40+ Mern Stack Developer Interview Questions and Answers

Given the head node of a singly linked list, return a pointer to the middle node. If there are an odd number of elements, return the middle element. If there are even elements, return the one farther from the head node.

• Traverse the linked list using two pointers, one moving one node at a time and the other moving two nodes at a time.

• When the fast pointer reaches the end of the list, the slow pointer will be at the middle node.

• If the number of elements is even, return the seco...read more

The task is to find the Huffman codes for each alphabet in an encoded message based on their frequencies.

• The Huffman code is a binary string that uniquely represents each character in the message.

• The total number of bits used to represent the message should be minimized.

• If there are multiple valid Huffman codes, any of them can be printed.

• The input consists of multiple test cases, each with an array of frequencies.

• Implement the function to find the Huffman codes and return 1 ...read more

The question asks to find the total amount of rainwater that can be trapped in the given elevation map.

• Iterate through the array and find the maximum height on the left and right of each bar.

• Calculate the amount of water that can be trapped at each bar by subtracting its height from the minimum of the maximum heights on the left and right.

• Sum up the amount of water trapped at each bar to get the total amount of rainwater trapped.

The task is to find the minimum number of edges that need to be reversed in a directed graph to find the path from a start node to an end node.

• The problem can be solved using graph traversal algorithms like Breadth First Search (BFS) or Depth First Search (DFS).

• Start by creating an adjacency list representation of the directed graph.

• Perform a BFS or DFS from the start node to the end node, keeping track of the number of edges reversed.

• Return the minimum number of edges revers...read more

Rearrange an unsorted array by moving all zeroes to the end while maintaining the order of non-zero elements.

• Iterate through the array and keep track of the count of non-zero elements encountered.

• While iterating, if the current element is non-zero, swap it with the element at the count index.

• After iterating, fill the remaining elements with zeroes.

Find the first repeated character in a given string composed of lowercase English letters.

• Iterate through the string and keep track of characters seen so far in a set.

• If a character is already in the set, return it as the first repeated character.

• If no repeated character is found, return '%'.

Redux middleware is a function that intercepts actions and can modify or dispatch new actions.

• Middleware can be used for logging, error handling, and asynchronous actions.

• Syntax: const middleware = store => next => action => { //middleware logic }

• Redux selectors are functions that extract specific data from the Redux store.

• Example: const getUserName = state => state.user.name;

While loop is used when the number of iterations is not known beforehand, while for loop is used when the number of iterations is known.

• While loop checks a condition before each iteration, for loop has initialization, condition, and increment/decrement expressions.

• For loop is more concise and easier to read for iterating over a range of values.

• While loop is more flexible and can be used for situations where the number of iterations is not fixed.

• Example: while loop - while(i <...read more

JWT token is a JSON web token used for authentication and authorization. Bearer token is a type of access token that is used to authenticate API requests.

• JWT token stands for JSON Web Token and is used for securely transmitting information between parties as a JSON object.

• Bearer token is a type of access token that is used to authenticate API requests.

• JWT token is self-contained and contains user information and metadata, while bearer token is simply a string that identifies ...read more

React-router-dom is a routing library for React applications.

• It allows for declarative routing in React applications.

• It provides components like BrowserRouter, Route, Switch, and Link.

• BrowserRouter is used to wrap the entire application and provide routing functionality.

• Route is used to define a route and the component to render when the route is accessed.

• Switch is used to render only the first matching route.

• Link is used to create links between different routes in the applic...read more

Context API provides a way to pass data through the component tree without having to pass props down manually. Redux is used for managing global state in complex applications.

• Context API allows you to share data across components without having to pass props down manually

• Redux is preferred for managing global state in large applications with complex data flow

• Context API is simpler to use for smaller applications with less complex state management

• Redux provides a centralized s...read more

NoSQL databases are non-relational databases that do not require a fixed schema, while SQL databases are relational databases that use structured query language.

• NoSQL databases are schema-less, allowing for flexibility in data storage

• SQL databases use structured query language to interact with data

• NoSQL databases are horizontally scalable, making them suitable for big data applications

• SQL databases are vertically scalable, typically running on a single server

• Examples of NoSQL...read more

Create a product cart with CRUD operations and suggestion feature.

• Create a database schema for products

• Implement CRUD operations for adding, updating, and deleting products

• Implement a suggestion feature based on user preferences or past purchases

useState is used to manage state in functional components, while useRef is used to persist values between renders.

