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Using $(body) to add single event listener on multiple elements [duplicate]

Where this is coming from
When I first learned jQuery, I normally attached events like this:
$('.my-widget a').click(function() {
$(this).toggleClass('active');
});
After learning more about selector speed and event delegation, I read in several places that "jQuery event delegation will make your code faster." So I started to write code like this:
$('.my-widget').on('click','a',function() {
$(this).toggleClass('active');
});
This was also the recommended way to replicate the behavior of the deprecated .live() event. Which is important to me since a lot of my sites dynamically add/remove widgets all the time. The above doesn't behave exactly like .live() though, since only elements added to the already existing container '.my-widget' will get the behavior. If I dynamically add another block of html after that code has ran, those elements will not get the events bound to them. Like this:
setTimeout(function() {
$('body').append('<div class="my-widget"><a>Click does nothing</a></div>');
}, 1000);
What I want to achieve:
the old behavior of .live() // meaning attaching events to not yet existent elements
the benefits of .on()
fastest performance to bind events
Simple way to manage events
I now attach all events like this:
$(document).on('click.my-widget-namespace', '.my-widget a', function() {
$(this).toggleClass('active');
});
Which seems to meet all my goals. (Yes it's slower in IE for some reason, no idea why?)
It's fast because only a single event is tied to a singular element and the secondary selector is only evaluated when the event occurs (please correct me if this is wrong here). The namespace is awesome since it makes it easier to toggle the event listener.
My Solution/Question
So I'm starting to think that jQuery events should always be bound to $(document).
Is there any reason why you would not want to do this?
Could this be considered a best practice? If not, why?
If you've read this whole thing, thank you. I appreciate any/all feedback/insights.
Assumptions:
Using jQuery that supports .on() // at least version 1.7
You want the the event to be added to dynamically added content
Readings/Examples:
http://24ways.org/2011/your-jquery-now-with-less-suck
http://brandonaaron.net/blog/2010/03/4/event-delegation-with-jquery
http://www.jasonbuckboyer.com/playground/speed/speed.html
http://api.jquery.com/on/

No - you should NOT bind all delegated event handlers to the document object. That is probably the worst performing scenario you could create.
First off, event delegation does not always make your code faster. In some cases, it's is advantageous and in some cases not. You should use event delegation when you actually need event delegation and when you benefit from it. Otherwise, you should bind event handlers directly to the objects where the event happens as this will generally be more efficient.
Second off, you should NOT bind all delegated events at the document level. This is exactly why .live() was deprecated because this is very inefficient when you have lots of events bound this way. For delegated event handling it is MUCH more efficient to bind them to the closest parent that is not dynamic.
Third off, not all events work or all problems can be solved with delegation. For example, if you want to intercept key events on an input control and block invalid keys from being entered into the input control, you cannot do that with delegated event handling because by the time the event bubbles up to the delegated handler, it has already been processed by the input control and it's too late to influence that behavior.
Here are times when event delegation is required or advantageous:
When the objects you are capturing events on are dynamically created/removed and you still want to capture events on them without having to explicitly rebind event handlers every time you create a new one.
When you have lots of objects that all want the exact same event handler (where lots is at least hundreds). In this case, it may be more efficient at setup time to bind one delegated event handler rather than hundreds or more direct event handlers. Note, delegated event handling is always less efficient at run-time than direct event handlers.
When you're trying to capture (at a higher level in your document) events that occur on any element in the document.
When your design is explicitly using event bubbling and stopPropagation() to solve some problem or feature in your page.
To understand this a little more, one needs to understand how jQuery delegated event handlers work. When you call something like this:
$("#myParent").on('click', 'button.actionButton', myFn);
It installs a generic jQuery event handler on the #myParent object. When a click event bubbles up to this delegated event handler, jQuery has to go through the list of delegated event handlers attached to this object and see if the originating element for the event matches any of the selectors in the delegated event handlers.
Because selectors can be fairly involved, this means that jQuery has to parse each selector and then compare it to the characteristics of the original event target to see if it matches each selector. This is not a cheap operation. It's no big deal if there is only one of them, but if you put all your selectors on the document object and there were hundreds of selectors to compare to every single bubbled event, this can seriously start to hobble event handling performance.
For this reason, you want to set up your delegated event handlers so a delegated event handler is as close to the target object as practical. This means that fewer events will bubble through each delegated event handler, thus improving the performance. Putting all delegated events on the document object is the worst possible performance because all bubbled events have to go through all delegated event handlers and get evaluated against all possible delegated event selectors. This is exactly why .live() is deprecated because this is what .live() did and it proved to be very inefficient.
So, to achieve optimized performance:
Only use delegated event handling when it actually provides a feature you need or increases performance. Don't just always use it because it's easy because when you don't actually need it. It actually performs worse at event dispatch time than direct event binding.
Attach delegated event handlers to the nearest parent to the source of the event as possible. If you are using delegated event handling because you have dynamic elements that you want to capture events for, then select the closest parent that is not itself dynamic.
Use easy-to-evaluate selectors for delegated event handlers. If you followed how delegated event handling works, you will understand that a delegated event handler has to be compared to lots of objects lots of times so picking as efficient a selector as possible or adding simple classes to your objects so simpler selectors can be used will increase the performance of delegated event handling.

