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title: Frequently asked MERN Interview questions

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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:-

### 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]('./ultimate-guide-to-MERN.md'), fullstack developer interview.