MERN stack developer Interview Questions and Answers for Freshers

A MERN stack developer is a web developer who specializes in using the MERN stack, which is a web development framework that includes MongoDB, Express, React, and Node.js. MERN stack developers use these technologies to build modern web pages and applications, and are in high demand because JavaScript is one of the world’s most popular programming languages

1.What is MERN stack?

A well-liked JavaScript-based technological stack for creating online applications is called MERN stack. It stands for Node.js, React.js, Express.js, and MongoDB. Every element has a distinct function during the development process:

• MongoDB: A NoSQL database that stores data in a flexible, JSON-like format.
• Express.js: A web application framework for Node.js that simplifies backend development.
• Angular: A front-end framework for building dynamic, single-page applications (SPA).
• Node.js: A runtime environment that allows JavaScript to be executed on the server side.

2 How does React work?

Document Object Model, or virtual DOM, is how React creates a thin replica of the real DOM. React updates the virtual DOM and verifies that it matches the real DOM whenever the application state changes. It then minimizes performance overhead by efficiently updating only the relevant portions of the actual DOM. We call this process reconciliation.

State and UI logic are contained into React components, which are building blocks. Nested ones can be used to build intricate user interfaces. When writing React components, JSX, a JavaScript syntax extension, is frequently utilized since it simplifies and organizes the UI code.

3.What Is Replication In MongoDB?

In MongoDB, replication is the procedure that guarantees data availability and consistency among many MongoDB instances. Data must be copied from a primary instance (or member of a replica set) to secondary instances (or members of a replica set). Because a secondary instance can take over as the new primary in the event of a primary instance failure, minimizing downtime and preventing data loss, this redundancy provides fault tolerance.

4. What do you mean by Asynchronous API?

You can send a request to a server using an asynchronous API and not have to wait for a response right away. The server will process your request while you carry out other duties. The server will let you know when it has a response so you can manage it. This enhances your application’s responsiveness and performance.

Example: Ordering meals from a restaurant is similar to using an asynchronous API. You can place your order and carry on menu browsing or chit-chatting with your companions, rather than waiting for the chef to finish your meal. The waiter will let you know when your food is ready so you can start eating.

5. Does MongoDB Support Foreign Key Constraints?

No, MongoDB does not support foreign key constraints. This is a key architectural decision that sets MongoDB apart from relational databases such as PostgreSQL or MySQL.

Foreign key constraints in relational databases guarantee that entries in one table relate to values that already exist in another table, preserving data integrity. Data corruption and inconsistencies may be avoided in this way. But MongoDB adopts a different strategy, emphasizing scalability and flexibility.

MongoDB provides embedded documents and references as tools to manage relationships between data, in place of foreign key limitations. While references serve as pointers to other documents, embedded documents let you keep related data inside of a single document. This method can frequently be more effective for specific use situations and allows you greater control over the structure of the data.

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6. What are the key features of React.js?

React.js is a powerful JavaScript library with key features:

• Component-based architecture: Component-based design, which divides the user interface into smaller, reusable components, is encouraged by React. Complex application management and maintenance are facilitated by this.
• Virtual DOM: A virtual DOM, which is a condensed replica of the real DOM, is used by React. React improves efficiency by efficiently updating only the essential portions of the actual DOM and updating the virtual DOM when the application state changes.
• JSX:With the JavaScript syntax extension JSX, you may construct structures that resemble HTML directly within your JavaScript code. It is now easier to understand and read as a result.
• Unidirectional data flow: Data flows from parent components to child components in React in a unidirectional fashion. This lessens the possibility of unexpected side effects and facilitates inference about the current state of your application.
• Hooks: Reacts hooks are a more recent feature that let you leverage state and other capabilities without having to write class components. This improves the readability and conciseness of your code.

7. What is the difference between Shadow DOM and Virtual DOM?

Two separate ideas with different applications in web development are Shadow DOM and Virtual DOM.

