Important Notice— Our url is 
MERN Stack Interview Questions and Answers- Part 4
Even experienced developers need to brush up on interview topics before a job switch. If you’ve been working with MERN Stack and are now preparing for interviews, this guide is for you. We’ve gathered some of the most relevant and frequently asked MERN Stack interview questions to help you get fully prepared.
From database queries in MongoDB to React hooks and Node.js event loops, this list covers all the important areas interviewers typically focus on. Interviews aren’t just about coding skills, they’re also about explaining your thought process clearly and showing your ability to solve real problems. That’s why each question includes a simple, effective answer to help you respond with confidence.
Use this guide as a quick refresher before your interviews or as part of your regular study routine. Being well-prepared can make all the difference in landing your next opportunity in full-stack development.
Answer:
In React, it is necessary to bind event handlers to “this” to ensure that the method has access to the correct instance of the component’s state and props. Without proper binding, “this” may refer to a different context, causing unexpected behavior or errors in the component.
Answer:
A Covered Query in MongoDB is significant because it can greatly improve query performance. It ensures that the query results only include the fields that are actually needed, reducing the amount of data transferred over the network and leading to faster response times.
Answer:
Without the specific code sample in question, it is impossible to provide details or context about its execution and result.
Answer:
The V8 engine compiles JavaScript code to machine code through a process known as Just-In-Time (JIT) compilation. Here are the key steps involved:
- Parsing: The JavaScript code is initially parsed into an abstract syntax tree (AST).
- Optimizations: V8 applies various optimizations to the code, including inline caching, dynamic recompilation, and more to improve performance.
- Compilation: Based on the optimized code, V8 generates machine code that can be executed directly by the computer’s processor.
- Execution: The compiled machine code is executed, providing the actual functionality of the JavaScript code.
Answer:
libuv is a library used by Node.js to provide cross-platform asynchronous I/O capabilities. Underneath, it employs platform-specific mechanisms (such as epoll on Linux, kqueue on macOS, and IOCP on Windows) to handle I/O operations efficiently. libuv abstracts these mechanisms and provides a consistent asynchronous API for Node.js, allowing it to manage events, timers, and non-blocking I/O operations.
Answer:
You can find a document with an array containing a specific value in MongoDB by using the `$elemMatch` operator within a query. Here’s an example:
“`javascript
db.collection.find({ fieldName: { $elemMatch: { $eq: “desiredValue” } } })
“`
Answer:
In the context of the CAP (Consistency, Availability, Partition Tolerance) theorem, MongoDB is classified as an AP (Availability-Partition tolerance) database. MongoDB prioritizes high availability and partition tolerance over strict consistency, which means it may sacrifice consistency in certain scenarios to maintain availability and tolerance to network partitions.
Answer:
Separating an Express app and server in Node.js offers several advantages:
- Modularity: It makes the codebase more modular, allowing you to manage the application logic and server configuration separately.
- Scalability: It enables the creation of multiple server instances to handle increased traffic or load balancing.
- Testing: It simplifies unit testing of the application logic and server components independently.
- Maintenance: It provides better organization and maintainability of the codebase as changes to the server can be made without affecting the core app logic.
Answer:
An Asynchronous API, in the context of Node.js and asynchronous programming, refers to an API or function that does not block the execution of code while waiting for a result. Instead, it allows the program to continue executing other tasks and registers a callback or uses promises to handle the result once it becomes available. This non-blocking nature of asynchronous APIs is fundamental to Node.js’s efficiency and scalability.
Question 70: Do Hooks in React replace the need for render props and higher-order components (HOCs)?
Answer:
Yes, Hooks in React can replace the need for render props and higher-order components (HOCs) in many cases. Hooks allow functional components to manage state and side effects, making them more flexible and capable of encapsulating reusable logic. This reduces the reliance on HOCs and render props for sharing behavior between components.
Answer:
The Reactor Pattern in Node.js is a design pattern used to handle asynchronous I/O operations efficiently. It involves using a central event loop (the “reactor”) that monitors and dispatches events, such as I/O events, to registered event handlers. This pattern allows Node.js to perform non-blocking I/O operations and efficiently manage concurrent tasks.
Answer:
Async/await can be used in Node.js to work with asynchronous code in a more synchronous and readable manner. To use async/await:
- Declare an asynchronous function using the `async` keyword.
- Inside the async function, use the `await` keyword before promises to pause execution until the promise resolves.
- Handle errors using try-catch blocks to gracefully handle exceptions.
Answer:
Node.js streams are objects that provide a way to read or write data in a continuous and efficient manner. There are four types of Node.js streams:
- Readable: For reading data.
- Writable: For writing data.
- Duplex: For both reading and writing.
- Transform: A special type of duplex stream that allows data transformation during processing.
Answer:
Node.js buffers are used to work with raw binary data. Buffers are especially useful when dealing with binary data in network protocols or file operations. Buffers can be thought of as a raw memory allocation where data is stored in a fixed-size, contiguous memory block. They are commonly used for reading and writing binary data.
Answer:
Middleware in software development serves as a bridge or intermediary between different components or services in an application. It provides common services, functionalities, and capabilities to applications, allowing them to communicate and interact efficiently. Middleware helps developers and operators build, deploy, and manage applications more effectively by abstracting complex operations and promoting modularity and reusability.
Answer:
| Method | Description |
|---|---|
| Buffer.alloc(size) | creates a buffer and allocates size to it. |
| Buffer.from(initialization) | initializes the buffer with given data. |
| Buffer.write(data) | writes the data on the buffer. |
| toString() | read data from the buffer and returned it. |
| Buffer.isBuffer(object) | checks whether the object is a buffer or not. |
| Buffer.length | returns the length of the buffer. |
| Buffer.copy(buffer,subsection size) | copies data from one buffer to another. |
| Buffer.slice(start, end=buffer.length) | returns the subsection of data stored in a buffer. |
| Buffer.concat([buffer,buffer]) | It concatenates two buffers. |
Answer:
There are four types of API functions in Node.js:
- Asynchronous, Non-blocking APIs: These functions, like `fs.readFile`, allow you to perform I/O operations asynchronously without blocking the event loop.
- Synchronous, Blocking APIs: Functions like `fs.readFileSync` perform I/O operations synchronously, potentially blocking the event loop until the operation completes.
- Simple Timers APIs: Node.js provides timer functions like `setTimeout` and `setInterval` for scheduling code execution at specified intervals.
- I/O for Networking: Functions like `http.get` are used for making network requests, such as HTTP GET requests.
Answer:
REPL stands for Read-Eval-Print Loop. It is an interactive programming environment provided by Node.js that allows you to enter commands and expressions, which are read, evaluated, and the results are printed to the console in a loop. This provides an interactive way to experiment with JavaScript code and test small code snippets.
Answer:
The `async.queue` function in Node.js takes two arguments:
- A worker function that defines the task to be executed for each item in the queue.
- The concurrency value, which determines how many tasks can be processed concurrently in the queue.
Answer:
The `module.exports` object is used in Node.js to define what a module exports to be used in other parts of the application. It allows you to expose functions, objects, or values from a module, making them accessible to other modules that require or import them. It is a way to encapsulate and share functionality between different parts of your Node.js application.