Engineering Blog

JavaScript Events: Bubbling, Capturing, and Propagation

A basic example of a JavaScript event

Events, in JavaScript, are occurrences that can trigger certain functionality, and can result in certain behaviour. A common example of an event, is a “click”, or a “hover”. You can set listeners to watch for these events that will trigger your desired functionality.

What is “propagation”?

Propagation refers to how events travel through the Document Object Model (DOM) tree. The DOM tree is the structure which contains parent/child/sibling elements in relation to each other. You can think of propagation as electricity running through a wire, until it reaches its destination. The event needs to pass through every node on the DOM until it reaches the end, or if it is forcibly stopped.

Event Bubbling and Capturing

Bubbling and Capturing are the two phases of propagation. In their simplest definitions, bubbling travels from the target to the root, and capturing travels from the root to the target. However, that doesn’t make much sense without first defining what a target and a root is.

The target is the DOM node on which you click, or trigger with any other event.

For example, a button with a click event would be the event target.

The root is the highest-level parent of the target. This is usually the document, which is a parent of the <body>, which is a (possibly distant) parent of your target element.

Capturing is not used nearly as commonly as bubbling, so our examples will revolve around the bubbling phase. As an option though, EventTarget.addEventListener() has an optional third parameter – which takes its argument as a boolean – which controls the phase of the propagation. The parameter is called useCapture, and passing true will cause the listener to be on the capturing phase. The default is false, which will apply it to the bubbling phase.

Once you trigger the event, it will propagate up to the root, and it will trigger every single event handler which is associated with the same type. For example, if your button has a click event, during the bubbling phase, it will bubble up to the root, and trigger every click event along the way.

This kind of behaviour might not sound desirable – and it often isn’t – but there’s an easy workaround…

Event.stopPropagation()

These two methods are used for solving the previously mentioned problem regarding event propagation. Calling Event.stopPropagation() will prevent further propagation through the DOM tree, and only run the event handler from which it was called.

<!--
-->
function first() {
  console.log(1);
}
function second() {
  console.log(2);
}
var button = document.getElementById("button");
var container = document.getElementById("container");
button.addEventListener("click", first);
container.addEventListener("click", second);

In this example, clicking the button will cause the console to print 1, 2. If we wanted to modify this so that only the button’s click event is triggered, we could use Event.stopPropagation() to immediately stop the event from bubbling to its parent.

function first(event) {
  event.stopPropagation();
  console.log(1);
}

This modification will allow the console to print 1, but it will end the event chain right away, preventing it from reaching 2.

How does this differ from Event.stopImmediatePropagation()? When would you use either?

<!--
-->
function first(event) {
  console.log(1);
}
function second() {
  console.log(2);
}
function third() {
  console.log(3);
}
var button = document.getElementById("button");
var container = document.getElementById("container");
button.addEventListener("click", first);
button.addEventListener("click", second);
container.addEventListener("click", third);

Let’s suppose we wanted to add a third function, which prints 3 to the console. In this scenario, we will also move the second function to also be on the button. We will apply third to the container now.

Long story short: we have two event handlers on the button, and clicking <div#container> will now print 3.

What will happen now? This will behave the same as before, and it will propagate through the DOM tree, and print 1, 2, 3, in that order.

How does this tie in to Event.stopPropagation() and Event.stopImmediatePropagation()?

Adding Event.stopPropagation() to the first function, like so, will print 1, 2 to the console.

function first(event) {
  event.stopPropagation();
  console.log(1);
}

This is because Event.stopPropagation() will immediately prevent all click events on the parent from being triggered, but it does not stop any other event handlers from being called.

As you can see in our example, our button has two click handlers, and it did stop the propagation to its parents. If you wanted to stop all event handlers after the first, that’s where Event.stopImmediatePropagation() comes in.

function first(event) {
  event.stopImmediatePropagation();
  console.log(1);
}

Now, the first function really will stop the parent click handlers, and all other click handlers. The console will now print just 1, whereas if we simply used Event.stopPropagation(), it would print 1, 2, and not including either would print 1, 2, 3.

Related Posts

Avatar

About Greg Sakai

Greg Sakai is a Software Developer at LoginRadius. He is currently studying Computer Science part time, and he loves all things JavaScript. In his spare time, he likes doing homework.