MERN Stack Interview Questions and Answers

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MERN consolidates four JS-based technologies that work together to build dynamic and responsive web applications. It is a prevalent technology used by developers to streamline faster web development. Due to the tremendous increase in the use of MERN Stack technology, there has been an escalating demand for skilled MERN Stack developers who can build robust applications. So, learning MERN Stack development and preparing MERN Stack interview questions can greatly improve your career prospects.

We have prepared a list of top MERN Stack interview questions to provide you with a deeper understanding of all these technologies. Whether you are a fresher trying to break into the web development industry or an experienced web developer, our listed questions will help you hone your MERN Stack skills.

Our MERN Stack interview questions and answers will enable you to evaluate your knowledge and expertise in MongoDB, React.js, Express.js, and Node.js. We have covered a wide range of MERN Stack concepts from basic to advanced levels. Preparing these questions can pave your way through your next MERN Stack interview.

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gradient_start_position=”0″ gradient_end_position=”100″ gradient_type=”linear” radial_direction=”center center” linear_angle=”180″ background_color=”” background_image=”” background_position=”left top” background_repeat=”no-repeat” background_blend_mode=”none” render_logics=”” filter_type=”regular” filter_hue=”0″ filter_saturation=”100″ filter_brightness=”100″ filter_contrast=”100″ filter_invert=”0″ filter_sepia=”0″ filter_opacity=”100″ filter_blur=”0″ filter_hue_hover=”0″ filter_saturation_hover=”100″ filter_brightness_hover=”100″ filter_contrast_hover=”100″ filter_invert_hover=”0″ filter_sepia_hover=”0″ filter_opacity_hover=”100″ filter_blur_hover=”0″ animation_type=”” animation_direction=”left” animation_speed=”0.3″ animation_offset=”” last=”true” border_position=”all” first=”true”][fusion_accordion type=”accordions” boxed_mode=”no” border_size=”1″ border_color=”” background_color=”” hover_color=”” divider_line=”” title_font_size=”” icon_size=”” icon_color=”” icon_boxed_mode=”” icon_box_color=”#4f4f4f” icon_alignment=”right” toggle_hover_accent_color=”#3ed9df” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” class=”interview_list” id=””][fusion_toggle title=”Question 1: Explain how ReactJS works.” open=”no” class=”” id=””]

Answer:

React.js is the most talked-about & popular JavaScript framework. It has played a vital role in web application development. ReactJS is used in building interactive elements on websites & applications.

[/fusion_toggle][fusion_toggle title=”Question 2: What are the props in ReactJS?” open=”no” class=”” id=””]

Answer:

Props is the abbreviation of properties used to pass data between React components. The React’s data flows between components in a unidirectional pattern from parent to child. It is an object that stores the attributes’ values of a tag & works just like HTML attributes. Props provide a route to pass data from one component to another, similar to function arguments. The props are fixed, so we cannot modify the props inside the component.

[/fusion_toggle][fusion_toggle title=”Question 3: What are the benefits of ReactJS?” open=”no” class=”” id=””]

Answer:

The following are some benefits of ReactJS:

• React helps to increase an application’s performance using Virtual DOM;
• JSX makes code easy to write & read;
• It renders both server-side & client-side development;
• JS is easy to integrate with other frameworks like Angular & Backbone.js, as it is only a view library;
• It is easy to write User Interface Test cases & integrate them with tools such as JEST.

[/fusion_toggle][fusion_toggle title=”Question 4: What is a Virtual DOM?” open=”no” class=”” id=””]

Answer:

The Virtual DOM in ReactJS is like a lightweight copy of the real or actual DOM. For each object in the real DOM, there is a copy of the objects in the Virtual DOM. However, it doesn’t possess the power to change the document’s layout directly. Manipulating the actual or real DOM is a slow process, but manipulating a Virtual DOM is faster as nothing gets drawn on the screen.