• useState is used for managing state in functional components and triggers re-renders when the state changes.

• useRef is used to persist values between renders without triggering re-renders.

• useState returns a tuple with the current state value and a function to update it, while useRef returns a mutable ref object.

• Example: useState can be used to manage a counter in a...read more

Implementing a multiselect file upload system for a MERN stack application.

• Use a library like Multer for handling file uploads in Node.js

• Create a form in React with multiple file input fields for selecting multiple files

• Send the selected files to the server using a POST request

• Handle the file uploads on the server side and save them to a designated folder

Exception handling is the process of handling errors and unexpected events in a program.

• Exceptions are thrown when an error occurs in the program.

• The try-catch block is used to handle exceptions.

• The catch block catches the exception and handles it appropriately.

• Finally block is used to execute code regardless of whether an exception was thrown or not.

• Exception handling helps in making the program more robust and prevents it from crashing.

The $shared operator in MongoDB is used to perform set intersection between arrays.

• Used to find common elements between arrays

• Returns an array containing elements that appear in all input arrays

• Syntax: { $shared: { arrays: [ , , ... ] } }

• Example: { $shared: { arrays: [ [1, 2, 3], [2, 3, 4] ] } } returns [2, 3]

Fibonacci series can be solved recursively by adding the previous two numbers in the series.

• Define a base case for when n is 0 or 1, return n in that case

• Otherwise, recursively call the function with n-1 and n-2 and add the results

• Example: Fibonacci(5) = Fibonacci(4) + Fibonacci(3) = Fibonacci(3) + Fibonacci(2) + Fibonacci(2) + Fibonacci(1) = 3 + 2 + 2 + 1 = 8

React's advanced feature is virtual DOM for efficient rendering, but it can be complex for beginners.

• Virtual DOM allows React to update only the necessary parts of the actual DOM, improving performance.

• Advanced state management with tools like Redux or Context API.

• Server-side rendering for better SEO and initial load performance.

• Component-based architecture for reusability and maintainability.

• Complex learning curve for beginners due to JSX syntax and component lifecycle metho...read more

Hooks are functions that let you use state and other React features without writing a class.

• useState() - for managing state in functional components

• useEffect() - for side effects in functional components

• useContext() - for accessing context in functional components

• useReducer() - alternative to useState() for more complex state logic

I have previously used the MERN stack which includes MongoDB, Express.js, React, and Node.js.

• MongoDB for database management

• Express.js for building the backend server

• React for building the frontend user interface

• Node.js for server-side scripting

To hash a password, use a secure hashing algorithm like bcrypt.

• Use a secure hashing algorithm like bcrypt to hash the password.

• Generate a salt to add to the password before hashing.

• Hash the password using the bcrypt library function.

• Store the hashed password and the salt securely in the database.

A full stack web app using React

• Set up a Node.js server with Express

• Create a database using MongoDB

• Build the frontend using React components

• Connect frontend to backend using API calls

• Implement user authentication and authorization

• Deploy the app on a hosting platform like Heroku

React is a JavaScript library for building user interfaces.

• React allows developers to create reusable UI components.

• It uses a virtual DOM for efficient rendering.

• React can be used for building single-page applications.

• React Native allows for building mobile applications using React.

• React is maintained by Facebook and a community of developers.

ES6 is a version of JavaScript with new features and syntax.

• ES6 introduced let and const for variable declaration

• Arrow functions were introduced in ES6

• Template literals were introduced in ES6

• ES6 introduced classes for object-oriented programming

• ES6 introduced the spread operator and destructuring

• JavaScript is the programming language, while ES6 is a version of it

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine, MongoDB is a NoSQL database that uses a document-oriented data model.

• Node.js is used for server-side scripting and building scalable network applications.

• MongoDB is a NoSQL database that stores data in flexible, JSON-like documents.

• Node.js and MongoDB are often used together in the MEAN/MERN stack for full-stack development.

• Node.js allows for asynchronous programming, making it efficient for handling mult...read more

Components re-rendering is the process of updating the UI when there is a change in the component's state or props.

• React re-renders the component and its child components when there is a change in state or props

• Re-rendering can be optimized using shouldComponentUpdate or React.memo

• Re-rendering can cause performance issues if not optimized properly

Node.js uses an event-driven, non-blocking I/O model to handle concurrency.

• Node.js uses an event loop to handle multiple requests concurrently.

• It employs a single-threaded event loop that can handle thousands of concurrent connections.

