100+ Mern Full Stack Developer Interview Questions and Answers

Creating CRUD APIs for sample data in a MERN stack application

• Create a POST API to add new data

• Create a GET API to retrieve all data

• Create a GET API to retrieve data by ID

• Create a PUT API to update data by ID

• Create a DELETE API to delete data by ID

Check if 2 strings are anagrams.

• Convert both strings to lowercase

• Remove all non-alphabetic characters

• Sort the characters in both strings

• Compare the sorted strings, if they are equal, they are anagrams

Foreign key in SQL is a field in a table that is a primary key in another table, establishing a relationship between the two tables.

• A foreign key constraint ensures referential integrity between two related tables.

• It helps maintain consistency and avoid orphan records.

• Example: In a database with tables 'orders' and 'customers', the 'customer_id' in 'orders' table can be a foreign key referencing the 'id' column in 'customers' table.

Lookup in MongoDB is used to perform a left outer join to retrieve documents from another collection.

• Lookup is used to combine data from two collections based on a common field.

• It is similar to a SQL join operation.

• Lookup is used in aggregation pipelines to fetch related documents from another collection.

• Example: db.collection.aggregate([{ $lookup: { from: 'anotherCollection', localField: 'field', foreignField: 'field', as: 'alias' } }])

React is a JavaScript library for building user interfaces, while Next.js is a framework built on top of React for server-side rendering and static site generation.

• React is a JavaScript library for building user interfaces, while Next.js is a framework built on top of React.

• Next.js provides features like server-side rendering, static site generation, and routing out of the box, while React does not.

• Next.js simplifies the process of building React applications by providing a m...read more

Slice is a method that returns a shallow copy of a portion of an array into a new array, while splice is a method that changes the contents of an array by removing or replacing existing elements.

• Slice does not modify the original array, while splice does.

• Slice takes two arguments - start and end index, while splice takes three arguments - start index, number of elements to remove, and optional elements to add.

• Example: const arr = [1, 2, 3, 4, 5]; arr.slice(1, 3) returns [2, 3...read more

Event loop is a mechanism in JavaScript that allows for asynchronous operations to be executed in a non-blocking way.

• Event loop is responsible for handling asynchronous operations in JavaScript.

• It allows for non-blocking execution of code by continuously checking the call stack and the callback queue.

• Event loop processes tasks in a queue and executes them one by one.

• Example: setTimeout() function in JavaScript uses event loop to schedule the execution of a function after a sp...read more

Oracle and Cerner are popular choices for healthcare organizations due to their robust features, scalability, and industry-specific functionalities.

• Oracle offers a comprehensive suite of healthcare solutions, including electronic health records (EHR) and population health management.

• Cerner is known for its interoperability capabilities, allowing seamless data exchange between different healthcare systems.

• Both Oracle and Cerner have a strong presence in the healthcare industry...read more

The for loop in JavaScript can be used with var, let, and const to iterate over arrays or objects.

• The 'var' keyword declares a variable globally or locally to an entire function regardless of block scope.

• The 'let' keyword declares a block-scoped variable that can be reassigned within the block it was declared.

• The 'const' keyword declares a block-scoped variable that cannot be reassigned or redeclared.

Functional programming is a programming paradigm that emphasizes on the use of functions to solve problems.

• Functions are treated as first-class citizens

• Pure functions are preferred over impure functions

• Avoidance of side effects

• Immutability is preferred over mutability

• Declarative programming style

• Examples: Haskell, Lisp, Clojure

HTML5 is the latest version of the HTML standard, introducing new features for web development.

• Introduction of new semantic elements like <header>, <footer>, <nav>, <article>, <section>, <aside>

• Support for audio and video elements without the need for plugins

• Canvas element for drawing graphics on the fly

• Local storage capabilities for storing data on the client side

• Improved form elements like date pickers, email validation, and placeholder text

JSX is a syntax extension for JavaScript used with React to describe what the UI should look like.

• JSX stands for JavaScript XML

• It allows developers to write HTML-like code in JavaScript

• JSX elements are transpiled into regular JavaScript function calls

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

• Hooks were introduced in React 16.8 to allow functional components to have state and lifecycle methods.

• useState() is a hook that allows you to add state to functional components.

• useEffect() is a hook that allows you to perform side effects in functional components.

• Custom hooks are reusable functions that contain logic and can be shared across multiple components.

State management is the process of managing and updating the state of an application in a predictable and efficient manner.

• State management involves storing and updating the state of an application in a centralized location.

• It helps in maintaining the consistency of data across different components of an application.

• State management libraries like Redux or Context API in React help in managing state effectively.

