Does Events Handlers in JavaScript for one event fired as FIFO, LIFO or in parallel?

Question

Does Events Handlers in JavaScript for one event fired as FIFO, LIFO or in parallel?

Answer 1

It's very simple. DOM Level 2 Events module does not specify order in which event listeners are to be fired. However, DOM Level 3 Events module does specify order to be the same as registration one.

First registered — first fired.

Note that DOM Level 3 Events module is still a draft, whereas Level 2 one is a recommendation. Nevertheless, modern browsers often follow semantics of exactly this draft Level 3 specification, instead of an older Level 2 one.

In practice, IIRC, only IE doesn't respect order of event listeners. All other browsers implement it as FIFO (and as described in DOM L3 Events). You can also find related tests online :)

Answer 2

Due the single threaded nature of JavaScript, events and timers are queued up and executed sequentially one at time, they run when there's been an opening in the execution.

This is best demonstrated with a diagram:

After the first block finishes its execution, there are two asynchronous events waiting, the Mouse Click Callback and the Timer, the browser picks the Mouse Click in this example, and the timer will wait until the next possible time, in order to execute.

Check this really good article about Timers and Events by John Resig.