MERN Stack Interview Questions



This is a comprehensive guide on MERN Stack interview questions. It has three sets of important MERN Stack interview questions and answers: Beginners, Intermediate and Advanced.

With the help of our sure-fire MERN Stack interview preparation guide, you can ace your upcoming interview and land your dream position as a MERN Stack developer.

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We have compiled the top MERN Stack developer technical interview questions for you to prepare for your next interview. We have divided the questions into three categories based on experience.

✔️ MERN Stack interview questions for Beginners
✔️ Intermediate MERN Stack interview questions
✔️ Advanced MERN Stack Interview Questions

We have selected the most asked MERN Stack questions in any interview. Each of these questions represents some core insights on MERN Stack Development.

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In this article, we present to you MERN developer interview questions and solutions that will get you started and assist you in passing your first job interview for a MERN developer.

Now, let’s learn top MERN Stack developer interview questions and answers. Let’s start.

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MERN Stack Interview Questions for Beginners

1. How does React work?

React builds a virtual DOM. A “diffing” mechanism is initially used when a component’s state changes to determine what has changed in the virtual DOM. Reconciliation is the next stage, in which the DOM is updated with the findings of the diff.

2. What are React Hooks?

In React 16.8, hooks are a brand-new feature. They provide state and other React capabilities without the need for class creation. You may remove stateful logic from a component using hooks so that it can be tested separately and used again. You may reuse stateful logic with hooks without altering the component structure. Sharing Hooks amongst several components or with the community is now simple as a result.

3. What Is Replication In MongoDB?

The practise of syncing data across several servers is known as replication.

Replication enhances data availability and adds redundancy. Replication offers a degree of fault tolerance against the loss of a single database server by storing numerous copies of the data on other database servers.

Replication can occasionally boost read capacity by allowing clients to transmit read operations to several servers. Data localization and availability for distributed applications can be improved by maintaining copies of the data in many data centres. You can also keep extra copies on hand for specific uses like backup, reporting, and disaster recovery.

4. What in React are Higher-Order Components (HOC)?

A function that accepts a component and produces another component is known as a higher-order component (HOC). In essence, it is a pattern that derives from the compositional structure of React. Since they may take any dynamically supplied child component, they are referred to as “pure” components since they don’t duplicate or change the behaviour of their input components.

HOC may be utilised in the following usage cases:
-Reuse of code, reasoning, and bootstrap abstraction
-Represent Highjacking
-State manipulation and abstraction
-Props manipulation

5. What distinguishes a class component from a functional component?

Class Components
-ES6 class syntax is used in class-based components. It may employ lifecycle methodologies.
-Components of a class extend React components.
-To access the properties and methods that you specify inside the class components in this section, you must use the keyword.
Functional Components
-Comparing functional components to class-based functions, they are easier.
-Functional Components primarily concentrate on the application’s user interface (UI), not on its behaviour.
-These are really render functions in the class component, to be more accurate.
-Utilizing Reach Hooks, Functional Components may simulate lifecycle events and have state.

6. By asynchronous API, what do you mean?

The Node.js library’s APIs are all synchronous and non-blocking. In essence, it implies that a server running Node.js never has to wait for an API to return data. After accessing an API, the server goes on to the next one, and Node.js’ Events notification system makes it possible for the server to get feedback from a previous API call.

7. What is Reconciliation in ReactJS?

React determines if a real DOM update is required when a component’s props or state change by comparing the newly returned element with the previously displayed one. React updates the DOM if they are not equal. Reconciliation is the name given to this procedure.

8. What is Sharding in MongoDB?

A technique for sharing data among several machines is sharding. To allow installations with very big data sets and high performance operations, MongoDB employs sharding. Sharding allows MongoDB to handle horizontal scalability. At the collection level, MongoDB shards data, dispersing it throughout the cluster’s shards.

Intermediate MERN Stack Interview Questions

With practice, you will be able to respond to basic MERN Stack interview questions with ease as a developer.

We have gathered some challenging MERN Stack interview questions for you in this part. You can get assistance from this section with these precise types of intermediate MERN Stack interview questions you might face while looking for work.

1. What is Callback Hell and what is the main cause of it?

JavaScript that is asynchronous or uses callbacks might be challenging to understand conceptually. Many pieces of code take on the following form:
MERN Stack Callback hell
Observe the pyramidal structure and all the }) at the conclusion? This is referred to as “callback hell.”

When individuals attempt to build JavaScript so that execution happens visibly from top to bottom, callback hell results. Many folks commit this error! Other programming languages like C, Ruby, or Python use the assumption that whatever occurs on line 1 will be completed before line 2’s code begins to run, and so on.

2. How can you achieve Transaction in MongoDB?

An operation on a single document in MongoDB is atomic.

This single-document atomicity eliminates the requirement for multi-document transactions for many practical use cases since you can utilise embedded documents and arrays to record connections between data in a single document structure rather than standardising over numerous documents and collections.

MongoDB enables multi-document (distributed) transactions for circumstances that call for the atomicity of reads and writes to many documents (in a single or several collections).