Event delegation is a technique to write your handlers before the element actually exist in DOM. This method has its own disadvantages and should be used only if you have such requirements.
When should you use event delegation?
When you bind a common handler for more elements that needs same functionality. (Ex: table row hover)
In the example, if you had to bind all rows using direct bind, you would end up creating n handler for n rows in that table. By using delegation method you could end up handling all those in 1 simple handler.
When you add dynamic contents more frequently in DOM (Ex: Add/remove rows from a table)
Why you should not use event delegation?
Event delegation is slower when compared to binding the event directly to element.
It compares the target selector on every bubble it hits, the comparison will be as expensive as it is complicated.
No control over the event bubbling until it hits the element that it is bound to.
PS: Even for dynamic contents you don't have to use event delegation method if you are bind the handler after the contents get inserted into DOM. (If the dynamic content be added not frequently removed/re-added)

Apparently, event delegation is actually recommended now. at least for vanilla js.
https://gomakethings.com/why-event-delegation-is-a-better-way-to-listen-for-events-in-vanilla-js/
"Web performance #
It feels like listening to every click in the document would be bad for performance, but it’s actually more performant than having a bunch of event listeners on individual items."

I'd like to add some remarks and counterarguments to jfriend00's answer. (mostly just my opinions based on my gut feeling)
No - you should NOT bind all delegated event handlers to the document
object. That is probably the worst performing scenario you could
create.
First off, event delegation does not always make your code faster. In
some cases, it's is advantageous and in some cases not. You should use
event delegation when you actually need event delegation and when you
benefit from it. Otherwise, you should bind event handlers directly to
the objects where the event happens as this will generally be more
efficient.
While it's true that performance might be slightly better if you are only going to register and event for a single element, I believe it doesn't weigh up against the scalability benefits that delegation brings. I also believe browsers are (going to be) handling this more and more efficiently, although I have no proof of this. In my opinion, event delegation is the way to go!
Second off, you should NOT bind all delegated events at the document
level. This is exactly why .live() was deprecated because this is very
inefficient when you have lots of events bound this way. For delegated
event handling it is MUCH more efficient to bind them to the closest
parent that is not dynamic.
I kind of agree on this. If you are 100% sure that an event will only happen inside a container, it makes sense to bind the event to this container, but I would still argue against binding events to the triggering element directly.
Third off, not all events work or all problems can be solved with
delegation. For example, if you want to intercept key events on an
input control and block invalid keys from being entered into the input
control, you cannot do that with delegated event handling because by
the time the event bubbles up to the delegated handler, it has already
been processed by the input control and it's too late to influence
that behavior.
This is simply not true. Please see this codePen: https://codepen.io/pwkip/pen/jObGmjq
document.addEventListener('keypress', (e) => {
e.preventDefault();
});
It illustrates how you can prevent a user from typing by registering the keypress event on the document.
Here are times when event delegation is required or advantageous:
When the objects you are capturing events on are dynamically
created/removed and you still want to capture events on them without
having to explicitly rebind event handlers every time you create a new
one. When you have lots of objects that all want the exact same event
handler (where lots is at least hundreds). In this case, it may be
more efficient at setup time to bind one delegated event handler
rather than hundreds or more direct event handlers. Note, delegated
event handling is always less efficient at run-time than direct event
handlers.
I'd like to reply with this quote from https://ehsangazar.com/optimizing-javascript-event-listeners-for-performance-e28406ad406c
Event delegation promotes binding as few DOM event handlers as possible, since each event handler requires memory. For example, let’s say that we have an HTML unordered list we want to bind event handlers to. Instead of binding a click event handler for each list item (which may be hundreds for all we know), we bind one click handler to the parent unordered list itself.
Also, googling for performance cost of event delegation google returns more results in favor of event delegation.
When you're trying to capture (at a higher level in your document)
events that occur on any element in the document. When your design is
explicitly using event bubbling and stopPropagation() to solve some
problem or feature in your page. To understand this a little more, one
needs to understand how jQuery delegated event handlers work. When you
call something like this:
$("#myParent").on('click', 'button.actionButton', myFn); It installs a
generic jQuery event handler on the #myParent object. When a click
event bubbles up to this delegated event handler, jQuery has to go
through the list of delegated event handlers attached to this object
and see if the originating element for the event matches any of the
selectors in the delegated event handlers.
Because selectors can be fairly involved, this means that jQuery has
to parse each selector and then compare it to the characteristics of
the original event target to see if it matches each selector. This is
not a cheap operation. It's no big deal if there is only one of them,
but if you put all your selectors on the document object and there
were hundreds of selectors to compare to every single bubbled event,
this can seriously start to hobble event handling performance.
For this reason, you want to set up your delegated event handlers so a
delegated event handler is as close to the target object as practical.
This means that fewer events will bubble through each delegated event
handler, thus improving the performance. Putting all delegated events
on the document object is the worst possible performance because all
bubbled events have to go through all delegated event handlers and get
evaluated against all possible delegated event selectors. This is
exactly why .live() is deprecated because this is what .live() did and
it proved to be very inefficient.
Where is this documented? If that's true, then jQuery seems to be handling delegation in a very inefficient way, and then my counter-arguments should only be applied to vanilla JS.
Still: I would like to find an official source supporting this claim.
:: EDIT ::
Seems like jQuery is indeed doing event bubbling in a very inefficient way (because they support IE8)
https://api.jquery.com/on/#event-performance
So most of my arguments here only hold true for vanilla JS and modern browsers.

