Top MERN Stack Interview Questions

 

MERN stack stands for MongoDB, Express, React and Node.js. MERN stack is one of the most popular framework for developing web and mobile applications. It provides database back-end while react is the client-side front-end web framework, express and node are middleware in MERN stack. The MERN stack enables fast development of applications that are easy to debug specially using JSON documents. In case you are considering MERN stack bootcamp to train for a progressing career then here are top MERN stack interview questions to consider and test your skill and knowledge on the MERN stack for cracking the technical interview with ease!

 

Q.1How does React work?

It creates a virtual DOM. When the condition changes in a component it initially runs a “diffing” algorithm, which identifies what has changed in the virtual DOM. The second step is reconciliation, where it updates the DOM with results of diff.

 

Q.2What is props in React?

Props are inputs to a React component. They are single values or objects containing a set of values which are passed to React components on creation using a naming convention similar to HTML-tag attributes. This implies they are data passed down from a parent component to a child component. The primary purpose of props in React is to providing component functionality- Pass custom data to React component.

 

Q.3What Is Replication In MongoDB?

Replication is the system of synchronized data across numerous services and offers redundancy and increased data availability. With several copies of data on different database servers, replication protects a database from loss of single server. It allows one to recover from hardware failure and service interruptions. MERN stack coding bootcamp can be helpful to learn several techniques and tools to ace in a fast-paced career.

 

Q.4What are Higher-Order components?

Higher-order component is a function that takes a component and returns a new component. Generally, it is a pattern that is derived from React compositional nature and one could call them as ‘pure’ components because one can accept any dynamically offer child component but they would not modify or copy any behaviour from input components.

 

Q.5 What do you mean by Asynchronous API?

All APIs of Node.js library are asynchronous that is non-blocking. It essentially means a Node.js depends on server never waits for a PI to return data. Server moves to next API after calling it and a notification mechanism of Events of Node.js assists in server to get response from previous API call. Coding Bootcamp is one of the way forward to gain insight into the fastest paced career.

 

Q.6 What is Callback Hell ?

The asynchronous function requires callbacks as a return parameter. When several asynchronous functions are chained together then callback hell situation turns up.

 

Q.7 What is Reconciliation?

When a component’s state change, React decides whether an actual DOM update is important, by comparing the newly returned element with earlier rendered one. When they are not equal, React would update the DOM. This process is known as reconciliation.

 

Q.8 Does MongoDB Support Foreign Key Constraints?

No. MongoDB does not support relationships. Database does not apply any constraints to the system, so there are no “cascading deletes or updates”. Generally, in a NoSQL database it is up to you to decide how to organise the data and its relations if there are any.

 

Q.9 How Node prevents blocking code?

By offering callback function. Callback function gets called when equivalent events are generated.

 

Q.10 How can you achieve transaction and locking in MongoDB?

For achieving concepts of transaction and locking in MongoDB, one can use the nesting of documents, also known as embedded or sub documents. MongoDB offers atomic operations within a single document.

 

Q.11 How does Node.js handle child threads?

Node.js in its real nature is a single thread process and does not represent child threads and thread management methods to the developer. Technically, Node.js does spawn child thread for certain tasks such as asynchronous I/O, but run behind the scenes and do not run any application JavaScript code, nor block main event loop. If threading support is needed in Node.js applications, there are tools available to enable it, such as ChildProcess module. 

 

Q.12 How to avoid Callback Hell in Node.js?

Node.js internally make use of a single-threaded event loop for processing queued events. This approach could lead to blocking entire process if in case task running for a longer stretch than expected. Node.js addresses this issue by incorporating callbacks known as higher order functions. So whenever a long running process ends its execution, it aims the callback associated. Often it could lead to complex and unreadable code, higher the numbers of callbacks, longer chain of returning callbacks would be. There are four remedies that can address the callback hell issue- Use async or await mechanism, use promises mechanism, use generators or make use of program modular. If you wish to know more about the same opt for MERN stack bootcamp.

 

Q.13 If Node.js is single threaded then how it handles concurrency?

Node provides a single thread to programmers so that code can be written easily and without bottleneck. Node internally uses multiple POSIX threads for various I/O operations such as File, DNS, Network calls etc. When Node gets I/O request it creates or uses a thread to perform that I/O operation and once the operation is done, it pushes the result to the event queue. On each such event, event loop runs and checks the queue and if the execution stack of Node is empty then it adds the queue result to execution stack.

 

Q.14 What are Pure Components?

Pure component is similar to Component but it can handle the should component update method for you. When props or state changes, PureComponent can do a shallow comparison on both props and state. Component, on one hand, won’t compare current props and state to next out of the box. Therefore, the component can further provide by default whenever shouldComponentUpdate is called.

 

Q.15 What are React Hooks?

Hooks are among the latest addition in React 16.8. They let us make use of state and other React features without writing a class. With hooks, one can extract stateful logic from a component so it can be tested independently and reused. Hooks allow one to reuse stateful logic without changing one component hierarchy. This makes it easy to share hooks among numerous components o with the community. So, in a nutshell, coding bootcamp are essentially the place to be at to be equipped with the latest technologies.

Also, Read This Blog: A complete guide to learning MERN Stack