3. How does Node.js handle Child Threads?

In its purest form, Node.js is a single thread process. The developer is not given access to child threads or thread management techniques. For some operations, such asynchronous I/O, js does create child threads, but they happen in the background and don’t interfere with the main event loop or execute any JavaScript code for the application.

There are tools available to provide threading functionality in a Node.js application, such as the ChildProcess module.

4. What are Pure Components?

With the exception of handling the shouldComponentUpdate function for you, PureComponent is identical to Component.

PureComponent does a quick comparison of both props and state when either changes. Component, on the other hand, will not, out of the box, compare current props and state to the next. Therefore, anytime shouldComponentUpdate is invoked, the component will automatically re-render.

5. How does concurrency work in Node.js?

With node.js, everything runs simultaneously with the exception of your code.

This implies that anytime you execute an async function, such as performing i/o operations on files, accessing databases, requesting urls, etc., those threads are used. In fact, there are many threads operating inside the Node.js virtual machine (or a thread pool, if you prefer).

Your code, on the other hand, uses a single thread to process events that are queued up in an event queue. Therefore, whenever an async action is complete, a new event is added to the event-queue with that callback when a callback is registered. This reference is really provided to the background worker thread.

The I/O operation is carried out by a thread that Node generates or utilises when it receives an I/O request, and after it is complete, Node pushes the outcome to the event queue. If the execution stack of the Node is empty, the event loop examines the queue and adds the queue result to the execution stack for each such event.

Concurrency is managed by Node in this way.

Advanced MERN Stack Interview Questions

This section contains some advanced interview questions for MERN Stack jobs. Read and Understand how these complex technologies work and how they help businesses.

1. Explain advantages of BSON over JSON in MongoDB?

Although BSON is intended to be space-efficient, it sometimes isn’t all that much more so than JSON. BSON occasionally consumes even more storage than JSON. This is due to traversability, one of the BSON design objectives. BSON enhances documents with certain “additional” data, such as string and subobject lengths. This speeds up traversal. Additionally, BSON is intended to be quickly encoded and decoded. Integers, for instance, don’t need to be processed into and out of text because they are kept as 32 (or 64) bit integers. For tiny numbers, this takes up more space than JSON but parses considerably more quickly. In addition to being more compact, BSON also introduces new data kinds that JSON does not support, most notably the BinData and Date data types.

2. How does Node.js handle Child Threads?

In its purest form, Node.js is a single thread process. The developer is not given access to child threads or thread management techniques. For some operations, such asynchronous I/O, js does create child threads, but they happen in the background and don’t interfere with the main event loop or execute any JavaScript code for the application.

There are tools available to provide threading functionality in a Node.js application, such as the ChildProcess module.

3. What is prop drilling and how can you avoid it?

A deeply nested component frequently has to consume data given by another component that is much higher in the hierarchy while developing a React application. Simply passing a prop from one component in the hierarchy to the next, from the source component to the deeply nested component, is the easiest method. Proper drilling is what this is.

Prop drilling’s main drawback is that it makes components that shouldn’t normally be aware of the data more complicated and difficult to maintain.

Using React context is a frequent strategy to prevent prop drilling. Using either a Consumer component or a useContext hook, this enables the definition of a Provider component that feeds data and enables nested components to consume context data.

4. What do you know about Dependency Injection?

You may create a slackly connected software by using dependency injection to separate client creation dependencies from activity. By utilising the components, it also enables customization of the behaviour of apps.

It enables injecting the service essentially in ways independent of client use. Therefore, it may be possible to stop the client from altering the dependencies that underlie service updates.

5. What is JSX in ReactJS?

The complete capability of JavaScript is available through JSX, which is a syntactic extension to JavaScript. React components are created via JSX. Any JavaScript expression may be included in JSX by being enclosed in curly braces. JSX expressions are converted to standard JavaScript objects after compilation. As a result, you may assign JSX to variables, take it as arguments, return it from functions, and utilise it inside of if statements and for loops.

6. What is the difference between Shadow DOM and Virtual DOM

Virtual DOM aims to prevent pointless DOM modifications, which are costly in terms of performance because they frequently need re-rendering of the page. As a result, not every modification necessitates a re-render, but rather, re-rendering only occurs once once a set of changes have been made to the DOM.

This is made possible by the virtual DOM. The main goal of Shadow DOM is to encapsulate the implementation. A single custom element is capable of implementing more-or-less complicated DOM and logic. A page can import a whole web application of any complexity, but it can also create simpler reusable and composable components as custom elements with the underlying representation concealed in the shadow DOM.


Whether you’re a developer getting ready for an interview or a hiring manager trying to find the ideal candidate, we believe these MERN Stack interview questions and answers will be a tremendous help to you during the process.

Keep in mind that technical proficiency is only one aspect of the hiring process. Both prior experience and soft skills are essential if you want to be hired for a high-paid web development position.

Keep in mind that many of the MERN Stack Stack interview questions are open-ended. Not just the answer you memorised, but also your reasoning will be of interest to the interviewer. Always be prepared to address any follow-up inquiries about how you came to your conclusion. Describe the way you think.

Good luck with your upcoming MERN Stack interview! You can go through our MERN Stak developers jobs here.