What is event loop in nodejs ?
Tags: node, javascript, express, mern
Published: 2023-08-01 10:59:32


In Node.js, the event loop is a crucial part of its architecture that allows it to handle asynchronous operations efficiently. Understanding the event loop is essential for writing scalable and non-blocking applications in Node.js. Let's break down how the event loop works:

**1. Single-Threaded Non-Blocking Model:**
Node.js operates on a single-threaded event loop model. Unlike traditional multi-threaded approaches, Node.js uses a single thread to handle all incoming requests and operations. This single thread does not get blocked by I/O operations, making it highly efficient in handling concurrent tasks.
**2. Event Loop Phases:**
The event loop in Node.js consists of multiple phases, each serving a specific purpose. These phases are executed in a loop, and during each iteration, the event loop performs the following tasks:

a. Timers: This phase executes callbacks scheduled by setTimeout() and setInterval() functions.

b. Pending I/O callbacks: Executes I/O related callbacks (e.g., networking, file system operations) that were deferred during a previous loop iteration.

c. Idle, prepare: These are internal phases, and they are generally not something developers need to be concerned about.

d. Poll: This is the most critical phase where I/O operations are processed. Node.js checks for new I/O events, such as incoming connections or data being available for reading. If there are pending I/O operations, they are executed in this phase. If there are no pending I/O operations, Node.js will wait here.

e. Check: Executes callbacks registered via setImmediate().

f. Close callbacks: Executes cleanup callbacks, such as socket.on('close', ...).

**3.Event Loop Flow:**
The event loop operates in a continuous loop, following these steps:

Executes any callbacks scheduled by setTimeout() and setInterval() that have met their timer criteria.

Checks for I/O events (networking, file system, etc.) and executes their callbacks.

Executes setImmediate() callbacks.

If there are no more events to process, Node.js waits in the "Poll" phase for new events.

After the "Poll" phase, Node.js performs some additional checks before starting the next loop iteration.

The process repeats, ensuring that the event loop keeps handling new incoming events without blocking.

**4. Callbacks Execution:**
Callbacks are functions that are registered to be executed when a particular event occurs. For example, when a network request is completed, the corresponding callback is triggered, allowing the application to respond to that event.

**5. Async and Non-Blocking:**
Node.js is designed to perform I/O operations asynchronously and in a non-blocking manner. When an I/O operation is initiated (e.g., reading from a file, making a network request), Node.js does not wait for the result. Instead, it continues processing other tasks and registers a callback to be executed once the I/O operation is completed.

This event loop architecture enables Node.js to handle a large number of concurrent connections efficiently. Developers can write code that maximizes the use of asynchronous functions and callbacks to prevent blocking operations and take advantage of Node.js's non-blocking capabilities.