Are events in JavaScript always fired even if there are no listeners attached?

Lets say "mousemove", I move the mouse but there are no listeners in the whole app, will the browser still construct a new Event and fire it, or will it optimize and consider the fact that if there are no event listeners, just ignore the data.

I assume that each browser works differently and I'm assuming they use patterns like observer and what not, but is there a spec around that states how it should be?

Are events in JavaScript always fired even if there are no listeners attached?

Lets say "mousemove", I move the mouse but there are no listeners in the whole app, will the browser still construct a new Event and fire it, or will it optimize and consider the fact that if there are no event listeners, just ignore the data.

I assume that each browser works differently and I'm assuming they use patterns like observer and what not, but is there a spec around that states how it should be?

• javascript
• events

• 7 This is like the "tree falls in the forest" question, but banning the use of microphones... – Katana314 Commented Dec 14, 2015 at 21:31
• Are you asking about browser events? – Amit Commented Dec 14, 2015 at 21:33
• This could be handled quite differently on a browser-by-browser, engine-by-engine basis. – JAAulde Commented Dec 14, 2015 at 21:35

1 Answer 1

Feel free to downvote this if you feel this is not correct but from my understanding and according to the DOM Level 2 Events Spec there is a sense that events are always constructed and executed but listeners need to be there, of course, to actually register them.

The reason I say "there is a sense that events are always constructed and executed" is because the Spec mentions that

This method allows the registration of event listeners on the event target. If an EventListener is added to an EventTarget while it is processing an event, it will not be triggered by the current actions but may be triggered during a later stage of event flow, such as the bubbling phase. If multiple identical EventListeners are registered on the same EventTarget with the same parameters the duplicate instances are discarded. They do not cause the EventListener to be called twice and since they are discarded they do not need to be removed with the removeEventListener method.

So if event listeners are dynamically added, there needs to be a way for the the page to know to register and listen to them. How each browser handles this is probably different as @JAAulde mentioned above but I do not think browsers would optimize for the fact that an event listener exists or not or at least nothing drastic.