DEV Community

Cover image for Node.js Event Loop
Ujjawal Kumar
Ujjawal Kumar

Posted on

Node.js Event Loop

The Node.js event loop is a crucial concept in understanding how Node.js manages asynchronous operations and handles concurrency. Node.js is designed to be non-blocking and asynchronous, which allows it to efficiently handle a large number of I/O-bound operations without getting blocked by any single operation.

Here's how the Node.js event loop works:

  1. Event Loop: The event loop is a central component of Node.js that manages all the asynchronous operations. It constantly checks for new events in the event queue and processes them in a loop.

  2. Event Queue: The event queue holds various types of events, such as callbacks, timers, and I/O events, that are generated as a result of asynchronous operations. These events are added to the queue when they are triggered but are not executed immediately.

  3. Callbacks: Callbacks are functions that are provided as arguments to asynchronous functions. They are executed once the corresponding asynchronous operation is completed and the event loop reaches the event in the queue. This mechanism allows Node.js to perform tasks without waiting for the completion of one task before moving on to the next.

  4. Timers: Node.js provides two types of timers: setTimeout() and setInterval(). These timers schedule callbacks to be executed after a specified amount of time or at regular intervals.

  5. I/O Operations: When Node.js performs I/O operations (e.g., reading from or writing to files, making network requests), it does not block the execution of the rest of the program. Instead, it delegates the I/O operation to the operating system and registers a callback to be executed when the operation is complete.

  6. Microtask Queue: In addition to the event queue, Node.js also maintains a microtask queue. Microtasks are tasks that have higher priority than regular callbacks and are executed before the next iteration of the event loop. Promises and certain APIs (e.g., process.nextTick()) add tasks to the microtask queue.

working of event Loop Node js
Here's a simplified step-by-step breakdown of how the event loop works:

  1. Check the event queue for pending events.

  2. If there are pending events, dequeue an event and execute its associated callback.

  3. Execute any microtasks that are in the microtask queue.

  4. Perform any ready I/O operations.

  5. Check if any scheduled timers have expired. If yes, execute their callbacks.

  6. Repeat the process from step 1.

The event loop allows Node.js to handle multiple tasks concurrently without creating a separate thread for each task. This concurrency model is efficient for I/O-bound operations, but it's important to note that CPU-bound operations can still block the event loop and should be offloaded to worker threads or other processes to maintain the responsiveness of the application.

Key Points:

  • The event loop is a single-threaded loop, which means that only one task can be executed at a time. However, the event loop can handle multiple tasks concurrently by running them in a non-blocking manner.

  • The event queue is a FIFO (first-in, first-out) queue, which means that events are processed in the order in which they are added to the queue.

  • Microtasks are executed before regular callbacks, which means that they have higher priority. This is because microtasks are typically used to perform tasks that are related to the event loop itself, such as scheduling timers and handling I/O events.

  • The event loop can be blocked by CPU-bound operations, which are operations that require a lot of processing power. To avoid blocking the event loop, CPU-bound operations should be offloaded to worker threads or other processes.

Hope you enjoyed this, Please like this article. Follow for more.

Top comments (2)

fanycastro profile image
Estefania Castro Vizoso

Every Nodejs developer should know this.

endeavourmonk profile image
Ujjawal Kumar

thanks 👍