• Node.js uses non-blocking I/O operations, allowing it to efficiently handle multiple requests without blocking the execution of other code.

• It utilizes callbacks and event emitters to handle asynchronous operations.

• Node.js also provides the 'clus...read more

useMemo and useEffect are hooks in React used for optimizing performance and managing side effects respectively.

• useMemo is used to memoize the result of a function and cache it for future use.

• useEffect is used to perform side effects like fetching data, subscribing to events, or updating the DOM.

• useMemo is used when you want to optimize expensive calculations or avoid unnecessary re-rendering.

• useEffect is used when you want to perform actions after rendering or when certain d...read more

State and props are two important concepts in React for managing data and passing data between components.

• State is a built-in feature in React components that allows components to have their own internal data that can change over time.

• Props (short for properties) are used to pass data from a parent component to a child component.

• State is mutable and can be changed within the component, while props are read-only and cannot be changed within the component.

• State is typically use...read more

Axios interceptor is a middleware that allows you to intercept requests or responses before they are handled by the application.

• Axios interceptors can be used to modify requests or responses globally for all requests.

• You can add interceptors to handle requests, responses, errors, and more.

• Interceptors can be used for tasks like adding headers, logging, or handling authentication.

• Example: axios.interceptors.request.use() for handling requests, axios.interceptors.response.use()...read more

Create a stopwatch using React

• Create a state variable to hold the time

• Use setInterval to update the time every second

• Render the time in the component

• Add buttons to start, stop, and reset the stopwatch

A palindrome is a word, phrase, number, or other sequence of characters that reads the same forward and backward.

• Palindrome is a sequence of characters that reads the same backward as forward

• Examples: 'racecar', 'level', 'deified', 'madam', 'civic'

• Palindrome can also be a phrase or a number, such as 'A man, a plan, a canal, Panama!' or '121'

• Palindrome is commonly used in programming to check if a string is the same when reversed

bodyparser is a middleware used in Node.js to parse incoming request bodies

• Used to parse incoming request bodies in Node.js

• Helps in accessing the request body data in a more structured format

• Commonly used with Express.js to handle POST requests

• Example: app.use(bodyParser.urlencoded({ extended: true }))

• Example: app.use(bodyParser.json())

HTML is a markup language used to create the structure of a website.

• HTML stands for HyperText Markup Language

• It is used to create the structure of web pages by using tags

• Tags are enclosed in angle brackets like <tag>

• Example: <html>, <head>, <body>, <p>

To connect an API, you can use libraries like Axios or Fetch to make HTTP requests and retrieve data from the API.

• Use Axios or Fetch library to make HTTP requests to the API endpoint

• Provide necessary headers, parameters, and authentication tokens when making the request

• Handle the response data accordingly, such as parsing JSON data or handling errors

Closures are functions that have access to their own scope, as well as the scope in which they were defined.

• Closures allow functions to access variables from their parent function even after the parent function has finished executing.

• They are commonly used in event handlers, callbacks, and for data privacy.

• Example: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerVar); }; }

Provide coding based code for Mern Stack Developer interview

• Use Node.js for backend development

• Utilize Express.js for creating APIs

• Implement React.js for frontend development

• Use MongoDB for database management

SQL is a relational database management system, while NoSQL is a non-relational database management system.

• SQL databases are table-based and have a predefined schema, while NoSQL databases are document-based, key-value pairs, graph databases, or wide-column stores.

• SQL databases are good for complex queries and transactions, while NoSQL databases are better for hierarchical data storage.

• Examples of SQL databases include MySQL, Oracle, and PostgreSQL, while examples of NoSQL da...read more

Microservices are a software development technique where applications are composed of small, independent services that communicate with each other.

• Microservices break down applications into smaller, loosely coupled services that can be developed, deployed, and scaled independently.

• Each microservice is responsible for a specific business function and communicates with other services through APIs.

• Microservices promote flexibility, scalability, and resilience in software develop...read more

The salary of 4 lakh per annum is below market average for a Mern Stack Developer.

• Research the average salary for Mern Stack Developers in your area to negotiate for a higher salary.

• Consider the cost of living in the area when evaluating the salary offer.

• Factor in your experience, skills, and qualifications when determining your worth in the job market.

Pipeline in Express is a series of functions that are executed in a specific order during the request-response cycle.

• Middleware functions are added to the pipeline using app.use() or app.METHOD() functions.

• Each middleware function can modify the request and response objects, or end the request-response cycle.

• Middleware functions can be used for tasks like logging, authentication, error handling, etc.