What is Event Loop In Node js?
Published: 2024-01-08 16:02:09

The event loop is a pivotal component in Node.js, facilitating asynchronous, non-blocking I/O operations and ensuring the efficiency and scalability of applications. At its core, the event loop continuously monitors the execution stack, callback queue, and the event-driven architecture that characterizes Node.js. When a task is completed or an event occurs, the associated callback is queued for execution in the callback queue. Subsequently, the event loop dispatches these callbacks to the execution stack, allowing for parallel processing and preventing the blocking of the main thread. This mechanism ensures that Node.js applications remain responsive, handling multiple operations concurrently without waiting for each to complete. Mastery of the event loop empowers developers to optimize performance and create robust, event-driven applications in the Node.js environment.

## Event Loop Overview:

1. Node.js is designed to be non-blocking and asynchronous. The event loop is the core mechanism that enables this behavior.

2. Node.js runs in a single-threaded event-driven model, but it can efficiently handle concurrent operations through non-blocking I/O and an event-driven architecture.

## Event Loop Visualisation

Visualizing the event loop is crucial for understanding its operation in Node.js. Imagine a circular loop where each iteration represents a cycle of checking for events and executing associated callbacks. At the center is the "Event Loop Core," constantly monitoring the execution stack and callback queue. When a non-blocking operation or event occurs, it triggers a callback function, which is then placed in the callback queue.

The event loop, resembling a continuous spin, efficiently grabs callbacks from the queue and executes them one by one, allowing the application to remain responsive to other tasks. This visualization emphasizes the asynchronous nature of Node.js, illustrating how it efficiently handles I/O operations without blocking the main thread. Overall, the event loop's cyclic process ensures optimal performance and responsiveness in Node.js applications.



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/byrzvqx7sp8ke6s2cva8.gif)
## Phases of the Event Loop:

- Timers: Execute scheduled callbacks (e.g., setTimeout or setInterval).

- I/O Callbacks: Handle I/O events (e.g., reading from files, making network requests).
- Idle, Prepare: Internal phases for Node.js to perform internal operations.
- Poll: Retrieve new I/O events and execute their callbacks. If there are no pending I/O events, it will wait for events to arrive.
- Check: Execute setImmediate callbacks.
- Close Callbacks: Execute callbacks registered with process.nextTick and handle 

### Event Loop Execution:

1. The event loop starts by processing the timers phase. It executes callbacks scheduled by setTimeout and setInterval.
2. It then moves to the I/O callbacks phase, where it executes callbacks related to completed I/O operations. 
3. The event loop continues through the remaining phases, checking for events and executing corresponding callbacks.

## Non-Blocking I/O:

- Node.js relies on non-blocking I/O operations to prevent the event loop from being blocked while waiting for I/O to complete.
- When an asynchronous operation is initiated (e.g., reading a file or making an HTTP request), Node.js registers a callback function for the event loop.
- The event loop can continue to process other tasks while waiting for the I/O operation to complete.
- When the I/O operation finishes, the callback is added to the callback queue, and the event loop will eventually execute it.

## Microtasks Queue:

. Indeed, alongside the callback queue, Node.js features a microtasks queue, often referred to as the "nextTick queue" or "Promise queue." This queue holds tasks that are scheduled to run after the current phase of the event loop completes, but before moving on to the next event loop cycle. It ensures that certain tasks, like resolved promises and process.nextTick callbacks, take precedence and are executed before other callbacks.

This microtasks queue helps maintain the order and priority of tasks, offering a finer level of control over asynchronous operations. Developers can leverage this queue strategically to ensure that critical tasks are executed promptly, influencing the overall flow of the Node.js application. Understanding and utilizing the microtasks queue is essential for precise and efficient event-driven programming in Node.js.
. Promises and tasks scheduled with process.nextTick are executed in the microtasks queue.
. Microtasks have higher priority than timers or I/O callbacks, allowing for more immediate execution.

## Here's a simplified representation of the event loop cycle:

1. Start
2. Timers
3. I/O Callbacks
4. Idle, Prepare
5. Poll
6. Check
7. Close Callbacks
8. Microtasks Queue
9. Repeat


## recipe
Understanding the event loop is essential for writing efficient and responsive Node.js applications. It allows you to leverage asynchronous operations and avoid blocking the main thread, enabling your application to handle many concurrent connections without significant performance degradation.