Shadow DOM : Developers can encapsulate portions of the DOM tree within a shadow root using the browser API known as Shadow DOM. In doing so, conflicts with other portions of the DOM are avoided by creating a private, separated zone for styling and scripting. The main purpose of Shadow DOM is to enable the creation of unique elements and components by encapsulating their behaviour and styling, hence enabling their reusability and independence.

Virtual DOM : Angular, React, and Vue are just a few of the JavaScript frameworks that employ the programming notion of virtual DOM. It is a condensed, memory-based version of the real DOM. The library updates the virtual DOM and verifies that it matches the real DOM whenever the application state changes. It then minimizes performance overhead by efficiently updating only the relevant portions of the actual DOM. The main purpose of Virtual DOM is to improve the responsiveness and performance of dynamic user interface rendering.

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8.What’s the Event Loop?

React’s event loop is a technique for handling asynchronous operations and maintaining responsiveness of the user interface. It functions as a kind of queue that records all of the jobs that need to get done, like event processing, state updates, and component rendering.

The JSX code of a React component is compiled into a JavaScript function when it is rendered. The event loop of the browser gains this function. React re-renders a component when its state changes, which entails building a new virtual DOM representation and contrasting it with the old one. React then minimizes performance overhead by effectively updating only the relevant portions of the actual DOM. The event loop of the browser manages this process.

9. Explain the concept of RESTful APIs in MERN stack development.

RESTful APIs (Representational State Transfer APIs) A collection of architectural rules for developing web services that communicate with clients by representing various actions on resources through HTTP methods (such as GET, POST, PUT, PATCH, and DELETE) are part of the development of the MERN stack.

Example of RESTful API endpoints in a MERN stack application:

• GET /users: Retrieves a list of all users.
• POST /users: Creates a new user.
• GET /users/:id: Retrieves a specific user by ID.
• PUT /users/:id: Updates a specific user.
• DELETE /users/:id: Deletes a specific user.

10 . Explain the concept of stub in Node.js.

A stub is a placeholder function in Node.js that mimics the actions of an actual function or external service. In testing and development, stubs are frequently used to isolate the code being tested from dependencies that may be unstable or challenging to set up.

11. What is a Document in MongoDB

A document is the fundamental data storage unit in MongoDB. Though its structure is more flexible, it resembles a row in a relational database table. MongoDB differs from relational databases in that it has a flexible document structure that makes it ideal for storing complex, unstructured data.

12. How would you debug a MERN application?

In order to debug a MERN program, you must methodically find and fix any faults or problems in your code. This is a broad strategy that you can use.

• Browser console : Use the developer tools in your browser to examine network requests, failures, and logs.
• Node.js debugger: The Node.js debugger can be used to step through the execution of code and examine variables.
• Logging: To monitor application traffic and spot trouble spots, add logging statements.
• Test-driven development (TDD):  To find and isolate bugs early in the development process, write tests.

13. What are some common security vulnerabilities to consider when developing a MERN application?

Despite their strength, MERN apps can have security flaws if they are not adequately addressed. Typical vulnerabilities to take into account are:

Cross-Site Scripting (XSS): Malicious scripts that are injected into a web page can cause XSS attacks, which provide hackers access to user data, session hijacking, and uncontrolled code execution. Use output encoding, sanitize user input, and apply Content Security Policy (CSP) to stop XSS.

Injection Attacks: When untrusted data is introduced into queries, it can result in injection attacks like SQL injection and NoSQL injection, which can manipulate or grant unwanted access to data. Use prepared statements or parameterized queries, check user input, and refrain from concatenating user-provided data directly into queries to reduce the risk of injection attacks.

14. What are some common practices for building secure MERN applications?

Developing secure MERN apps necessitates addressing potential weaknesses in advance. The following are standard procedures to adhere to:

• Input validation and sensitization’s : stop injection attacks, make sure user inputs are validated and cleaned.
• Strong authentication: Put strong authentication procedures in place and use secure password hashing.
• Authorization: Depending on user responsibilities, utilize permission to restrict access to particular resources.
• Secure communication: Use HTTPS for secure data transmission.
• Regular security updates: Update all components and dependencies to patch vulnerabilities.

