We've seen that we can select and manipulate nodes in the DOM using JavaScript. We've even seen that we can create and remove elements. But wouldn't it be more interesting if we made those types of thing happen when we did something: double-clicked on something, pushed a button, shook our mouse in a circle. Those actions are called "events." In JavaScript we can "listen" for events and use them to call functions that update the DOM. In this lesson, we'll learn how to do just that.

1. Demonstrate using addEventListener() to a DOM node
2. Prevent default behavior
3. Explain the difference between bubbling and capturing events
4. Add stopPropogation() to a DOM node

If you are using the Learn IDE available in your browser, use httpserver to launch the web server. You will see the message Your server is running at ... followed by a string of numbers and dots. This string is your temporary IP address that is hosting your index.html file. Copy this string of numbers, open a new tab and past the string in to the URL bar.

If you are using an standalone text editor such as Sublime or Atom, before we get started, follow these instructions to manually fork and clone the lesson repository on GitHub. In your forked and cloned copy, you'll find the index.html file, which you can then manually open up in the browser. (For instructions on opening HTML files in the browser from the Learn IDE, see this Help Center article.)

To add an event we can call addEventListener() on the node. addEventListener() takes two arguments: the name of the event, and a function to handle the event.

Let's start by adding a listener for click events to the main#main element in index.html. Once you've opened index.html in the browser, copy and paste the following in the browser's JS console:

const main = document.getElementById('main')

main.addEventListener('click', function(event) {
  alert('I was clicked!')
})

Now if you click on the main element (you can click its text, "My ID is 'main'!"), you should see an alert: 'I was clicked!'. How does this work?

The first argument, 'click', is the name of the event we're listening for.

Click events make up the majority of events listened you'll use, but other events you might use include change, 'keydown', 'keyup', 'load', 'mouseover', 'mouseout'. You can find more possible events on MDN.

The second argument is a function that accepts an event object as its argument.

The event has a number of useful properties on it. keypress, keydown, and keyup events will have a which property that tells us which key was pressed. Try it out by copying and pasting the following in your js console in the browser:

const input = document.querySelector('input')
input.addEventListener('keydown', e => console.log(e.which))

Don't forget arrow functions!

You'll notice that, for example, pressing "enter" logs 13 in console, while pressing "a" logs 65. What do other keys log in the console?

Refresh the page. We've got a vendetta against the letter "g" (71), so we're going to prevent the input from receiving "g"s. Paste the following in your console:

NOTE: If you don't refresh your page between the previous section of code and the following, JavaScript will give you an error about attempting to re-define input. Check your understanding and make sure you understand what const is doing. We'll use a number of variable declarations in this exercise, so make sure you understand const and let.

const input = document.querySelector('input')

input.addEventListener('keydown', e => {
  if (e.which === 71) {
    return e.preventDefault()
  }
})

Now try to enter "g" in the input — no can do!

Every DOM event comes with a preventDefault property. preventDefault is a function that, when called, will prevent the default event from taking place. It provides us an opportunity to intercept and change user interactions, usually in more helpful ways than preventing them from typing "g".

You can probably think of a couple of web pages that have prevented you from ESC-keying their ad away or one that wouldn't let you paste your password on that "repeat to confirm" entry field. Yep, that was JavaScript.

Another related event property is called stopPropagation. Like preventDefault, stopPropagation is a function that, when called, interrupts the event's normal behavior. In this case, it stops the event from triggering other nodes in the DOM that might be listening for the same event.

Wait. Do we mean one action can trigger multiple events? We sure do.

DOM events propagate by bubbling (starting at the target node and moving up the DOM tree to the root) and capturing (starting from the target node's parent elements and propagating down the tree until it reaches the target) — by default. Events nowadays all bubble. We can show this behavior by putting listeners to those nested divs in index.html. Paste the following in your console:

let divs = document.querySelectorAll('div')

function bubble(e) {
  // remember all of those fancy DOM node properties?
  // we're making use of them to get the number
  // in each div here!

  // if `this` is a bit confusing, don't worry —
  // for now, know that it refers to the div that
  // is triggering the current event handler.
  console.log(this.firstChild.nodeValue.trim() + ' bubbled')
}

for (const aDiv of divs) {
  aDiv.addEventListener('click', bubble);
}

Now click on the div containing "5". You should see

5 bubbled
4 bubbled
3 bubbled
2 bubbled
1 bubbled

What just happened? Well, the event starts at div 5, and then it bubbles up to the topmost node. Along the way, it triggers any other nodes that are listening for the event -- in this case, 'click'.

Try clicking on a node that's not so deeply nested -- you should still see the event bubble up, starting at the node that you clicked and hitting every node up the tree until it reaches the top.

What about capturing? In order to capture, we need to set the third argument to addEventListener to true. Let's try it out.

divs = document.querySelectorAll('div')

function capture(e) {
  console.log(this.firstChild.nodeValue.trim() + ' captured')
}

for (const aDiv of divs) {
  // set the third argument to `true`!
  aDiv.addEventListener('click', capture, true)
}

Now click on div 5. You should see

1 captured
2 captured
3 captured
4 captured
5 bubbled
5 captured
4 bubbled
3 bubbled
2 bubbled
1 bubbled

Now, the event propagates from the top of the page towards the target node, triggering event handlers as appropriate along the way.

Notice that the target node is the last node to capture the event, whereas it's the first node to bubble the event up. This is the most important takeaway.

NOTE: Don't worry if bubbling and capturing seems a bit confusing or weird. The different event behaviors are the results of the browser wars of the 90s, but most of the time it's safe just to stick to the default behaviors(which, for the record, is bubbling). You can read more about bubbling and capture on StackOverflow and QuirksMode.

Now that you've learned a bit about the dangers and behavior of bubbling and capturing, you understand how events propagate through the DOM. Much of the time, since we're listening for very specific events, this doesn't matter: our events can propagate up or down, and they'll only trigger the event handler(s) that we want them to trigger. But sometimes, as with these divs, we have a fairly generic event that we want to only hit its target. That's where stopPropagation comes in.

Let's rewrite the bubbling example to stop propagation so that only one event is triggered (remember to reload the page before entering this code!):

const divs = document.querySelectorAll('div')

function bubble(e) {
  // stop! that! propagation!
  e.stopPropagation()

  console.log(this.firstChild.nodeValue.trim() + ' bubbled')
}

for (const aDiv of divs) {
  aDiv.addEventListener('click', bubble)
}

Now try clicking on any node — you should only see one log statement!

In order to move on from this lesson enter learn. If your code passes the test, use learn submit to close out this lab and move on.

We covered a lot in this lesson. Feel free to edit index.html, to write code directly in the document (just put it between <script></script> tags). Check out the MDN documentation and play around with the different events - this stuff might feel intimidating at first, but it's important to practice so you can get the hang of it!

You should understand how to add an event listener, how different event triggers work, and how to intercept user interactions with e.preventDefault() and e.stopPropagation().

• MDN - Web Events
• StackOverflow - Bubbling and Capturing
• QuirksMode - Event order