What is the difference between the event loop in JavaScript and async non-blocking I/O in Node.js?



In this answer to the question –

What is non-blocking or asynchronous I/O in Node.js?

the description sounds no different from the event loop in vanilla js. Is there a difference between the two? If not, is the Event loop simply re-branded as "Asynchronous non-blocking I/O" to sell Node.js over other options more easily?


The event loop is the mechanism. Asynchronous I/O is the goal.

Asynchronous I/O is a style of programming in which I/O calls do not wait for the operation to complete before returning, but merely arrange for the caller to be notified when that happens, and for the result to be returned somewhere. In JavaScript, the notification is usually performed by invoking a callback or resolving a promise. As far as the programmer is concerned, it doesn’t matter how this happens: it just does. I request the operation, and when it’s done, I get notified about it.

An event loop is how this is usually achieved. The thing is, in most JavaScript implementations, there is literally a loop somewhere that ultimately boils down to:

while (poll_event(&ev)) {

Performing an asynchronous operation is then done by arranging for the completion of the operation to be received as an event by that loop, and having it dispatch to a callback of the caller’s choice.

There are ways to achieve asynchronous programming not based on an event loop, for example using threads and condition variables. But historical reasons make this programming style quite difficult to realise in JavaScript. So in practice, the predominant implementation of asynchrony in JavaScript is based on dispatching callbacks from a global event loop.

Put another way, ‘the event loop’ describes what the host does, while ‘asynchronous I/O’ describes what the programmer does.

From a non-programmer’s bird’s eye view this may seem like splitting hairs, but the distinction can be occasionally important.

Answered By – user3840170

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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