Top 30 MERN Stack Interview Questions and Answers

The need for this MERN Stack Engineer position is increasing, and many businesses are hiring MERN stack developers, thus this blog is geared toward individuals who are interested. For the purpose of assisting candidates who are preparing for the MERN stack interview, we will discuss interview questions for MERN stack developers.

1. How MongoDB is functioning in the MERN stack?

MongoDB is a document-oriented database executive that is used to store data. MongoDB holds data in binary JSON format, which embodies the anthology and documentation theory. MongoDB is a NoSQL database with great performance, scalability, and flexibility in querying and indexing. 2. What is the purpose of Express JS in this technology? ExpressJS is a web application framework that was created to facilitate and host NodeJS projects. ExpressJS is an open-source framework licensed under the MIT license. It coordinates the workflow between the front end and the database, ensuring that data is transferred quickly and securely. Furthermore, ExpressJS has strong error-handling web design functionality for streamlining online development processes. 3. Explain the work of ReactJS? React produces a virtual document object model (DOM). When a component’s state changes, it first performs a “diffing” method to determine what has changed in the virtual DOM. The second stage is reconciliation, which involves updating the DOM with the differences found in the diff. 4. How is NodeJS functioning in this technology? NodeJS is a cross-platform open-source framework for developing server-side and networking applications that use a single-threaded JavaScript framework. The MERN stack’s backbone is known as NodeJS. Apart from JavaScript, NodeJS uses C and C++ programming languages. It comes with a web server that allows MongoDB and cloud apps to be deployed quickly. 5. What do you know about Mongoose? The object document mapper, or mongoose, is used to define objects using a strongly typed schema that can be mapped to a MongoDB document. Mongoose, as a result, provides a schema-based solution for modeling application data. Typecasting, validation, query construction, business logic hooks, and other functionality were also integrated into Mongoose. 6. How does REPL work under NodeJS? The Read Eval Print Loop (REPL) is a program that accepts commands, evaluates them, and prints the results. As a result, REPL works in the same way that Unix or Linux did in terms of creating an environment for entering commands and systems that respond to the output. 7. How will you utilize Data Modeling in the MERN stack? In the context of Mongoose and MongoDB, this word is employed. Data modeling is the process of developing a data model for the data at hand before storing it in a database. The data model depicts the data object, its relationship, and the rules that govern how the data is connected. Data visualization can be portrayed while business standards, compliance, and policy are enforced through data modeling. Data modeling is used to ensure that conventions, values, semantics, security, and data quality are all consistent. 8. What distinguishes React from AngularJS(1.x)? AngularJS (1.x) works by extending HTML syntax and injecting numerous components (such as Directives, Controllers, and Services) during runtime. As a result, AngularJS has strong opinions about your application’s overall architecture — these abstractions are certainly valuable in some situations, but they come at the expense of flexibility. React, on the other hand, is solely concerned with the construction of components and has few (if any) thoughts on the architecture of an application. This gives a developer a lot of flexibility in selecting the architecture they think is “optimal.” 9. What do you know about Higher-Order Components? A function that takes a component and returns a new component is known as a higher-order component (HOC). It’s essentially a pattern developed from React’s compositional nature. They’re called “pure” components since they can accept any dynamically provided child component but don’t change or copy the behavior of their input components. const EnhancedComponent = higherOrderComponent(WrappedComponent); HOC can be used in a variety of scenarios, as seen below.

• Bootstrap abstraction, logic, and code reuse
• Manipulation and abstraction of states
• Manipulation of objects

10. Tell us about Replication in MongoDB? The practice of synchronizing data across several servers is known as replication. Data availability is increased and redundancy is provided via replication. Replication protects a database from the loss of a single server by storing several copies of data on various database servers. You may also recover from hardware failures and service interruptions using replication. 11. Explain Asynchronous API? The Node.js library’s APIs are all asynchronous, or non-blocking. It means that a Node.js server never has to wait for an API to return data. After accessing the next API, the server moves on to the next one, and the notification mechanism of Node.js Events assists the server in receiving the response from the previous API call. 12. Tell me about Reconciliation? When a component’s props or state change, React compares the newly returned element to the previously rendered one to determine whether an actual DOM update is required. React will update the DOM if they are not equal. This is referred to as reconciliation. 13. What do you know about CallBack Hell? As a return parameter, the asynchronous function expects callbacks. A callback hell condition occurs when numerous asynchronous functions are chained together. 14. When should we use MongoDB to embed one document inside another? You should consider embedding documents for the following reasons:

• Includes inter-entity relationships
• One-to-many connections are the most common type of relationship.
• Reasons for poor performance

