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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:-


  • 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


  • 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.


  • 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.

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