Event Loop in Node.js

Deep understanding of the JavaScript Event Loop: call stack, Web APIs, task queues, microtasks, macrotasks, execution order, and real-world behavior.

Why the Event Loop Exists

JavaScript is single-threaded, meaning it can execute only one piece of code at a time. The Event Loop allows JavaScript to handle asynchronous operations without blocking execution.

1. JavaScript is Single-Threaded

Only one call stack.

Core Components of Event Loop

The Event Loop coordinates execution between multiple internal components.

1. Event Loop Architecture

Mental model.

Call Stack

The call stack is a LIFO (Last In, First Out) structure that keeps track of function calls.

1. Call Stack Execution

Nested function calls.

Web APIs

Web APIs are provided by the environment (browser or Node.js), not JavaScript itself.

1. setTimeout and Web APIs

Delegation to environment.

Task Queue (Macrotasks)

Macrotasks include setTimeout, setInterval, I/O callbacks.

1. Macrotask Execution Order

Queued after stack clears.

Microtask Queue

Microtasks run immediately after the current stack, before macrotasks.

1. Promise Callbacks

Microtask priority.

Event Loop Algorithm

The Event Loop follows a strict cycle to decide what runs next.

1. Event Loop Steps

Execution rules.

Nested Async Operations

Async callbacks can schedule other async callbacks.

1. Nested Async Example

Complex ordering.

Node.js Event Loop (Brief)

Node.js has multiple event loop phases unlike browsers.

1. Node Event Loop Phases

High-level overview.

Common Pitfalls

Misunderstanding event loop leads to bugs.

1. Blocking the Event Loop

CPU-heavy tasks.