15. Tell me the advantages of BSON over JSON in MongoDB? BSON is supposed to be space-efficient, however, it isn’t always more efficient than JSON. BSON needs considerably more space than JSON in some circumstances. Another of BSON’s design aims is traversability, which explains why. BSON adds “additional” information to documents, such as string and subobject lengths. This speeds up traversal. BSON is also made to encode and decode quickly. Integers, for example, are kept as 32 (or 64) bit integers and do not require parsing to and from the text. For small integers, this takes up more space than JSON, but it is significantly faster to parse. 16. Explain how Node JS handles child threads? In essence, Node.js is a single-threaded application. The developer is not given access to child threads or thread management mechanisms. Although Node.js does launch child threads for certain activities like asynchronous I/O, these run in the background and do not execute any JavaScript code or disrupt the main event loop. There are tools available to enable threading functionality in a Node.js application, such as the ChildProcess module. 17. How Pure Components Work? PureComponent is identical to Component, with the exception that it takes care of the shouldComponentUpdate method for you. PureComponent does a shallow comparison on both props and state when they change. Out of the box, Components will not compare current props and state to the next. As a result, anytime shouldComponentUpdate is triggered, the component will re-render by default. 18. Tell us about React Hooks? In React 16.8, hooks are a brand-new feature. They make it possible to use state and other React capabilities without having to create a class. Hooks allow you to abstract stateful logic from a component so that it may be tested and reused separately. Hooks allow you to reuse stateful functionality without having to change the hierarchy of your components. This makes it simple to distribute Hooks across several components or to the community. 19. What is Key and why should you use it in your lists? When generating lists of elements, a key is a particular string attribute that must be included. React uses keys to figure out which items have changed, been added to, or been removed. We frequently use IDs from your data as keys, for example. const todoItems = todos.map((todo) => <li key={todo.id}> {todo.text} </li> ); You can use the item index as a key as a last resort if you don’t have reliable IDs for rendered items: const todoItems = todos.map((todo, index) => <li key={index}> {todo.text} </li> ); 20. What do you mean by a Blocking Code? If an application needs to wait for an I/O operation before continuing its execution, the code responsible for waiting is referred to as blocking code. 21. How does Node.js handle concurrency if it is single-threaded? Node gives programmers access to a single thread, allowing them to write code quickly and efficiently. For different I/O activities such as File, DNS, Network calls, and so on, Node uses many POSIX threads internally. When Node receives an I/O request, it creates or utilizes a thread to complete the operation, and then it pushes the result to the event queue. On each such event, the event loop runs and checks the queue, adding the queue result to the execution stack of Node if it is empty. 22. Explain Shadow DOM? The main goal of Shadow DOM is to encapsulate the implementation. A single custom element can mix more or less complex logic with a more or less sophisticated DOM. An import and <my-app>/my-app> tag can be used to bring a whole web application of any complexity to a page. Simpler reusable and composable components, such as <date-picker>/date-picker>, can likewise be implemented as custom elements with their internal representation concealed in the shadow DOM. 23. What are the smart component and dumb component? Smart components manage their state or are connected to the Redux store in a Redux context. Dumb components are fully controlled by the props they receive from their parent and have no state of their own. 24. What is the usage of createRef? createRef is always going to make a new ref. When building a class-based component, you’d usually place the ref in an instance property (e.g. this.input = createRef()). This option isn’t available in a function component. 25. Explain the procedure of the Event loop in Node JS? Although JavaScript is single-threaded, the event loop allows Node.js to conduct non-blocking I/O operations by transferring operations to the system kernel whenever available. Every I/O requires a callback, which is then pushed into the event loop to be executed. Because most current kernels are multi-threaded, they can handle many background operations. When one of these tasks is completed, the kernel notifies Node.js, allowing the appropriate callback to be added to the poll queue and executed later. 26. Tell us about Prop drilling and the ways to avoid it? When developing a React application, it’s common to need a deeply nested component to access data from a component far higher in the hierarchy. The most straightforward method is to simply pass a prop from one component to the next in the hierarchy, from the source component to the deeply nested component. This is referred to as prop drilling. Prop drilling’s main downside is that it makes components that should not be aware of data overly complicated and difficult to maintain. A typical technique to avoid prop drilling is to leverage React context. This enables the definition of a data-supply Provider component, as well as the consumption of context data by nested components via either a Consumer component or a useContext hook. 27. How will you query with the “like” operator in MongoDB? The below query can be simply converted as a MongoDB query as select * from users where name like ‘%m%’ db.users.find({“name”: /.*m.*/}) // or db.users.find({“name”: /m/}) 28. Do you know about Occasion Driven Programming? The programming viewpoint of occasion-driven creating computer programs is one in which the program stream is controlled by events such as notifications from other projects and strings. The application engineering framework of event-driven programming is divided into two parts: event selection and event handling. 29. Modify this code into Async/Await. function asyncTask() { return functionA() .then((valueA) => functionB(valueA)) .then((valueB) => functionC(valueB)) .then((valueC) => functionD(valueC)) .catch((err) => logger.error(err)) } Answer: async function asyncTask() { try { const valueA = await functionA() const valueB = await functionB(valueA) const valueC = await functionC(valueB) return await functionD(valueC) } catch (err) { logger.error(err) } } 30. What is an event emitter in NodeJS? In NodeJS, the event emitter module allows you to create and destroy custom events. The game class emitter, which may handle and raise custom events, is included in this event. In addition, if the emitter encounters an issue, the event will contain some errors. When a new user is added, the event emitter, on the other hand, will send the demonstration with the new user’s name. When the user is no longer present, the demonstration is referred to as a remote ad user. CONCLUSION: The top MERN Stack interview questions are now complete. We hope the MERN Stack interview questions for freshers and MERN Stack interview questions for experienced prove useful in helping you prepare for your forthcoming MERN Stack job interviews. Data Science is a popular job title these days. Consider taking this popular MERN Stack course from NSCHOOL Academy for more MERN Stack Interview Questions. All the best for your MERN Stack Interview!