DOM Events

Review: The DOM

The Document-Object Model is the 'glue' that connects Javascript to the data on web pages.

So far we've been working with the "element heirarchy", a set of interconnected objects that represent the text and HTML tags on a web page. When we change a property of one of these objects, the web page is updated to match.

var theParagraph = document.getElementById('para1'); = '#333'; = '#eee'; = '10px';

By itself this is boring and actually bad code! We should just write this directly into our HTML/CSS instead of using JavaScript, right?

What if we change stuff on the page when the person viewing the web page does something interesting?

DOM Events

Events are another part of the DOM that we haven't talked about yet. They let Javascript programs respond to user interactions such as clicks, mouse movements, etc. Test Link

var clickCallback = function() {
  log('The link was clicked.');

var linkEl = document.getElementById('testLink');
linkEl.addEventListener('click', clickCallback);

All we have to do is create a function that we want to run, and then attach it to the DOM using the addEventListener method that's bulit-in to DOM element objects.

Note: In IE versions 8 and below, you have to use the attachEvent function instead of addEventListener. Don't worry about that for now, because we're going to learn jQuery soon, and it handles this detail behind the scenes.

Available Events

Mouse events

  • click
  • dblclick
  • mousedown
  • mouseup
  • mousemove
  • mouseover
  • mouseout

Keyboard Events

  • keydown
  • keypress
  • keyup

Form Element Events

  • submit
  • reset

input/select/button Events

  • select
  • change
  • submit
  • focus
  • blur

document Object Events

  • load
  • unload

DOM Events

One more detail: JavaScript provides your event handler with an object of information about the event that happened. More on this later, but for right now you need to know one thing: event.preventDefault()

Go To Facebook

var handler = function(event) {
  log('Get off of Facebook')
  log('and pay attention!');

  // Stop the browser from following the link.

var linkEl = document.getElementById('testLink2');
linkEl.addEventListener('click', handler);

The "default action" is different depending on the element you're listening to:

  • For <a> tags, the "default action" is to open the link.
  • For <form> tags, the "default action" is to submit the form. You'll learn more about forms later.