JavaScript Internals: At what interval does the event loop run?

Question

This is a question about JavaScript internals.

Lets say I have 10 async tasks that all take x number of seconds to run. Whilst waiting for a response the script is idle.

In the background the JavaScript engine is asking "Is there anything on the task queue". To my understanding this is a loop. Hence, event loop. I know in Node this is implemented with Libuv. I've read this article which explains a little: https://nikhilm.github.io/uvbook/basics.html

Do JavaScript engines place any restriction on how often this event loop runs in order to balance out performance of the application? Does it run at a set interval?

If I'm off with anything, please correct me. I was purely interested at what interval this event loop runs.

if you setTimeout with zero from a function with itself, it fires about 250 times a second in V8. other sources of interruption (eg. ajax) are typically not "pinged" at all, but use lower-level flow controls with sub-ms granularity. — dandavis
– dandavis, Commented Aug 23, 2016 at 19:34
"restriction on how often this event loop runs in order to balance out performance of the application" - do you you mean to ask whether they are artificially slowing it down? No, why would they? — Bergi
– Bergi, Commented Aug 23, 2016 at 19:35
@Bergi: setTimeout (et al) is about the only "js internal" where a granularity applies... — dandavis
– dandavis, Commented Aug 23, 2016 at 19:41
@Bergi according to MDN, if the delay value is 0, the function is executed on the next event cycle. — Celsiuss
– Celsiuss, Commented Dec 1, 2020 at 4:52

saghul · Accepted Answer · 2016-08-24 07:23:49Z

2

There is no loop per se in the JavaScript side. There is one in libuv though. Basically libuv will wait until the closest timer hits or an i/o operation happens. Then it will fire a callback in C, which calls the C++ function Node passed and then this triggers JavaScript code to be executed.

Have a look at this presentation, specially the section starting at slide 33.

answered Aug 24, 2016 at 7:23

saghul

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