00a0It 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n The virtual DOM is a core concept that makes React fast.\u00a0The “real DOM” is the browser’s representation of our webpage. Changing the real DOM is very slow. The virtual DOM is a lightweight\u00a0copy\u00a0of the real DOM, kept in memory.\u00a0When 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\u00a0exact\u00a0differences. Finally, it only updates those specific, changed parts of the real DOM. This process is called “reconciliation”.\u00a0\u00a0\u00a0<\/p>\n\n\n\n These are the two ways we create components in React.<\/p>\n\n\n\n This is one of the most fundamental questions in React.<\/p>\n\n\n\n Hooks are special functions that “hook into” React’s features.\u00a0They were introduced in React 16.8 and let us use state and other React features in functional components. The two most important hooks are\u00a0 The\u00a0 The\u00a0 Prop drilling is a common problem in React applications.\u00a0It 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.\u00a0This makes the code messy and hard to maintain.\u00a0\u00a0\u00a0<\/p>\n\n\n\n The Context API is React’s built-in solution to the “prop drilling” problem.\u00a0It 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n Redux is another solution for state management, but it is a separate, external library.\u00a0Like Context, it solves prop drilling. Redux is more powerful and complex. It keeps the\u00a0entire\u00a0state 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n Both are hooks for managing state.<\/p>\n\n\n\n A Higher-Order Component (HOC) is an advanced React pattern. It is not a component, but a\u00a0function\u00a0that takes a component as an argument and returns a\u00a0new\u00a0component.\u00a0The 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n There are two main ways we optimize React performance.<\/sup> <\/p>\n\n\n\n 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\u00a0Server-Side Rendering (SSR)<\/strong>, the server runs the React components itself and sends a\u00a0fully rendered\u00a0HTML page to the browser.\u00a0This makes the initial page load much faster and is much better for SEO, because search engines can read the content immediately.\u00a0\u00a0\u00a0<\/p>\n\n\n\n A React component must return a single element. This means if we want to return two paragraphs, we have to wrap them in a\u00a0 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.<\/p>\n\n\n\n Node.js is\u00a0not\u00a0a language or a framework. It is a\u00a0JavaScript runtime environment<\/strong>.\u00a0It 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n The event loop is the absolute core of Node.js. It is what allows Node.js to be asynchronous and “non-blocking”.\u00a0Node.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\u00a0not\u00a0wait. 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n A callback function is a function that we pass as an argument to another function.\u00a0In Node.js, we use callbacks to handle asynchronous operations. We tell a function (like\u00a0 Node.js has two main module systems.<\/p>\n\n\n\n The\u00a0 Streams are a powerful concept in Node.js for handling data.\u00a0They are a way to read or write data\u00a0in chunks, one piece at a time, instead of loading the entire thing into memory.\u00a0This 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n We did it. We have covered 51 of the most important questions. I hope this guide to\u00a0MERN stack interview questions\u00a0helps 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.<\/p>\n\n\n\n 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:<\/p>\n\n\n\n A full-stack MERN interview surprise shared by a developer: Discussion on what companies look for when hiring a MERN\/React developer:General MERN Stack Interview Questions<\/h2>\n\n\n\n
1. What is the MERN Stack?<\/h3>\n\n\n\n
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2. How do the MERN stack components work together?<\/h3>\n\n\n\n
