20+ Mern Stack Developer Interview Questions and Answers for Freshers

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 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

The task is to find all distinct triplets in an array that add up to a given sum.

• Iterate through the array and fix the first element of the triplet.

• Use two pointers approach to find the other two elements that sum up to the remaining target.

• Handle duplicates by skipping duplicate elements while iterating.

• Return the list of valid triplets.

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 determine the number of ways to decode a given number string using a specific encoding.

• Iterate through the string and use dynamic programming to calculate the number of ways to decode at each position.

• Consider different cases such as single digit decoding, double digit decoding, and invalid decoding.

• Use modulo 10^9+7 to handle potentially large numbers of decode ways.

• Example: For '226', there are 3 possible decodings: 'BZ', 'BBF', 'VF'.

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 manages application state, while JWT securely transmits information between parties.

• Redux is a state management library for JavaScript applications, particularly with React.

• It uses a single store to hold the entire state of the application, making state predictable.

• Actions are dispatched to modify the state, which are processed by reducers.

• Example: An action could be 'ADD_TODO', and the reducer updates the state with a new todo item.

• JSON Web Token (JWT) is a compact, UR...read more

Java does not support multiple inheritance, but it can be achieved through interfaces.

• Java does not allow multiple inheritance of classes due to the diamond problem.

• However, multiple inheritance can be achieved through interfaces.

• A class can implement multiple interfaces, which can have their own default implementations.

• Example: class A implements Interface1, Interface2 { }

Java keywords like 'this' and 'super' are essential for object-oriented programming, enabling access to class members and inheritance.

• 'this' refers to the current object instance, allowing access to instance variables and methods.

• Example: 'this.variableName' differentiates between instance variables and parameters.

• 'super' is used to access superclass methods and constructors, facilitating inheritance.

• Example: 'super()' calls the parent class constructor, while 'super.methodNa...read more

MongoDB queries allow for flexible data retrieval and manipulation using a rich query language tailored for document databases.

• Find Documents: Use `db.collection.find({})` to retrieve documents. Example: `db.users.find({ age: { $gt: 18 } })` fetches users older than 18.

• Insert Documents: Use `db.collection.insertOne({})` or `db.collection.insertMany([{}])` to add new documents. Example: `db.products.insertOne({ name: 'Laptop', price: 999 })`.

• Update Documents: Use `db.collectio...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

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

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

I am motivated by challenges, continuous learning, and the impact of my work on users and the community.

• I thrive on solving complex problems, like optimizing a slow API response time to enhance user experience.

• Continuous learning drives me; I regularly explore new technologies, such as GraphQL, to improve my skill set.

• I find motivation in user feedback; for instance, receiving positive comments on a feature I developed inspires me to innovate further.

• Collaboration with my tea...read more

I enjoy coding, playing video games, and exploring new technologies, which helps me stay creative and sharp in my development skills.

• Coding personal projects, like building a weather app using React and Node.js.

• Playing video games, especially strategy games like StarCraft, which enhance my problem-solving skills.

• Exploring new technologies, such as learning about GraphQL and its integration with MongoDB.