• State management is crucial for building complex applications wit...read more

React is a JavaScript library for building user interfaces. State is a JavaScript object that stores data in a component. useEffect is a React hook for handling side effects.

• React is a library for building UI components

• State is a JavaScript object that stores data within a component

• useEffect is a hook used for handling side effects in React components

Redux is a predictable state container for JavaScript apps.

• Redux is a state management tool commonly used with React.

• It helps manage the state of an application in a predictable way.

• Redux stores the entire state of an application in a single immutable object.

Aggregation is the process of combining data from multiple sources to provide a summary or overview.

• Aggregation in databases involves grouping and summarizing data to provide insights.

• In MongoDB, aggregation pipelines are used to process data and return computed results.

• Examples of aggregation functions include sum, average, count, and group by.

SDLC stands for Software Development Life Cycle, a process used by software developers to design, develop, and test software.

• SDLC is a structured process that consists of several phases such as planning, analysis, design, implementation, testing, and maintenance.

• Each phase in SDLC has its own set of activities and deliverables to ensure the successful development of software.

• SDLC helps in improving the quality of software, reducing development time and cost, and ensuring that...read more

addEventListener is used to attach an event handler to an element.

• addEventListener is a method used to listen for a specified event on an element.

• The 'useCapture' parameter determines the order in which event handlers are called.

• Example: element.addEventListener('click', myFunction);

A basic function component in React

• Create a function component using the 'function' keyword

• Return JSX elements within the component function

• Example: const MyComponent = () => { return

  Hello World!
  ; }

Virtual DOM is a lightweight copy of the actual DOM that allows for efficient updates and rendering in web applications.

• Virtual DOM is a concept used in frameworks like React to improve performance by minimizing direct manipulation of the actual DOM.

• When changes are made to the virtual DOM, a comparison is done with the actual DOM to identify the minimal number of updates needed.

• This approach reduces the number of DOM manipulations, leading to faster rendering and improved us...read more

Callback functions are functions passed as arguments to other functions to be executed later.

• Callback functions are commonly used in asynchronous programming.

• They allow for functions to be executed after another function has finished.

• Examples include event listeners in web development.

To solve CORS issue, configure server to allow cross-origin requests.

• Set appropriate headers on server side to allow requests from specific origins

• Use middleware like cors in Node.js to handle CORS

• Consider using proxy server to bypass CORS restrictions

The event loop is a mechanism in JavaScript that allows for asynchronous operations to be executed in a non-blocking way.

• Event loop is responsible for handling asynchronous operations in JavaScript.

• It continuously checks the call stack and the callback queue to see if there are any functions that need to be executed.

• If the call stack is empty, the event loop will take a function from the callback queue and push it onto the call stack for execution.

• Example: setTimeout() functi...read more

Browsers interpret HTML, CSS, and JavaScript to display web pages to users.

• Browsers receive HTML, CSS, and JavaScript files from a web server.

• The browser parses the HTML to create the Document Object Model (DOM).

• CSS is used to style the DOM elements, while JavaScript adds interactivity.

• The browser then renders the final web page for the user to view.

Props are read-only properties that are passed from a parent component to a child component in React.

• Props allow data to be passed down the component tree.

• Props are immutable and cannot be changed by the child component.

• Props are accessed in the child component using this.props.

EventLoop is a mechanism in JavaScript that allows for asynchronous operations to be executed in a non-blocking manner.

• EventLoop is responsible for handling asynchronous operations in JavaScript.

• It continuously checks the call stack and the task queue, moving tasks from the queue to the stack when the stack is empty.

• EventLoop ensures that JavaScript remains single-threaded and non-blocking, allowing for efficient handling of asynchronous tasks.

Create a UI clone of a given website for a Mern Full Stack Developer position.

• Study the layout, design, and functionality of the given website thoroughly.

• Use HTML, CSS, and JavaScript to replicate the UI elements and features.

• Ensure responsiveness and compatibility with different devices and browsers.

• Consider using frameworks like React for efficient development.

• Test the clone extensively to ensure it matches the original website.

var, let, and const are all used to declare variables in JavaScript, but they have different scopes and mutability.

• var has function scope and can be redeclared and reassigned

• let has block scope and can be reassigned but not redeclared

• const has block scope and cannot be reassigned or redeclared

Event emitter in Node.js is a module that allows objects to emit and listen for events.

• Event emitter is a built-in module in Node.js

• It allows objects to emit named events that cause Function objects to be called

• Example: const EventEmitter = require('events');

• Example: const myEmitter = new EventEmitter();

Variables in JavaScript are used to store data values.

