Node.js Interview Questions and Answers

This section covers frequently asked Node.js interview questions, focusing on its architecture, features, and common challenges in Node.js development.

What is Node.js?

Node.js is a JavaScript runtime environment that executes JavaScript code outside a web browser. It's built on Chrome's V8 JavaScript engine and is used for building scalable server-side applications.

Licensing and Cost of Node.js

Node.js is open-source and free to use, licensed under the MIT license.

Single-Threaded Nature of Node.js

Node.js is single-threaded but highly scalable due to its event-driven, non-blocking I/O model. It uses an event loop to handle asynchronous operations efficiently.

Purposes of Node.js

• Real-time web applications (chat, collaboration tools)
• Network applications
• Distributed systems
• General-purpose applications

Advantages of Node.js

• Asynchronous and Event-Driven: Non-blocking I/O model improves efficiency and scalability.
• Fast: Uses the high-performance V8 JavaScript engine.
• Scalable: Handles many concurrent connections effectively.
• No Buffering: Data is streamed directly; no buffering delays.

Node.js Web Application Architecture

A typical Node.js web application has these layers:

1. Client Layer: Web browsers or mobile apps sending requests.
2. Server Layer: The Node.js web server receiving and processing requests.
3. Business Layer: Application logic (often in services).
4. Data Layer: Databases or other data sources.

I/O (Input/Output)

I/O refers to operations involving external resources (like the file system, network, or databases). In Node.js, I/O operations are typically asynchronous.

Types of Node.js API Functions

• Asynchronous (Non-blocking): The function returns immediately without waiting for the operation to complete. A callback function handles the result.
• Synchronous (Blocking): The function waits for the operation to complete before returning.

First-Class Functions in JavaScript

In JavaScript, functions are first-class citizens; they can be passed as arguments to other functions (callbacks), returned from functions (higher-order functions), and assigned to variables.

JavaScript vs. Node.js

How Node.js Works

1. Clients send requests to the server.
2. Requests are added to an event queue.
3. The event loop processes non-blocking requests (immediately).
4. Blocking requests are handled by a thread pool.
5. Responses are sent back to clients.

Package Management in Node.js

Tools like npm (Node Package Manager) and yarn manage project dependencies (installing, updating, and removing packages). package.json and package-lock.json manage package versions.

Why Node.js is Single-Threaded

Node.js uses a single thread and an event loop to handle multiple requests concurrently. This approach often outperforms multi-threaded models for typical web application workloads by avoiding thread management overhead and context switching.

Callback Hell

Callback hell is a coding anti-pattern where nested callbacks make code difficult to read and maintain. Promises and async/await help mitigate this problem.

Node.js vs. Other Frameworks

Node.js's advantages include:

• Simplicity (due to its event-driven model).
• High performance (V8 engine).
• JavaScript for both front-end and back-end.
• Rich ecosystem of modules via npm.

Common Node.js Applications

• Real-time applications (chat, collaboration tools).
• Streaming applications.
• Single-page applications (SPAs).
• Microservices.
• Internet of Things (IoT) applications.

Node.js Timing Features

• setTimeout()/clearTimeout()
• setInterval()/clearInterval()
• setImmediate()/clearImmediate()
• process.nextTick()

Forking in Node.js

Forking creates child processes using the child_process module. This allows utilizing multiple CPU cores for improved performance.

Ensuring Consistent Code Style

Use linters like ESLint to enforce coding style conventions and improve code quality.

Front-End vs. Back-End Development

The i_am_a_dummy Flag

The i_am_a_dummy flag (a MySQL setting) forces the use of a WHERE clause in UPDATE and DELETE statements. This prevents accidentally modifying or deleting all rows in a table without specifying a condition.

Getting the Current Date in MySQL

SELECT CURRENT_DATE();

MySQL Security Alerts

Key security measures for MySQL:

• Install antivirus software.
• Configure a firewall.
• Avoid running MySQL as root.
• Use strong passwords.
• Restrict remote access.
• Regular security audits.

Changing MySQL Passwords

mysqladmin -u root -p password "new_password"

UNIX vs. MySQL Timestamps

Both store timestamps as integers, but MySQL typically displays them in YYYY-MM-DD HH:MM:SS format, while UNIX timestamps represent seconds since January 1, 1970.

Finding the Nth Highest Salary (MySQL)

SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT n-1, 1;

MySQL Default Port

The default port for MySQL is 3306.

REGEXP in MySQL

The REGEXP operator performs pattern matching using regular expressions.

Maximum Columns in an Index

A standard MySQL index can have a maximum of 16 columns.

Async/Await in Node.js

The async/await pattern makes asynchronous code look and behave a bit more like synchronous code, making it easier to read and understand.

async function myFunction() {
    try {
        let result = await someAsyncOperation();
        // ... process result ...
    } catch (error) {
        // ... handle error ...
    }
}

Modules in Node.js

Modules are JavaScript files that contain reusable functions and classes. You use the require() function to import modules.

Some core Node.js modules:

• http: Creates HTTP servers.
• util: Utility functions.
• url: URL parsing.
• fs: File system access.
• stream: Working with streams of data.
• querystring: Parsing query strings.
• zlib: Compression and decompression.

