JavaScript Events: Bubbling, Capturing, and Propagation
Tags: javascript, beginners, tutorial, programming
Published: 2022-12-02 09:16:30

##**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 , 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.

```
<!--Try it-->
function first() {
  console.log(1);
}
function second() {
  console.log(2);
}
let button = document.getElementById("button");
let 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?**

```
<!--Try it-->
function first(event) {
  console.log(1);
}
function second() {
  console.log(2);
}
function third() {
  console.log(3);
}
let button = document.getElementById("button");
let 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 **.

**That's it!! #HappyCoding**
![cheers](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/khh0u7guia7qpv8u21aj.jpeg)