Let's take a look at how these models play into the "event delegation" model used by modern web frameworks. Suppose you have this HTML structure:. Using addEventListeneryou can choose whether each listener participates in the capturing phase or the bubbling phase. Fortunately, Netscape Navigator is dead. A keypress in a text field can do one thing while a keypress not in any text field does another. Event objects have a property event.

In the example here, that's always the SPAN. The property event.

event bubbling vs event delegation

Event delegation is not a browser feature, but a popular technique built into libraries like jQuery. Many blogs get confused talking about it or equate it with bubbling, but I hope the following description is clear.

Suppose you want to respond to a click on any A tag on the page, even if the set of A tags changes over time. So, taking advantage of bubbling, you bind a single event handler to the BODY, and from this handler you look at event.

Be careful, though, because event. You need to not just check if the event's target is an A tag, but also walk up the DOM tree in a simple simulation of bubbling.

event bubbling vs event delegation

This is event delegation. To that end, the final step in event delegation at least in jQuery and Meteor is to set event. The BODY element is not really important, so it is nowhere to be found. They are the event target; the simulated currentTarget where we pretend the handler was called; and the actual currentTarget where the browser called into the delegation code. In many situations, the first two will be the same e.

In summary, the steps of event delegation are:. However, not all events bubble! How can we delegate non-bubbling events? Most events that people care about and might want to delegate are bubbling events, and if you squint, you can make the rest fit. Traditional non-bubbling events fall into the following categories:. So until recently, any event you might want to delegate — in particular UI events like mouse, keyboard, focus, etc.

Recently, however, HTML5 technologies have been introducing a lot of new events, including non-bubbling ones.

event bubbling vs event delegation

As far as I know, jQuery provides no way to delegate such non-bubbling events. Originally posted on Quora. Sign in. Browser events: bubbling, capturing, and delegation. Meteor Software Follow. Meteor Blog Product releases, community news, and project updates for….Help to translate the content of this tutorial to your language! When an event happens on an element, it first runs the handlers on it, then on its parent, then all the way up on other ancestors.

For instance, a focus event does not bubble. The most deeply nested element that caused the event is called a target element, accessible as event.

For instance, if we have a single handler form. A bubbling event goes from the target element straight up. For instance, here body. If an element has multiple event handlers on a single event, then even if one of them stops the bubbling, the other ones still execute.

In other words, event.

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After it no other handlers execute. Bubbling is convenient. Sometimes event. A task that seemingly requires that may be solved by other means. Also we can write our data into the event object in one handler and read it in another one, so we can pass to handlers on parents information about the processing below.

It is rarely used in real code, but sometimes can be useful. The standard DOM Events describes 3 phases of event propagation:. Before we only talked about bubbling, because the capturing phase is rarely used.

Bubbling and capturing

Normally it is invisible to us. To catch an event on the capturing phase, we need to set the handler capture option to true :. The code sets click handlers on every element in the document to see which ones are working. If we addEventListener If we have multiple event handlers on the same phase, assigned to the same element with addEventListenerthey run in the same order as they are created:.

Any event handler can stop the event by calling event. The capturing phase is used very rarely, usually we handle events on bubbling. In real world, when an accident happens, local authorities react first. They know best the area where it happened.

Then higher-level authorities if needed.

What's the difference between JavaScript event delegation, bubbling, and capturing?

The same for event handlers. The code that set the handler on a particular element knows maximum details about the element and what it does. Then its immediate parent also knows about the context, but a little bit less, and so on till the very top element that handles general concepts and runs the last. We want to make this open-source project available for people all around the world. Tutorial map. Almost all events bubble. For instance: We create a nested menu.

Some analytic systems do that. Usually the code uses document. To remove the handler, removeEventListener needs the same phase.Help to translate the content of this tutorial to your language! Capturing and bubbling allow us to implement one of most powerful event handling patterns called event delegation.

The idea is that if we have a lot of elements handled in a similar way, then instead of assigning a handler to each of them — we put a single handler on their common ancestor. In the handler we get event. In the handler table. The first idea may be to assign a separate handler to each button. We can add a handler for the whole menu and data-action attributes for buttons that has the method to call:. Please note that this. We could also use classes. And we can use it in CSS rules too.

If we click a button — its value is increased.

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Not buttons, but the general approach is important here. There can be as many attributes with data-counter as we want. We can add new ones to HTML at any moment. When we assign an event handler to the document object, we should always use addEventListenernot document.

One more example of behavior. That may become really convenient — no need to write JavaScript for every such element. Just use the behavior.

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The document-level handler makes it work for any element of the page. Open a sandbox for the task. Open the solution in a sandbox. In the example above the first column has numbers, and the second one — strings. The sorting function should handle sort according to the type. Only "string" and "number" types should be supported. In this task we assume that all elements with data-tooltip have only text inside.

No nested tags yet. After the behavior is implemented, even people unfamiliar with JavaScript can add annotated elements. We want to make this open-source project available for people all around the world. Tutorial map.Handling events is a crucial part of any modern web application.

Every developer who works with JavaScript in the browser would benefit from having a deeper knowledge of how events work, what is event bubbling and how to use delegation to optimize the event handling in an app. Well, this is exactly what we are gonna see in this article. So, have a good read! In JavaScript, when users interact with the page, their actions are captured in form of events.

This could be done like this:. What will happen when you click on the header? How about the two links? Well, you can check it by yourself with this live example:. Event bubbling is the propagation of an event from its origin towards the root element. If any of these elements has one or more registered listeners, they will be called.

All of this is easy to see in the previous sandbox.