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3. Why choose the MERN stack?<\/h3>\n\n\n\n
4. What is the difference between MERN and MEAN?<\/h3>\n\n\n\n
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5. What is a RESTful API?<\/h3>\n\n\n\n
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6. What is the MVC architecture?<\/h3>\n\n\n\n
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7. How do you pass data from the frontend (React) to the backend (Express)?<\/h3>\n\n\n\n
fetch<\/code> API or a library like axios<\/code>. If we are creating new data, we use a POST<\/code> request. We attach the data (like a username and password) to the body<\/code> of the request. We usually format this data as a JSON string.<\/sup> <\/p>\n\n\n\n8. How do you deploy a MERN stack application?<\/h3>\n\n\n\n
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MongoDB and Mongoose Interview Questions<\/h2>\n\n\n\n
9. What is MongoDB?<\/h3>\n\n\n\n
10. How is MongoDB different from SQL databases?<\/h3>\n\n\n\n
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11. What is Mongoose?<\/h3>\n\n\n\n
12. What is a schema in Mongoose?<\/h3>\n\n\n\n
name<\/code>\u00a0or\u00a0email<\/code>), what data type they should be (like\u00a0String<\/code>\u00a0or\u00a0Number<\/code>), and add validation rules (like\u00a0required: true<\/code>\u00a0or\u00a0unique: true<\/code>).\u00a0\u00a0\u00a0<\/p>\n\n\n\n13. What is indexing in MongoDB?<\/h3>\n\n\n\n
14. What is an aggregation pipeline in MongoDB?<\/h3>\n\n\n\n
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$match<\/code>: This filters the documents, like a find()<\/code> query. <\/li>\n\n\n\n$group<\/code>: This groups documents together to perform calculations, like finding a sum or an average.<\/li>\n\n\n\n$sort<\/code>: This sorts the documents.<\/li>\n<\/ul>\n\n\n\n15. What is Replication in MongoDB?<\/h3>\n\n\n\n
16. What is Sharding in MongoDB?<\/h3>\n\n\n\n
17. What is the difference between embedding and referencing in MongoDB?<\/h3>\n\n\n\n
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_id<\/code>) to a document in another collection. For example, a blog post document might have a\u00a0userId<\/code>\u00a0field that “refers” to a document in the\u00a0users<\/code>\u00a0collection.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n18. How do you update a document in MongoDB?<\/h3>\n\n\n\n
db.collection.updateOne()<\/code>\u00a0method.\u00a0This method takes two main arguments. The first is a\u00a0filter\u00a0to find the document we want to update. The second is an\u00a0update operator, like\u00a0$set<\/code>, which tells MongoDB what fields to change. For example:\u00a0db.users.updateOne({ name: \"Alice\" }, { $set: { age: 30 } })<\/code>.\u00a0\u00a0\u00a0<\/p>\n\n\n\nExpress.js Interview Questions<\/h2>\n\n\n\n
19. How does Express.js work in the MERN Stack?<\/h3>\n\n\n\n
20. What is middleware in Express.js?<\/h3>\n\n\n\n
req<\/code>), the response object (res<\/code>), and a special function called\u00a0next<\/code>.\u00a0We use middleware for many tasks, such as:\u00a0\u00a0\u00a0<\/p>\n\n\n\n\n
21. What is the purpose of next() in Express middleware?<\/h3>\n\n\n\n
next()<\/code>\u00a0function is very important. When a middleware function finishes its job, it must call\u00a0next()<\/code>\u00a0to pass control to the\u00a0next\u00a0middleware function in the line.\u00a0If we forget to call\u00a0next()<\/code>, the request will get “stuck” in our middleware, and the user will never get a response.\u00a0\u00a0\u00a0<\/p>\n\n\n\n22. How do you define a route in Express.js?<\/h3>\n\n\n\n
app<\/code>\u00a0object that matches an HTTP method, like\u00a0app.get()<\/code>,\u00a0app.post()<\/code>,\u00a0app.put()<\/code>, or\u00a0app.delete()<\/code>. This method takes two arguments: the URL path and a callback function that runs when that path is requested.\u00a0For example:\u00a0app.get('\/users', (req, res) => { res.send('Hello users'); });<\/code>.\u00a0\u00a0\u00a0<\/p>\n\n\n\n23. How do you handle routing in Express.js?<\/h3>\n\n\n\n
