Events (v1)

Introduction

Now that we have a handle on manipulating the DOM with JavaScript, the next step is learning how to make that happen dynamically, or on demand! Events are how you make that magic happen on your pages. Events are actions that occur on your webpage such as mouse-clicks or keypresses, and using JavaScript we can make our webpage listen and react to these events.

There are three primary ways to go about this: you can attach functions attributes directly on your HTML elements, you can set the "on_event_" property on the DOM object in your JavaScript, or you can attach event listeners to the nodes in your JavaScript. Event listeners are definitely the preferred method, but you will regularly see the others in use, so we’re going to cover all three.

Learning Objectives

  • How do "events" and "listeners" work? What are three ways to use events in your code?
  • How does "bubbling" work?

We’re going to create 3 buttons that all alert "Hello World" when clicked. Try them all out using your own HTML file, or using something like CodePen.

Method 1 (Not recommended)

<button onclick="alert('Hello World')">Click Me</button>

This solution is less than ideal because we’re cluttering our HTML with JavaScript. Also, we can only have 1 "onclick" event per element.

Method 2 (Not recommended)

<!-- the html file -->
<button id="btn">Click Me</button>
// the JavaScript file
const btn = document.querySelector('#btn');
btn.onclick = () => alert("Hello World");

(need to review arrow functions? Arrow Functions Basics)

This is a little better. We’ve moved the JS out of the HTML and into a JS file, but we still have the problem that a DOM element can only have 1 "onclick" property.

Method 3 (Recommended)

<!-- the html file -->
<button id="btn">Click Me Too</button>
// the JavaScript file
const btn = document.querySelector('#btn');
btn.addEventListener('click', () => {
  alert("Hello World");
});

Now, we maintain separation of concerns, and we also allow multiple event listeners if the need arises. Method 3 is much more flexible and powerful, though it is a bit more complex to set up.

Note that all 3 of these methods can be used with named functions like so:

<!-- the html file -->
<!-- METHOD 1 -->
<button onclick="alertFunction()">CLICK ME BABY</button>
function alertFunction() {
  alert("YAY! YOU DID IT!");
}

// METHOD 2
btn.onclick = alertFunction;

// METHOD 3
btn.addEventListener('click', alertFunction);

Using named functions can clean up your code considerably, and is a really good idea if the function is something that you are going to want to do in multiple places.

With all three methods we can access more information about the event by passing a parameter to the function that we are calling. Try this out on your own machine:

btn.addEventListener('click', function (e) {
  console.log(e);
});

Note that function (e) is a callback from addEventListener. Further explanation of callbacks can be found HERE.

The e in that function is an object that references the event itself. Within that object you have access to many useful properties and functions such as which mouse button or key was pressed, or information about the event’s target – the DOM node that was clicked.

Try this:

btn.addEventListener('click', function (e) {
  console.log(e.target);
});

and now this:

btn.addEventListener('click', function (e) {
  e.target.style.background = 'blue';
});

pretty cool eh?

Attaching listeners to groups of nodes

This might seem like a lot of code if you’re attaching lots of similar event listeners to many elements. There’s a few ways to go about doing that more efficiently. We learned above that we can get a nodelist of all of the items matching a specific selector with querySelectorAll('selector'). In order to add a listener to each of them we simply need to iterate through the whole list like so:

<div id="container">
    <button id="1">Click Me</button>
    <button id="2">Click Me</button>
    <button id="3">Click Me</button>
</div>
// buttons is a node list. It looks and acts much like an array.
const buttons = document.querySelectorAll('button');

// we use the .forEach method to iterate through each button
buttons.forEach((button) => {

  // and for each one we add a 'click' listener
  button.addEventListener('click', () => {
    alert(button.id);
  });
});

This is just the tip of the iceberg when it comes to DOM manipulation and event handling, but it’s enough to get you started with some exercises. In our examples so far we have been using the ‘click’ event exclusively, but there are many more available to you.

Some useful events include:

  • click
  • dblclick
  • keypress
  • keydown
  • keyup

You can find a more complete list with explanations of each event on this page.

Avoiding Confusion

As you’ve seen, you can define an event handler in 3 different ways.

  1. Using the addEventListener method (Recommended)
document.querySelector(".box").addEventListener( "click", function( event ){ ... });
document.querySelector("form").addEventListener( "submit", function( event ){ ... });
document.body.addEventListener( "keyup", function( event ){ ... });
  1. Using the on<EVENTNAME> property syntax (Not recommended)
document.querySelector(".box").onclick = function( event ){ ... };
document.querySelector("form").onsubmit = function( event ){ ... };
document.body.addEventListener.onkeyup = function( event ){ ... };
  1. Using an attribute with the on<EVENTNAME> directly in an HTML element (Not recommended):
<div class="box" onclick="clickHandler()">Click me</div>
<form onsubmit="submit()">
    <!-- CONTENT -->
</form>
<body onkeyup="handleKey()">

Events in the first syntax are used like this (nouns): ‘click’, ‘submit’, ‘change’, ‘scroll’, etc.

While using the 2nd syntax, are prefixed with the ‘on’ keyword: ‘onclick’, ‘onsubmit’, ‘onchange’, ‘onscroll’, etc.

The second syntax corresponds to element properties and should not be confused with the event names used in the first case.

Additional Resources

LOOKING FOR HELP?

When looking for help, try doing so in the following order:

  • Did you try everything you could?
  • Did you read the documentation?
  • Did you Google for it?
  • Did you post your question on Slack/Forum?
  • Did you ask your fellow students for help?
  • Did you ask your Mentors for help?
  • Did you leave a comment on the comments section of this page?
  • Did you ask your Instructor for help?
    • Did you arrange and appointment with your instructor using Calendly? Visit this URL and set up an appointment: https://calendly.com/kostasx
    • Is it urgent? Did you try reaching him on Slack

Feedback

  • Is there anything we can help with up to this point? Do you have something to suggest about this chapter? Let us know in the comments below.

UPDATED: 06.05.2021