• Variables are declared using the 'var', 'let', or 'const' keywords.

• Variables can store different types of data such as numbers, strings, arrays, objects, etc.

• Variables can be reassigned with new values.

• Variables are case-sensitive.

• Example: var x = 5; let name = 'John'; const PI = 3.14;

I used React.js as the framework in my last project.

• React.js is a popular JavaScript library for building user interfaces.

• It allows for the creation of reusable UI components.

• React.js uses a virtual DOM for efficient rendering.

• Example: I used React.js to build a dynamic dashboard for tracking sales data.

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine.

• Used for building scalable network applications

• Event-driven, non-blocking I/O model

• Allows for server-side scripting with JavaScript

• NPM (Node Package Manager) for easy installation of packages

• Examples: Netflix, LinkedIn, Walmart, PayPal

React JS is a JavaScript library for building user interfaces.

• Developed by Facebook

• Uses a virtual DOM for efficient rendering

• Component-based architecture

• Can be used for both web and mobile app development

• Supports server-side rendering

• Popular in the MERN stack

The previous project architecture was based on a microservices design pattern with a front-end built using React and a back-end using Node.js and Express.

• Microservices architecture was used to break down the application into smaller, independent services that communicate with each other through APIs.

• Front-end was developed using React, a popular JavaScript library for building user interfaces.

• Back-end was built using Node.js, a runtime environment for executing JavaScript cod...read more

Node.js follows a single-threaded event-driven architecture, utilizing non-blocking I/O operations.

• Node.js uses an event loop to handle asynchronous operations.

• It employs the V8 JavaScript engine to execute code.

• Node.js modules are organized using CommonJS module system.

• It allows for the creation of scalable and high-performance applications.

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.

• State and props are used to manage data and pass it between components.

A closure is a function that has access to its own scope, as well as the outer scope in which it was defined.

• A closure allows a function to access variables from an outer function even after the outer function has finished executing.

• Closures 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); }; }

Components are reusable and independent units of code that encapsulate functionality and can be combined to build larger applications.

• Components are the building blocks of a software application.

• They are reusable and can be used multiple times in different parts of the application.

• Components can have their own state and can communicate with other components through props.

• Examples of components include buttons, forms, navigation bars, and cards.

Flow of redux store involves actions being dispatched, reducers updating the state, and components subscribing to the updated state.

• Actions are dispatched by components or middleware

• Reducers receive the actions and update the state immutably

• Components subscribe to the updated state using mapStateToProps

Creating a prototype in JavaScript

• Use the Object.create() method to create a prototype object

• Add properties and methods to the prototype object

• Use the prototype object to create new instances of an object

Var, let, and const are used to declare variables in JavaScript.

• Var is function-scoped and can be redeclared and updated.

• Let is block-scoped and can be updated but not redeclared.

• Const is block-scoped and cannot be updated or redeclared.

Use the built-in reverse() method to reverse an array in JavaScript.

• Use the reverse() method on the array to reverse its elements in place.

• Example: let arr = ['apple', 'banana', 'cherry']; arr.reverse(); // ['cherry', 'banana', 'apple']

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

• SQL databases are table-based and have a predefined schema, making them suitable for complex queries and transactions.

• NoSQL databases are document-based, key-value pairs, graph databases, or wide-column stores, making them more flexible for unstructured data.

• Examples of SQL databases include MySQL, PostgreSQL, and Oracle, while exam...read more

Oops concept in Java refers to Object-Oriented Programming principles like Inheritance, Encapsulation, Polymorphism, and Abstraction.

• Inheritance allows a class to inherit properties and behavior from another class.

• Encapsulation involves bundling data and methods that operate on the data into a single unit.

• Polymorphism allows objects to be treated as instances of their parent class.

• Abstraction hides the implementation details and only shows the necessary features to the outsid...read more

Event Loop in JavaScript manages the execution of multiple tasks in a single-threaded environment.

• Event Loop is responsible for handling asynchronous operations in JavaScript.

• It continuously checks the call stack and the task queue, moving tasks from the queue to the stack when the stack is empty.

• Tasks like setTimeout, setInterval, and AJAX requests are handled by the Event Loop.

• Event Loop ensures that JavaScript remains non-blocking and responsive.

Indexing in Node.js is a way to optimize data retrieval by creating indexes on specific fields in a database.

• Indexing in Node.js can be done using libraries like Mongoose for MongoDB databases.

• Indexes can improve query performance by allowing the database to quickly locate the requested data.

• Example: Creating an index on a 'username' field in a user collection can speed up queries searching for a specific user.