Node.js  

What is the Event Loop in Node.js, and How Does It Work?

🔍 Introduction

When developers hear that Node.js is single-threaded but still handles thousands of concurrent operations, the magic behind it is the Event Loop. This mechanism ensures that Node.js executes code efficiently without blocking other operations, even with just one main thread.

🌍 Why the Event Loop Exists

JavaScript was designed to run in browsers for interactive web pages, handling events like clicks and keystrokes. In a browser, the event loop ensures smooth interactions without freezing the UI.

Node.js brought JavaScript to the server side, where it deals with I/O operations like reading files, querying databases, or sending network requests. The Event Loop makes it possible to manage these without stopping the execution of other code.

🧠 How the Event Loop Works in Node.js

The Event Loop is managed by libuv, a C library that provides asynchronous I/O. Here’s the step-by-step process:

  1. Call Stack Execution: Node.js runs your synchronous code first.
  2. Delegating Tasks: When asynchronous functions like setTimeout or fs.readFile are called, they are handed over to background APIs or the thread pool.
  3. Callback Queue: Once the background task is done, its callback is added to the queue.
  4. Event Loop Processing: The event loop checks if the call stack is empty and then pushes the next callback from the queue to be executed.

📊 Event Loop Phases

The Node.js Event Loop runs in phases:

  1. Timers: Executes callbacks from setTimeout and setInterval.
  2. Pending Callbacks: Executes callbacks for system operations.
  3. Idle, Prepare: Internal use only.
  4. Poll: Retrieves new I/O events; executes I/O callbacks.
  5. Check: Executes setImmediate callbacks.
  6. Close Callbacks: Executes close events (e.g., socket.on('close')).

Microtasks (like process.nextTick() and resolved promises) run between these phases, before moving to the next phase.

💻 Example: Event Loop in Action

Example:

console.log("Start");

setTimeout(() => {
  console.log("Timeout callback");
}, 0);

Promise.resolve().then(() => {
  console.log("Promise callback");
});

console.log("End");

Output:

Start
End
Promise callback
Timeout callback

Explanation:

  • Promise callback runs before Timeout callback because promises are microtasks, which have higher priority than macrotasks like setTimeout.

🛠 Understanding Microtasks vs. Macrotasks

  • Microtasks: process.nextTick(), Promise.then(). Run immediately after the current operation.
  • Macrotasks: setTimeout(), setImmediate(), I/O callbacks. Run in the normal event loop phases.

📌 Key Points to Remember

  • Node.js is single-threaded for JavaScript execution.
  • The Event Loop allows asynchronous, non-blocking operations.
  • Microtasks always run before the next macrotask.
  • libuv handles background tasks and the thread pool.

📝 Summary

The Event Loop is the heart of Node.js's asynchronous programming model. It ensures that even though JavaScript runs on a single thread, Node.js can handle thousands of concurrent tasks without blocking. By delegating I/O operations to the background and using a queue system for callbacks, it keeps applications fast and responsive. Understanding the Event Loop is essential for writing efficient Node.js applications.

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