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When you click on the Sign in link, for example, not only its listener is executed, but the listeners of its ancestors as well. Besides, since the body the white part has no registered click handlers, nothing happens when you click directly on it. In the above example, when the user clicks on the Sign in button, only its own listener s will be executed.

Modern applications heavily rely on events to provide interactive interfaces to users. If you have, for example, a color palette component and you need to perform some action when the user clicks on a given color, you could just add a click handler to each individual color in the palette. This could be easily done during the initialization of the component. Our palette has colors and, as a consequence, we are attaching listeners to handle all of them. Besides, if our component had a feature that allowed the user to add custom colors, we would need to add our listener to each new color added to the palette.

Well, all of this look like a silly concerns, since computers are much faster nowadays. The faster computers get, the more complex applications become as well. Every performance gain matters. Ok, but how to solve this problem? How to handle events for dozens, hundreds or even thousands of elements without adding individual handlers to each one of them?

To solve the problem mentioned in the above section, we will put together everything we have learned so far to make use of a technique called event delegation. In simple words, it leverages the bubbling effect to extract the handling logic of an event to a common ancestor of the elements where this event is triggered.

This way, every time the user clicks on a color, the bubbling effect causes the event to propagate to the palette element, triggering the execution of its listener. You can identify each color by using the e.

Do you want to become a JavaScript master?Yesterday, I wrote about why event delegation is better than attaching events to specific elements. In response, my buddy Andrew Borstein asked:. This is a great question that I get fairly often. The terms are often used interchangeably sometimes by me, oops!

Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it. I often use listening to all clicks in the document as an example, but it can be any element on the page.

For example, if you wanted to detect any time any field changed in value inside a specific form, you could do this:. The event starts are the element that triggered it saying, changing the email field in our example above. Using a form field as an example, the event would bubble up to the parent form, then any containers or divs the form was in, then the bodythen the html element, then the documentthen the window.

The last argument in addEventListener is called useCapture. We almost always set it to false. Skip to main content Accessibility Feedback. Event Delegation Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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What is the difference between event bubbling and capturing? When should one use bubbling vs capturing? Event bubbling and capturing are two ways of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event.

The event propagation mode determines in which order the elements receive the event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. With capturing, the event is first captured by the outermost element and propagated to the inner elements. Back in the old days, Netscape advocated event capturing, while Microsoft promoted event bubbling. On the other hand, the performance of event bubbling may be slightly lower for complex DOMs.

We can use the addEventListener type, listener, useCapture to register event handlers for in either bubbling default or capturing mode. To use the capturing model pass the third argument as true. In capturing model, the event will be handled by the div first click event handlers in the div will fire firstthen in the ulthen at the last in the target element, li.

In the bubbling model, the opposite will happen: the event will be first handled by the lithen by the uland at last by the div element. In the example below, if you click on any of the highlighted elements, you can see that the capturing phase of the event propagation flow occurs first, followed by the bubbling phase.

Another example at JSFiddle. In a nutshell copied from quirksmode :. It depends on what you want to do. There is no better. The difference is the order of the execution of the event handlers.

Event Delegation in Javascript - UI/Frontend Interview Question

Most of the time it will be fine to fire event handlers in the bubbling phase but it can also be necessary to fire them earlier. If there are two elements element 1 and element 2. Element 2 is inside element 1 and we attach an event handler with both the elements lets say onClick. Now when we click on element 2 then eventHandler for both the elements will be executed. Now here the question is in which order the event will execute.

If the event attached with element 1 executes first it is called event capturing and if the event attached with element 2 executes first this is called event bubbling. As per W3C the event will start in the capturing phase until it reaches the target comes back to the element and then it starts bubbling.

The capturing and bubbling states are known by the useCapture parameter of addEventListener method. I have found this tutorial at javascript.

And its 3-points summary at the end is really talking to the crucial points. I quote it here:.Instead, bind the single handler to their parent, and get the child from event. This site provides useful info about how to implement event delegation.

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Delegation is a technique where an object expresses certain behavior to the outside but in reality delegates responsibility for implementing that behaviour to an associated object. This sounds at first very similar to the proxy pattern, but it serves a much different purpose.

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Delegation is an abstraction mechanism which centralizes object method behavior. Generally spoken: use delegation as alternative to inheritance. Inheritance is a good strategy, when a close relationship exist in between parent and child object, however, inheritance couples objects very closely. Often, delegation is the more flexible way to express a relationship between classes. This pattern is also known as "proxy chains". Several other design patterns use delegation - the State, Strategy and Visitor Patterns depend on it.

Using a delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked. DOM event delegation is a mechanism of responding to ui-events via a single common parent rather than each child, through the magic of event "bubbling" aka event propagation. When an event is triggered on an element, the following occurs :.

The event is dispatched to its target EventTarget and any event listeners found there are triggered. Bubbling events will then trigger any additional event listeners found by following the EventTarget 's parent chain upwardchecking for any event listeners registered on each successive EventTarget. This upward propagation will continue up to and including the Document. Event bubbling provides the foundation for event delegation in browsers.

event bubbling vs event delegation

Now you can bind an event handler to a single parent element, and that handler will get executed whenever the event occurs on any of its child nodes and any of their children in turn.

This is event delegation. Here's an example of it in practice:. With event delegation you don't need to do anything. With event delegation the number of event bindings can be drastically decreased by moving them to a common parent element, and code that dynamically creates new elements on the fly can be decoupled from the logic of binding their event handlers. Another benefit to event delegation is that the total memory footprint used by event listeners goes down since the number of event bindings go down.