The Node.js event loop is one of the most important concepts to understand when working with Node.js. It's what allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded.

## What is the Event Loop?

The event loop is what allows Node.js to perform non-blocking I/O operations by offloading operations to the system kernel whenever possible. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background.

## Event Loop Phases

The event loop has several phases, each with a FIFO queue of callbacks to execute:

1. **Timers Phase**: Executes callbacks scheduled by setTimeout() and setInterval()
2. **Pending Callbacks**: Executes I/O callbacks deferred to the next loop iteration
3. **Idle, Prepare**: Used internally
4. **Poll Phase**: Retrieves new I/O events and executes I/O related callbacks
5. **Check Phase**: setImmediate() callbacks are invoked here
6. **Close Callbacks**: Executes close callbacks like socket.on('close')

## How It Works

When Node.js starts, it initializes the event loop and processes the provided input script. The script may make async API calls, schedule timers, or call process.nextTick(), then begins processing the event loop.

Deep dive into how the Node.js event loop works, including phases, timers, and microtasks.

Understanding Node.js Event Loop: A Complete Guide

Building RESTful APIs is one of the most common use cases for Node.js. Express.js provides a robust set of features for web and mobile applications, making API development straightforward and efficient.

## Setting Up Express

First, let's set up a basic Express server with proper middleware configuration.

## REST API Best Practices

When building RESTful APIs, following best practices ensures your API is maintainable, scalable, and easy to use:

- Use proper HTTP methods (GET, POST, PUT, DELETE, PATCH)
- Implement proper status codes
- Version your API
- Use proper error handling
- Implement request validation
- Add rate limiting and security measures

## Middleware in Express

Middleware functions are functions that have access to the request object, response object, and the next middleware function in the application's request-response cycle.

Learn how to build production-ready RESTful APIs using Node.js and Express framework.

Building RESTful APIs with Node.js and Express

Asynchronous programming is at the heart of Node.js. Understanding the difference between Promises and async/await is crucial for writing clean, maintainable code.

## Understanding Promises

Promises represent the eventual completion or failure of an asynchronous operation. They provide a cleaner alternative to callbacks and help avoid callback hell.

## Async/Await Syntax

Async/await is syntactic sugar built on top of Promises. It makes asynchronous code look and behave more like synchronous code, making it easier to read and maintain.

## Error Handling

Both approaches have different error handling mechanisms. Promises use .catch(), while async/await uses try/catch blocks.

Compare async/await and Promises in Node.js and learn when to use each approach.

Async/Await vs Promises in Node.js

This comprehensive guide covers 50 essential Node.js interview questions that every backend developer should know. These questions cover fundamental concepts, event loop, async programming, modules, streams, and production-ready patterns commonly asked in technical interviews.

## Core Concepts

Understanding Node.js core concepts is essential for any backend developer. These questions test your knowledge of the event loop, non-blocking I/O, and how Node.js handles concurrency.

## Event Loop & Async Programming

The event loop is the heart of Node.js. Master these questions to demonstrate your understanding of asynchronous programming, promises, async/await, and callback patterns.

## Modules & Package Management

Node.js uses CommonJS and ES modules for code organization. Understanding module systems, npm, and package management is crucial for building scalable applications.

## Streams & File System

Streams are powerful for handling large data efficiently. These questions cover readable, writable, and transform streams, along with file system operations.

## Production & Best Practices

Production-ready Node.js applications require proper error handling, security practices, performance optimization, and deployment strategies.

Comprehensive collection of 50 essential Node.js interview questions covering event loop, async programming, modules, streams, and backend concepts. Free Node.js, NodeJS, Node JS interview questions with answers. Node.js interview prep guide.

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Node.js Interview Questions: 50 Essential Questions for Developers

TypeScript with Node.js is becoming the standard for professional backend development. It catches bugs before they reach production and makes your code way easier to maintain.

## Why TypeScript?

TypeScript finds errors while you're coding, not when users complain. Your IDE becomes smarter, autocomplete works better, and refactoring becomes safe. For backend code, this is a game changer.

## Setting It Up

Don't worry, it's not complicated. Install TypeScript, create a config file, and you're good to go. I'll show you exactly what to do, step by step.

## Typing Express Routes

Once TypeScript is set up, you can type everything - your routes, request bodies, query parameters. Your IDE will tell you if you make a mistake, and you'll catch bugs before they happen.

## Real Examples

I'll show you how to type database models, middleware, and complex Express routes. These are patterns you'll use in real projects.

Build type-safe Node.js apps with TypeScript. Learn how to set it up, type your Express routes, and catch errors before they reach production. This is how professionals build backends in 2025.

Node.js with TypeScript: Type-Safe Backends

CORS errors are one of the most common issues when building web applications. If you're getting "Access-Control-Allow-Origin" errors, this tutorial will help you fix them.

## What is CORS?

CORS (Cross-Origin Resource Sharing) is a security feature that browsers use to control which websites can make requests to your API. When your frontend (React app) tries to call your backend (Node.js API) from a different origin, the browser blocks it unless you configure CORS properly.

## Why CORS Errors Happen

CORS errors occur when:
- Your frontend runs on http://localhost:3000
- Your backend runs on http://localhost:5000
- Browser blocks the request because origins don't match

## Fixing CORS in Express

The easiest way to fix CORS is using the cors middleware. Install it and configure it properly.

## Basic CORS Setup

For development, you can allow all origins. For production, specify exact origins for security.

## Advanced CORS Configuration

Learn how to configure specific headers, methods, and credentials. This is important for production applications.

## Common CORS Issues

I'll show you how to handle preflight requests, credentials, and custom headers. These are the most common CORS problems developers face.

Learn how to fix CORS errors in Node.js and Express. Understand CORS, configure it properly, and handle cross-origin requests. Essential for building APIs that work with frontend applications.

Fixing CORS Errors in Node.js and Express

Learn Node.js, NodeJS, and Node JS from basics to advanced concepts including async programming, APIs, and backend development.

Node.js Tutorials - NodeJS Node JS Programming

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