server.js<\/code>\u00a0file. For a larger application, this becomes messy. The best practice is to use\u00a0express.Router()<\/code>.\u00a0This allows us to create separate files for our routes (like\u00a0userRoutes.js<\/code>\u00a0or\u00a0postRoutes.js<\/code>) and then import them into our main\u00a0server.js<\/code>\u00a0file. This keeps our code clean and organized.\u00a0\u00a0\u00a0<\/p>\n\n\n\n24. How do you handle errors in Express.js?<\/h3>\n\n\n\n
(err, req, res, next)<\/code>.\u00a0We place this middleware at the very end of our middleware stack. If any route or middleware calls\u00a0next()<\/code>\u00a0with an error, Express will skip all other middleware and go straight to this error-handling function.\u00a0\u00a0\u00a0<\/p>\n\n\n\n25. What is CORS and how do you enable it in Express.js?<\/h3>\n\n\n\n
my-react-app.com<\/code>\u00a0(our frontend) to\u00a0my-api.com<\/code>\u00a0(our backend) by default. We fix this in Express by using the\u00a0cors<\/code>\u00a0middleware package. We simply install it and add\u00a0app.use(cors())<\/code>\u00a0to our Express app, which tells the browser that our backend allows requests from other domains.\u00a0\u00a0\u00a0<\/p>\n\n\n\n26. What is the difference between app.use() and app.get()?<\/h3>\n\n\n\n
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app.get()<\/code>\u00a0is used for\u00a0routing. It specifically matches only\u00a0GET<\/code>\u00a0requests to a particular URL path.\u00a0\u00a0\u00a0<\/li>\n\n\n\napp.use()<\/code>\u00a0is used for\u00a0middleware. It is more general. It will match\u00a0any\u00a0HTTP method (GET, POST, etc.) for a given path. We use it to apply middleware to all requests or all requests under a certain path.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n27. How do you handle file uploads in MERN?<\/h3>\n\n\n\n
multipart\/form-data<\/code>. On our Express backend, we cannot use our normal body-parser for this. We must use a special middleware called\u00a0multer<\/code>.\u00a0multer<\/code>\u00a0is designed to parse this\u00a0multipart\/form-data<\/code>, extract the file, and make it available in our route handler (for example, as\u00a0req.file<\/code>) so we can save it to our server or a cloud storage service.\u00a0\u00a0\u00a0<\/p>\n\n\n\nReact.js Interview Questions<\/h2>\n\n\n\n
28. What is React?<\/h3>\n\n\n\n
29. What is JSX in React?<\/h3>\n\n\n\n
30. What is the virtual DOM in React?<\/h3>\n\n\n\n
31. What distinguishes a class component from a functional component?<\/h3>\n\n\n\n
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React.Component<\/code>\u00a0and use lifecycle methods (like\u00a0componentDidMount<\/code>) and manage state with\u00a0this.state<\/code>.\u00a0\u00a0\u00a0<\/li>\n\n\n\nuseState<\/code>), we can now manage state and side effects in functional components. They are cleaner and easier to read.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n32. What is the difference between props and state in React?<\/h3>\n\n\n\n
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useState<\/code>), the component re-renders itself.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n33. What are hooks in React?<\/h3>\n\n\n\n
useState<\/code>\u00a0and\u00a0useEffect<\/code>.\u00a0\u00a0\u00a0<\/p>\n\n\n\n34. What is the purpose of the useState hook?<\/h3>\n\n\n\n
useState<\/code>\u00a0hook is how we add local state to a functional component.\u00a0When we call\u00a0useState<\/code>, 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.\u00a0\u00a0\u00a0<\/p>\n\n\n\n35. What is the useEffect hook used for in React?<\/h3>\n\n\n\n
useEffect<\/code>\u00a0hook is used for handling “side effects”.\u00a0A side effect is anything in our component that is not part of the main render. The most common examples of side effects are:\u00a0\u00a0\u00a0<\/p>\n\n\n\n\n