Buffers in Node.js

Buffers are used to handle binary data, providing a way to work with raw byte arrays. They're essential for network programming and file I/O.

Error-First Callbacks

Error-first callbacks place the error object (if any) as the first argument, followed by the data. This pattern simplifies error handling in asynchronous code.

Asynchronous APIs in Node.js

Node.js APIs are largely asynchronous, meaning they don't block execution while waiting for a result. Callbacks or promises handle the results when they become available.

Avoiding Callbacks

To make asynchronous code more manageable, you can use:

• Modularization: Break down complex callback structures into smaller, more manageable functions.
• Promises: Use promises to represent the eventual result of an asynchronous operation.
• Async/await: Write asynchronous code that looks synchronous (using promises under the hood).

Node.js Debugger

Node.js has a built-in debugger that you can invoke from the command line. It allows inspecting variables, setting breakpoints, and stepping through code.

node debug myScript.js

Control Flow Functions

Control flow functions are used to manage the execution sequence of multiple asynchronous operations, ensuring that they run in the correct order and handle results appropriately. They collect results, manage concurrency, and orchestrate the flow of asynchronous tasks. Libraries like Async.js provide helpful functions.

Accessing the DOM in Node.js

You cannot directly access the DOM (Document Object Model) in Node.js because it's a server-side environment; the DOM is a browser-side concept.

Asynchronous Tasks and the Event Loop

The Node.js event loop handles I/O operations asynchronously. Heavy computations should generally be offloaded to worker processes or other strategies to avoid blocking the event loop.

REPL (Read-Eval-Print Loop) in Node.js

REPL is an interactive environment where you can execute JavaScript code and see the results immediately. It's useful for testing and debugging.

REPL Components

• Read: Reads user input.
• Eval: Evaluates the input.
• Print: Prints the result.
• Loop: Repeats the process.

Evaluating Expressions in Node REPL

Yes, Node's REPL is suitable for evaluating JavaScript expressions.

The Underscore (_) Variable

In the Node.js REPL, the underscore (_) variable holds the result of the last executed expression.

Cryptography in Node.js

Node.js's crypto module provides cryptographic functions (hashing, HMAC, encryption, decryption, signing, verification).

Tools for Consistent Code Style

• ESLint
• JSHint
• JSLint
• JSCS

Operational vs. Programmer Errors

Operational errors: Problems outside your code (network issues, server failures). Programmer errors: Bugs in your code.

Global vs. Local Dependencies

Global dependencies: Installed globally, accessible from any project. Local dependencies: Installed in the project's node_modules folder, only accessible to that project.

The Buffer Class

The Buffer class handles raw binary data, essential for working with streams and network protocols in Node.js.

Assertions with Node.js assert

The assert module provides simple assertion methods for testing. It only reports failures; passing tests don't produce output.

Streams in Node.js

Streams are objects that provide a way to read from a source and write to a destination, allowing efficient handling of data. Types include:

• Readable
• Writable
• Duplex
• Transform

Event-Driven Programming

Node.js is event-driven; it waits for events and executes corresponding callback functions. This enables highly scalable and efficient I/O handling.

Events vs. Callbacks

Callbacks are functions passed to asynchronous functions. Events use an observer pattern; listeners are registered for events, and those listeners are called when the event is triggered.

Punycode in Node.js

Punycode is an encoding scheme that converts Unicode (UTF-8) strings to ASCII. It's essential for handling internationalized domain names (IDNs) because domain names are restricted to ASCII characters. Punycode is built into Node.js v0.6.2 and later.

const punycode = require('punycode');
let ascii = punycode.encode('myunicodeString');

Node.js TTY Module

The TTY (Teletypewriter) module provides access to the terminal's input and output streams. It's typically used for interacting with the terminal itself. In many cases, you don't need to use this module directly as Node.js handles basic terminal I/O automatically. The module provides the `tty.ReadStream` and `tty.WriteStream` classes.

const tty = require('tty');

Angular vs. Node.js

Operational vs. Programmer Errors

Operational errors: External issues (network problems, hardware failures, etc.) that impact the application's operation. Programmer errors: Bugs in the application's code.

EventEmitters in Node.js

EventEmitter is a class that allows objects to emit named events. Listeners (functions) can be attached to those events. When an event is emitted, the registered listeners are executed.

const EventEmitter = require('events');
let myEmitter = new EventEmitter();
myEmitter.on('myEvent', () => { /* ... */ });
myEmitter.emit('myEvent'); // Triggers the event listener

readFile() vs. createReadStream()

fs.readFile() reads the entire file into memory at once (blocking). fs.createReadStream() streams the file in chunks (non-blocking), making it more efficient for large files.

Punycode

Punycode encodes Unicode characters into ASCII. It's used to represent internationalized domain names (IDNs) in a format that's compatible with the DNS system.

const punycode = require('punycode');
let encoded = punycode.encode('myUnicodeString');

Improving Node.js Performance with Clustering

Clustering in Node.js starts multiple Node processes, allowing you to utilize multiple CPU cores. A master process manages these worker processes.

Thread Pools in Node.js (libuv)

Node.js uses the libuv library for asynchronous I/O. libuv manages a thread pool for handling blocking operations, preventing the main event loop from being blocked.