Is there a downside to calling jquery .on(document.body) rather than the specific element

I am upgrading the version of jQuery used in a project to the latest and greatest. The .live function was used in numerous places to change dynamically created display and worked great, but the .live functionality was removed in later versions of jQuery thus necessitating a retool.
As a way of standardizing the coding I am considering calling the function as:
$(document.body).on('click', '#clickableElement', function(){})
rather than
$("#clickableElement').on('click', function(){})
even if the click method will not be performed on a dynamically created object.
Do you see any downfalls to this idea considering it is an internal website, small number of users, and all elements are provided id's.

Event delegation is a recommended pattern because it helps avoid memory leaks that are possible when event handlers are attached to DOM elements that are later removed from the DOM.
It it also faster in most cases where selectors force JS to travel through the DOM tree, identify all matches, and attach a new instance of an event handler function to each matching element.
While neither may be the case in a very specific ID example, there is no drawback to using it as a general approach. The only real downside to delegation at the root of the document is the risk of something canceling the event before it gets to the body which prevents it from bubbling to your handler.

Event delegation does not always make your code faster. In some cases, it's is advantageous and in some cases not.
You should use event delegation when you actually need event delegation and when you benefit from it. Otherwise, you should bind event handlers directly to the objects where the event happens as this will generally be more efficient.
Also, you should not bind all delegated events at the document level. This is exactly why .live() was deprecated because this is very inefficient when you have lots of events bound this way. For delegated event handling it is much more efficient to bind them to the closest parent that is not dynamic.

Event delegation on mouseover

When creating click events, I do my best to bind them only once – generally to a parent shared by all the nodes expected to trigger the event. I'm curious, however, what the best practice is with mouseover events: Does it still make sense to bind an event to a parent when the result would be the constant firing of the event on mouseover? What's the most efficient practice?

In order to provide some closure to this question, I'm going to paraphrase/quote some relevant notes from this answer: 'Should all jquery events be bound to $(document)?', which was referenced above by #Faust:
Event delegation does not always make your code faster. Unless you're binding to dynamic elements or a ton of elements, you should bind event handlers directly to the objects where the event happens as this will generally be more efficient.
More specifically, here are times when event delegation is required or advantageous:
When the objects you are capturing events on are dynamically created/removed and you still want to capture events on them without having to explicitly rebind event handlers every time you create a new one.
When you have lots of objects that all want the exact same event handler (where lots is at least hundreds). In this case, it may be more efficient at setup time to bind one delegated event handler rather than hundreds or more direct event handlers. Note, delegated event handling is always less efficient at run-time than direct event handlers.
When you're trying to capture (at a higher level in your document) events that occur on any element in the document.
When your design is explicitly using event bubbling and stopPropagation() to solve some problem or feature in your page.
Original answer by #jfriend00