36. What is prop drilling?<\/h3>\n\n\n\n
37. How does the Context API work in React?<\/h3>\n\n\n\n
38. What is Redux and when do you use it?<\/h3>\n\n\n\n
39. What is the difference between useState and useReducer in React?<\/h3>\n\n\n\n
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useState<\/code>\u00a0for simple state, like a number, a string, a boolean, or a simple object.\u00a0\u00a0\u00a0<\/li>\n\n\n\nuseReducer<\/code>\u00a0for 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.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n40. What are higher-order components (HOCs) in React?<\/h3>\n\n\n\n
41. How do you optimize React applications for performance?<\/h3>\n\n\n\n
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React.memo<\/code>\u00a0to wrap components so they only re-render if their props change. We can use the\u00a0useMemo<\/code>\u00a0and\u00a0useCallback<\/code>\u00a0hooks to memoize values and functions.\u00a0\u00a0\u00a0<\/li>\n\n\n\nReact.lazy<\/code>\u00a0and\u00a0Suspense<\/code>\u00a0to load components\u00a0only\u00a0when they are needed (for example, when a user clicks a button to open a modal), instead of loading all the code at once.\u00a0\u00a0\u00a0<\/li>\n<\/ol>\n\n\n\n42. What is server-side rendering (SSR) in React?<\/h3>\n\n\n\n
43. What are React fragments?<\/h3>\n\n\n\n
div<\/code>. This adds an extra, unnecessary\u00a0div<\/code>\u00a0to the DOM.\u00a0Fragments<\/strong>\u00a0solve this.\u00a0They let us group a list of children elements without adding any extra node to the DOM. We can write\u00a0<React.Fragment><\/code>\u00a0or use the short syntax:\u00a0<>... <\/><\/code>.\u00a0\u00a0\u00a0<\/p>\n\n\n\nNode.js Interview Questions<\/h2>\n\n\n\n
44. What is Node.js?<\/h3>\n\n\n\n
45. Explain the event loop in Node.js.<\/h3>\n\n\n\n
46. What is a callback function in Node.js?<\/h3>\n\n\n\n
fs.readFile<\/code>) to “read this file, and when you are done, run this callback function with the data.”\u00a0\u00a0\u00a0<\/p>\n\n\n\n47. What is the difference between synchronous and asynchronous methods in Node.js?<\/h3>\n\n\n\n
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fs.readFileSync<\/code>)\u00a0block\u00a0the event loop.\u00a0When we call one, our entire application freezes and waits for it to finish. We should almost never use synchronous methods in Node.js.\u00a0\u00a0\u00a0<\/li>\n\n\n\nfs.readFile<\/code>) are\u00a0non-blocking.\u00a0They 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.\u00a0\u00a0\u00a0<\/li>\n<\/ul>\n\n\n\n48. How do you export and import modules in Node.js?<\/h3>\n\n\n\n
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module.exports = myFunction<\/code>\u00a0to export a function, and\u00a0const myFunction = require('.\/myFile')<\/code>\u00a0to import it.\u00a0\u00a0\u00a0<\/li>\n\n\n\nexport default myFunction<\/code>\u00a0to export, and\u00a0import myFunction from '.\/myFile'<\/code>\u00a0to import. Modern Node.js supports both systems.<\/li>\n<\/ol>\n\n\n\n49. What is the purpose of the package.json file?<\/h3>\n\n\n\n
package.json<\/code>\u00a0file is the “manifest” or “metadata” file for our Node.js project.\u00a0It holds important information. Its two most important jobs are:\u00a0\u00a0\u00a0<\/p>\n\n\n\n\n
dependencies<\/code> and devDependencies<\/code> (the libraries our project needs to run).<\/li>\n\n\n\nscripts<\/code> (like npm start<\/code> or npm test<\/code>) that we can run from the command line.<\/li>\n<\/ol>\n\n\n\n50. What are streams in Node.js?<\/h3>\n\n\n\n
Conclusion<\/h2>\n\n\n\n
References<\/strong><\/h2>\n\n\n\n
https:\/\/www.reddit.com\/r\/developersIndia\/comments\/1lfdf1p\/got_surprised_in_my_full_stack_interview_mern\/<\/a><\/p>\n\n\n\n
https:\/\/www.reddit.com\/r\/javascript\/comments\/ve8mab\/askjs_if_you_were_hiring_a_mernjsreact_dev_what\/<\/a><\/p>\n\n\n\n