[/fusion_toggle][fusion_toggle title=”Question 5: Define the key features of MongoDB.” open=”no” class=”” id=””]

Answer:

Below are some of the essential features of MongoDB:

• Schema-less Database: It is a prominent feature of MongoDB. A Schema-less Database is a collection that can hold various documents in it. The documents may contain a different number of fields, size & content. MongoDB provides much flexibility to databases by using this feature.
• Document-oriented: In MongoDB, all data is stored in the documents instead of tables like RDBMS. Each document has its unique object ID, and the data is stored in fields rather than columns or rows, making it much more flexible than RDBMS.
• Indexing: In the MongoDB database, each document field is indexed with primary & secondary indices that make it easier & take less time to search data from the big data pool. If the data is not indexed, the database searches each document with a specified query that takes much time and is inefficient.
• Scalability: MongoDB renders horizontal scalability with Sharding. Sharding refers to the distribution of data on multiple servers. A large amount of data is partitioned into chunks with a shard key. Those data chunks are distributed evenly across shards that reside in many physical servers. It also adds new machines to a running database.
• Replication: MongoDB provides redundancy & high availability through replication. It creates multiple copies of data & sends those copies to a different server so that if a server fails, the data is retrieved from another server.
• Aggregation: It enables developers to perform operations on grouped data & get a computed or single result. Aggregation is just like the SQL GROUP BY clause. It provides three different aggregations: map-reduce function, aggregation pipeline, & single-purpose aggregation.
• High Performance: MongoDB’s performance is very high & in data persistence because of its features like indexing, scalability, replication, etc.

[/fusion_toggle][fusion_toggle title=”Question 6: How is ReactJS different from AngularJS?” open=”no” class=”” id=””]

Answer:

The table below highlights the differences between ReactJS & AngularJS:
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[/fusion_toggle][fusion_toggle title=”Question 7: What do you mean by replication in MongoDB?” open=”no” class=”” id=””]

Answer:

Replication refers to synchronizing data around multiple servers. It provides redundancy & increases data availability. Having multiple copies of data on various database servers, replication in MongoDB protects a database from the single server’s loss. Replication also enables you to recover data from service interruptions & hardware failure.

[/fusion_toggle][fusion_toggle title=”Question 8: What is the main difference between a functional component & a class component?” open=”no” class=”” id=””]

Answer:

The following are the differences between a functional component & a class component:

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[/fusion_toggle][fusion_toggle title=”Question 9: What are the key features of Node.js?” open=”no” class=”” id=””]

Answer:

Here are some of the key features of Node.js:

• Asynchronous event helps in handling Concurrent Requests- In Node.js, APIs are asynchronous. It means that if a Node receives a request for some Input or Output operation, it will execute that operation in the background while processing other requests simultaneously. Hence, it will not wait for any response from the previous requests.
• Executes Code Faster- Node.js deploys the V8 JS Runtime engine, which Google Chrome also uses. It has a wrapper over the JavaScript engine that makes the runtime engine faster, thus enabling faster processing of requests within Node.js.
• js library uses JavaScript- It is another fundamental aspect of Node.js. Developers who are well-versed in JavaScript can easily develop in Node.js.
• Highly Scalable- Node.js utilizes a single thread for event looping. The response from those events may or may not reach the server immediately. However, it does not block other operations, which makes Node.js highly scalable. The traditional servers generate limited threads to manage requests, while Node.js only creates a single thread that provides service to a much larger no. of such requests.
• Vibrant & Robust Community for Node.js framework- Node.js has an active community. It keeps the framework updated with the latest trends in web & application development.
• No Buffering- Node.js applications never buffer any data. It outputs the data in chunks.

