- Adding jQuery to a Project 2:43
- Spoiler Revealer: Breaking it Down 6:28
- Understanding Unobtrusive JavaScript 4:38
- Adding New Elements to the DOM 5:06
- Progressive Enhancement and Unobtrusive JavaScript 5 questions
- Adding New Elements to the DOM 2 objectives
- Using on() for Event Handling 2:26
- Using Events with Dynamically Added Elements 4:41
- The Event Object 4:44
- Events with jQuery 2 objectives
- What is Traversal? 8:42
- DOM Traversal with jQuery 2 objectives
- jQuery Events and DOM Traversal Review 5 questions
Preview
Video Player
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The Event Object
4:44Use the event object to obtain useful information about the event that has just occurred. This can come in handy for quite a few programming tasks!
Sometimes when a user clicks or
interacts with an element,
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we'll need to get more information about
that element to know how to proceed.
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Let me show you what I mean,
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what if I wanted to put two
spoiler revealers on this page?
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I can add another Star Wars
spoiler by going the index.html,
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copying and pasting the spoiler,
and typing a new one.
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So here we'll say, Jar Jar Binks is Sith,
save and preview that.
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Now when we click either button,
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though, both buttons disappear, and
both spoilers are revealed, why is that?
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Because in our code, we're saying,
when any button element
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inside of any element with the class
of spoilers clicked, do something.
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I can fix this easily by giving
the second spoiler a unique class or ID.
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But what if I wanted to add 5, or 10,
or 20 spoilers to my spoiler revealer?
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Then I'd have to repeat a lot of code,
and adding and maintaining unique IDs and
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event listeners on each element
could become a real pain.
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We can use something called
the event object to fix this issue,
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to control which button is hidden.
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So far, we haven't used it, but
every time an event is triggered,
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the event object gets passed
automatically into the event handler.
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To access the event object, we have to
define a parameter in our event handler,
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like this.
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What exactly is an event object,
and why is it useful?
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The event object contains a bunch of
information about the event that has
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just occurred.
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And some of this information can come
in really handy, as we shall see.
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Let's take a look at it in the console
to see what information it gives us.
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Open up the workspace for this video,
if you don't have it open already.
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And inside of this click handler,
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let's pass the event object,
and console.log the event.
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Now let's open Chrome Dev Tools.
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When I click the button,
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it will reveal to us the innards
of this mysterious event object.
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So this is all the information we
have access to when an event occurs.
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In this case, the event being, when the
button was clicked, reveal the spoiler.
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As you can see,
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there's a ton of info, a lot of
which we don't care about right now.
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But notice that the event object contains
a couple of key value pairs here
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that could be really useful.
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It tells us the type of event that
occurred, for example, in this case,
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it was a click.
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It also tells us the target of the event,
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which is the specific
element that was clicked on.
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Let's console.log event.target,
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Refresh, you can see it returns to us
the specific button that was clicked on.
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Note that the event.target is not
the parent spoiler element, but
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the specific button that was clicked,
that's event propagation in action.
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So here, where we're hiding the button, or
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in this case all buttons,
we can use event.target instead.
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So what we're saying here is
that when the button is clicked,
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hide the target of the event.
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In other words, hide the specific button
that was clicked on, and not just any and
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all elements on the page that
are nested within the spoiler.
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One thing to note here, in this example,
we're using the name event.
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Another common name that developers
use for event variables is evt.
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Or simply e.
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It doesn't actually matter
what you call it, but
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it's convention to use
one of these variants.
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I like to be explicit, so
I use the full word event.
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Let's save and preview this,
don't need this anymore.
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Now the button disappears for
whatever element we click on.
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However, both spoilers are still revealed,
because we have this line of code here.
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This line of code is programmed to show
on click any element with a class of
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spoiler that has a child span element.
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We'll fix that issue in an upcoming video,
for now,
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we can just comment out that line,
save, go back and refresh the page.
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And you'll see now that
when we click a button,
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only the button we've clicked disappears,
pretty handy.
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The event object is also handy for
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preventing default browser behavior
using the prevent default method.
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We'll learn about that
later in this course.
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