15. What is the Mongo Shell?

A well-known NoSQL database called MongoDB offers a command-line interface (CLI) called the Mongo Shell. It enables direct interaction with MongoDB databases, enabling you to carry out CRUD (create, read, update, and delete) actions on data.

A powerful environment for managing and exploring your MongoDB data is offered by the Mongo Shell. JavaScript commands can be used to generate indexes, query and alter data, and carry out administrative functions. Additionally, it provides tools to improve your development experience, like as error handling and autocompletion.

16. How do you test a MERN application?

An essential first step in guaranteeing a MERN application’s dependability and quality is testing. It entails confirming that the program runs effectively, securely, and as intended.

Unit Testing: This entails testing distinct parts or features separately. Early bug detection is beneficial during the development process. Mocha or Chai for Node.js and Express, and Jest for React are popular tools for unit testing in MERN.

Integration Testing: Verifying the interactions between several components is the main goal of integration testing. It guarantees that all of the features of the application operate as planned. Testing API endpoints and their interactions with the frontend can be done with tools such as Supertest.

End-to-End Testing: End-to-end testing makes sure the entire program functions as a coherent whole by simulating actual user interactions. Testing the complete application flow—from user input to the output at the end—is part of it. For end-to-end testing, programs like Cypress or Selenium can be utilized.

Performance Testing: Performance testing assesses the scalability, speed, and responsiveness of the application under different load scenarios. Performance metrics can be measured and various user loads can be simulated using tools such as JMeter or Artillery.

17. What is a Blocking Code?

Code that pauses a program’s execution until a predetermined condition is satisfied or a particular task is finished is known as blocking code. Performance problems may result from this, particularly in programs that have to manage several tasks at once.

The program thread stops processing new tasks when a blocking operation occurs, stopping it altogether. Delays, sluggish user interfaces, and ineffective resource usage may arise from this. Developers frequently use strategies like multithreading or asynchronous programming to prevent blocking behavior by allowing the software to carry out other tasks while it waits for blocking processes to complete.

18. What is Stream and what types of Streams are available in Node.js?

Streams are a potent abstraction in Node.js that allows for effective, non-blocking data manipulation. They serve as a representation of a data sequence that may be written to or read from a source. In contrast to conventional techniques for reading or writing data, streams handle data in chunks, making it possible to handle huge datasets and use memory more effectively.

19. Which Template Engines Does Express Support

A versatile framework for creating web apps, Express.js facilitates the rendering of dynamic HTML content using a variety of template engines. Several popular template engines that are compatible with Express are as follows:

• EJS (Embedded JavaScript):JavaScript code may be directly embedded into your HTML templates with the help of EJS, a lightweight and straightforward template engine. It offers a simple and easy-to-understand syntax for creating dynamic content.
• Pug (formerly Jade):Pug is a well-liked template engine that has a clear and understandable syntax. It makes templates simpler to create and update by using indentation to specify the HTML structure.
• Handlebars: Handlebars is a powerful template engine that uses Mustache-style syntax for templating. It offers features like partials, helpers, and conditional logic to create dynamic and reusable templates.
• Mustache: Mustache is a simple template language with an emphasis on templating without logic. It offers a straightforward syntax for incorporating logic and variables into your HTML layouts.

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

Props : Props are attributes that a parent component imparts to its offspring components. They are employed to transfer configuration and data between components. Props cannot be changed directly within the child component because they are immutable. Usually, they are employed to transfer configuration settings or user input from a parent component to its offspring.

State : Contrarily, state is a method used by a component itself to manage and save data. It permits components to save their own internal states, which are capable of dynamic rendering and updating. State is modifiable, which means the set State function within the component can be used to update it. User-generated data, application state, and other dynamic values that must be modified and re-rendered are frequently stored in states.