Tuesday, 7 May 2013

What does “event bubbling” mean? [In context of CSS]

“Delegation” and “bubbling” are terms that gets thrown round a lot in JavaScript; but what exactly do these terms mean?

Event Bubbling

In JavaScript, events bubble. This means that an event propagates through the ancestors of the element the event fired on. Lets show what this means using the HTML markup below;
<div>
    <h1>
        <a href="#">
            <span>Hello</span>
        </a>
    </h1>
</div>
Lets assume we click the span, which causes a click event to be fired on the span; nothing revolutionary so far. However, the event then propagates (or bubbles) to the parent of the span (the <a>), and a click event is fired on that. This process repeats for the next parent (or ancestor) up to the document element.
You can see this in action here. Click “Hello” and see the events as they get fired.
That’s all event bubbling is; an event fired on an element bubbles through its ancestor chain (i.e. the event is also fired on those elements). It’s important to note that this isn’t a jQuery feature, nor is it something that a developer must turn on; it’s a fundamental part of JavaScript that has always existed.
Ok, that’s a little bit of a lie… sort of.
By default, not all events bubble. For instance submit does not normally bubble, nor does change. However, jQuery masks this in the event handling code using all sorts of voodoo, so it will seem that they do bubble when using jQuery.
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