So, I know this question is long dead, but I figured I might as well answer with a way to do this.
With dynamic-elements, you can establish a mousemove listener on the parent div/container, and then query within the div for elements with a :hover attribute.
For example:
<div class="list-container">
<ul class="dynamic-list-content">
<!-- actual list elements provided by js -->
</ul>
</div>
Then:
var listContainer = document.querySelector('.list-container');
listContainer.addEventListener('mousemove', function(e) {
var hovered = listContainer.querySelector('li:hover');
// do something with the hovered element here.
});
Note that (as you mentioned) this will fire a lot, but no more than if you added a mousemove event listener to the individual entries. And you could debounce this a bit, using data-attributes, unique ids, etc. From my tests though, it's pretty performant in Chrome.

you can also stop the propagation of events. More info here: http://api.jquery.com/event.stoppropagation/ and here event.preventDefault() vs. return false

Does having a large number of event handlers degrade performace?

I have a page with some 30+ event handlers.
Similar to this:
$section.on("click", "a.unselect-all", function () {
var $s = $(this).siblings("select");
$s.find("option").removeAttr("selected");
$s.val("");
});
The page itself is not all that large.
But I feel that it's response time has become more sluggish with more event handlers.

First off, here's a similar question/answer with some pertinent info to understanding this issue: Should all jquery events be bound to $(document)?.
Here are some notes on large numbers of event handlers and performance:
30+ is NOT a large number of event handlers. In most cases, you shouldn't have to worry about 30+ event handlers at all (see possible exception below if you have nasty selectors).
If event handlers are attached directly to the intended object (the most efficient way for them to be processed at the actual event time), you will maximize your run-time performance.
If you are using delegated event handling, then event handlers should be placed on a parent that is as close as possible to the actual object receiving the event. Putting all delegated event handlers on a top level object like document or document.body is generally a bad idea because that makes the code have to look at more selectors for every event than if the event handlers are closer to the actual object that triggers the event thus avoiding evaluating most of the selectors for most events.
If you are using delegated event handling, then the performance of the secondary selector (the 2nd argument to .on()) is important. If this is a complicated selector that is not fast to evaluate and you have a lot of these all attached to the same object for the same event, you could start to see some performance degradation. How much performance degradation will depend upon the exact circumstances.
So, the worst case is if you have a lot of delegated event handlers all attached to the same object that all have a complicated secondary selector. This is the worst case because the event will have to bubble up to this parent object and then jQuery will have to go through every one of the delegated events for this particular event and it will have to evaluate the secondary selector argument to see if it matches the target object. If you have a lot of these delegated event handlers on the same object and you the secondary selector is somewhat complicated, it's possible that the time to process all this could start to be noticeable.
As with any performance issue, if this is really important to you, then you need to do your own performance testing. Install 30 delegated 'click' handlers with secondary selectors like you will be using to the document object in a sample page and try it. See if you can see any performance degradation at all between the click and the response to the click. Try it with 100 event handlers, 1000 event handlers, etc... until you see where it becomes noticeable.

Does jQuery class selector event binding attach eventHandler to each instance?

Im learning jQuery and when I read about $('.className').bind('click',function(){}) method one doubt arose.
Does this method attach my event handler to each instance of className in the DOM? If so would it be a good approach -- like attaching an event handler to lot of instances on a page is an overhead right?
Or is it making use of event delegation -- say, the handler would be bound to some common parent which would make use of e.target equalTo className and execute the click event, so that handler is bound to only one DOM element?
Please help me understand. Thanks.
One more doubt. If I do attach an event to each and every dom element where does the overload come into effect? Will it cause extra load to the browser while execution or would it make the dom heavy (by heavy I mean, difficulty in parsing dom)?

It sure does bind your callback to each and every one of those elements. If you have a lot of elements matching the selector, it is definitely better to use event delegation.
However, you don't have to manually fiddle around with e.target. jQuery has the overloaded .on() method specifically for this scenraio:
$('#parent').on('click', '.children', function (){});
You should always attach the event handler to the closest parent possible, so that the least amount of elements will be affected by the event listener.

$('.className').bind('click',function(){}) does attach the event handler to each available element in the dom... event delegation also has its share of issues like instead of listening to only events with in the target elements it need to evaluate all the click events happening within the delegated parent again the delegation target selector...
that said, I prefer event delegation if there are a lot of elements.... but prefer to attach the handler to the most immediate common ancestor element instead of binding it to body or document