Frequently asked MERN Interview questions

As a MERN stack developer you must have a clear understanding of underlying language - JavaScript that is powering the technologies like Node, Express and React.

Most interviewers will ask questions related to core JavaScript concepts first and then a few questions from frameworks or libraries like Express or React. You must have a firm grasp on Javascript Fundamental concepts like -

Core JavaScript

• Why Javascript ?
• What are different data types in javascript ?
• Explain hoisting in javascript.
• Explain implicit and explicit coercion.
• What is NaN in javascript ?
• What are higher order functions?
• What are IIFEs?
• Difference between var, let and const.
• Explain this keyword
• Explain scope and closure with examples
• What are promises in javascript?
• Explain rest parameter and spread operator
• What is an AJAX request?

MERN stack questions will check your understanding about different technologies used to create a full stack application.

They will ask you some web fundamental questions too, for example -

Web Fundamentals

• What is client side, server side architecture?
• What is a request-response cycle?
• What is DOM?
• What is the role of HTML vs CSS?
• What is a stateless protocol?
• What is REST? Define REST principles.
• What is CSRF?

Here are few interview questions related to underlying technologies used in MERN stack listed technology wise:-

Node/Express

• What is Node.js?
• Explain few features of Node.js?
• What is event loop & how it manages tasks in Node.js?
• What is blocking code? Give examples of blocking and non-blocking code.
• List few libraries used underneath by node platform
• Javascript is single threaded then how node handles concurrency?
• What is callback hell & how to avoid this?
• What are streams in node and how is it different from normal read operation?
• What are EventEmitters in Node? Give example of an EventEmitter.
• What is REPL in Node?
• What are globals in Node? Give 2 examples
• What is CORS issue and how to handle it?
• How would you define API?
• What are middlewares in expresss? Explain different types of middlewares
• What is next function in express middlewares ?
• What are templating engines ? How is it different from rendering HTML ?
• What are JWT token ? Explain different parts of it
• Explain JWT authentication
• Difference between Session and Token based authentication

MongoDB

• Explain important features of MongoDB ?
• How to insert multiple documents in mongoDB at once ?
• What are indexes in mongoDB and how are they helpful ?
• What is aggregation framework in mongoDB ?
• What are associations in mongoDB ?
• Explain different types of association with example.
• What is the difference between associating and referencing ?
• Explain text based index in mongoDB ?
• What is replication in mongoDB ?
• How to update array field in mongoDB ?
• How to find multiple documents using an array of unique Ids ?
• How to update integer values in mongoDB ?
• What are the parameters passed to update method ?
• Differentiate between relational and non-relational database.

React

• Why React ?
• What is props in React?
• What are pure components?
• What are higher order components in React?
• What is virtualDOM ? How is it helpful in React?
• Explain lifecycle method and their sequence of execution?
• What is prop drilling and how to avoid it?
• What is key and benefit of using it in the lists?
• Difference between smart ad dumb components
• Dfference between functional and class component
• What is the purpose of super(props)?
• Why we bind event handlers to this?
• Explain Context API in React?
• Explain advanced hooks in React?

The above questions should get you a pretty good idea on what you might be asked in a MERN stack, fullstack developer interview.