What is DOM Event delegation?

Question

Can anyone please explain event delegation in JavaScript and how is it useful?

Answer 1

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 upward, checking 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. 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:

<ul onclick="alert(event.type + '!')">
    <li>One</li>
    <li>Two</li>
    <li>Three</li>
</ul>

With that example if you were to click on any of the child <li> nodes, you would see an alert of "click!", even though there is no click handler bound to the <li> you clicked on. If we bound onclick="..." to each <li> you would get the same effect.

So what's the benefit?

Imagine you now have a need to dynamically add new <li> items to the above list via DOM manipulation:

var newLi = document.createElement('li');
newLi.innerHTML = 'Four';
myUL.appendChild(newLi);

Without using event delegation you would have to "rebind" the "onclick" event handler to the new <li> element, in order for it to act the same way as its siblings. With event delegation you don't need to do anything. Just add the new <li> to the list and you're done.

This is absolutely fantastic for web apps with event handlers bound to many elements, where new elements are dynamically created and/or removed in the DOM. 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). It may not make much of a difference to small pages that unload often (i.e. user's navigate to different pages often). But for long-lived applications it can be significant. There are some really difficult-to-track-down situations when elements removed from the DOM still claim memory (i.e. they leak), and often this leaked memory is tied to an event binding. With event delegation you're free to destroy child elements without risk of forgetting to "unbind" their event listeners (since the listener is on the ancestor). These types of memory leaks can then be contained (if not eliminated, which is freaking hard to do sometimes. IE I'm looking at you).

Here are some better concrete code examples of event delegation:

• How JavaScript Event Delegation Works
• Event Delegation versus Event Handling
• jQuery.delegate is event delegation + selector specification
• jQuery.on uses event delegation when passed a selector as the 2nd parameter
• Event delegation without a JavaScript library
• Closures vs Event delegation: takes a look at the pros of not converting code to use event delegation
• Interesting approach PPK uncovered for delegating the focus and blur events (which do not bubble)

Answer 2

Event delegation allows you to avoid adding event listeners to specific nodes; instead, the event listener is added to one parent. That event listener analyzes bubbled events to find a match on child elements.

JavaScript Example :

Let's say that we have a parent UL element with several child elements:

<ul id="parent-list">
  <li id="post-1">Item 1</li>
  <li id="post-2">Item 2</li>
  <li id="post-3">Item 3</li>
  <li id="post-4">Item 4</li>
  <li id="post-5">Item 5</li>
  <li id="post-6">Item 6</li>
</ul>

Let's also say that something needs to happen when each child element is clicked. You could add a separate event listener to each individual LI element, but what if LI elements are frequently added and removed from the list? Adding and removing event listeners would be a nightmare, especially if addition and removal code is in different places within your app. The better solution is to add an event listener to the parent UL element. But if you add the event listener to the parent, how will you know which element was clicked?

Simple: when the event bubbles up to the UL element, you check the event object's target property to gain a reference to the actual clicked node. Here's a very basic JavaScript snippet which illustrates event delegation:

// Get the element, add a click listener...
document.getElementById("parent-list").addEventListener("click", function(e) {
  // e.target is the clicked element!
  // If it was a list item
  if(e.target && e.target.nodeName == "LI") {
    // List item found!  Output the ID!
    console.log("List item ", e.target.id.replace("post-"), " was clicked!");
   }
});

Start by adding a click event listener to the parent element. When the event listener is triggered, check the event element to ensure it's the type of element to react to. If it is an LI element, boom: we have what we need! If it's not an element that we want, the event can be ignored. This example is pretty simple -- UL and LI is a straight-forward comparison. Let's try something more difficult. Let's have a parent DIV with many children but all we care about is an A tag with the classA CSS class:

// Get the parent DIV, add click listener...
document.getElementById("myDiv").addEventListener("click",function(e) {
// e.target was the clicked element
  if(e.target && e.target.nodeName == "A") {
    // Get the CSS classes
    var classes = e.target.className.split(" ");
    // Search for the CSS class!
    if(classes) {
        // For every CSS class the element has...
        for(var x = 0; x < classes.length; x++) {
            // If it has the CSS class we want...
            if(classes[x] == "classA") {
                // Bingo!
                console.log("Anchor element clicked!");
                // Now do something here....
            }
        }
    }
  }
});

http://davidwalsh.name/event-delegate