[/fusion_toggle][fusion_toggle title=”Question 10: Mention some limitations of ReactJS.” open=”no” class=”” id=””]

Answer:

• The following are some limitations of ReactJS:
• ReactJS accelerates so fast that it cannot properly document the project. Thus, the developer tries to write the instructions himself.
• ReactJS only focuses on the view part of the MVC, i.e., the User Interface of a web application. Thus, we need to use other tooling set technologies to focus on the other parts of development.
• As some of the syntax or command in ReactJS has to be a part of JavaScript as CONDITIONAL BLOCKS, it is not allowed in JSX of the documents.
• In ReactJS, some standard functions cannot use the state effectively; instead, they use ES6.
• Extension of the files in ReactJS can only be emitted to the JavaScript files to ensure that each CSS file has been imported in ReactJS.
• Some new developers believe JSX to be a barrier as they complain about the complexity of the learning curve.

[/fusion_toggle][fusion_toggle title=”Question 11: What is the meaning of Asynchronous API?” open=”no” class=”” id=””]

Answer:

All APIs of Node.js are asynchronous or non-blocking. It means that Node.js-based servers do not wait for an API to return data. Instead, the server moves to the following API after calling it. A notification mechanism of Events helps the server to receive a response from the previous API call.

[/fusion_toggle][fusion_toggle title=”Question 12: What do you mean by Callback Hell?” open=”no” class=”” id=””]

Answer:

The asynchronous functions require callbacks as a return parameter. The callback hell situation arises when two or more asynchronous functions are chained together.

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[/fusion_toggle][fusion_toggle title=”Question 13: What is the meaning of Reconciliation?” open=”no” class=”” id=””]

Answer:

Reconciliation is the process through which ReactJS updates the DOM. When the state of a component changes, ReactJS calculates if it is essential to update the DOM. It executes this by creating a virtual DOM and comparing it with the current DOM. In this context, the virtual DOM contains the new state of the component.

[/fusion_toggle][fusion_toggle title=”Question 14: Why is the use of Redux?” open=”no” class=”” id=””]

Answer:

Redux is a predictable state container that can help you write JavaScript applications that act consistently across server, client, & native environments. While Redux is used as a state management tool with React, one can also use it with any other JavaScript library or framework.

[/fusion_toggle][fusion_toggle title=”Question 15: What is the difference between an Element & a Component in ReactJS?” open=”no” class=”” id=””]

Answer:

An element implies what you want to see on the screen. It is an object representation of some User Interface. On the other hand, a component is an independent and reusable bit of code. Its objective is similar to the JavaScript function; however, it works in isolation and returns HTML through a render() function.

[/fusion_toggle][fusion_toggle title=”Question 16: When to embed a document within another in MongoDB?” open=”no” class=”” id=””]

Answer:

One should embed a document within another document when:

• It contains relationships between entities
• One-to-many relationships
• Or due to performance reasons

[/fusion_toggle][fusion_toggle title=”Question 17: Does MongoDB support Foreign Key Constraints?” open=”no” class=”” id=””]

Answer:

MongoDB does not support foreign key constraints. It encourages you to put relevant data in a single collection instead of putting it into multiple collections & then mapping those multiple collections by foreign key relationships. Having relative data in a single table makes fetching data from a database faster- a single query instead of multiple queries or joining.

[/fusion_toggle][fusion_toggle title=”Question 18: What are the benefits of BSON in MongoDB?” open=”no” class=”” id=””]

Answer:

BSON renders the ability to derive a document-based data model. It helps to store the data in a very rich manner while being capable of holding other documents & arrays. Due to the structuring of the BSON format, no complex joins are required.

[/fusion_toggle][fusion_toggle title=”Question 19: How does Node.js prevent the blocking code?” open=”no” class=”” id=””]

Answer:

Node.js prevents blocking code by enabling other codes to be executed in the meantime. Rather than code reading top to bottom procedurally, async programs can execute different functions at different times based on the speed or order of earlier functions.

[/fusion_toggle][fusion_toggle title=”Question 20: How to achieve transaction & locking in MongoDB?” open=”no” class=”” id=””]

Answer:

One can use the nesting of documents or embedded documents to achieve transactions and locking in MongoDB. It supports atomic operations within a single document.

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