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Using Callbacks on Text Inputs
4:26In this video, we'll use event handlers for the focus and blur events.
You should already be somewhat familiar
with adding event listeners to the dom.
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Here's a refresher.
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An element has a method
called addEventListener.
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It takes two arguments,
the first, the event type.
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The event type can range
from a mouse click to
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when a user loses focus on a text field.
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Secondly, the callback of the event
handler, which is a function
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that gets executed when the matching
event is triggered by the computer.
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Here's what en event handler looks like,
written as a regular function or
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an arrow function.
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It's a function that has one argument,
the event object.
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The event object has
a number of properties.
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The most important ones are type and
target.
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type lets you know, as the developer, what
type of event triggered the call back.
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target is the element that
the event was triggered on.
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This allows you to use the same callback
for multiple types of events or elements.
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We'll see more about this later.
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Here's the app we're building.
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We have a form with an input field and
a text area.
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Our job is to apply the highlight
class name to the input field and
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text area when the user gives
focus to these elements.
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When the user navigates
away from the elements or
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loses focus,
it should remove the highlight class name.
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The highlight class name gives
this gold outline to the elements.
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Launch the work space with this video.
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In the app.js file,
we have two elements selected,
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the nameInput and the
messageTextArea field.
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We'll work with the nameInput field first.
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Let's add a click listener
to the nameInput.
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First I call the addEventListener method.
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Then I pass in the event type, click.
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And then then the event handler.
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Now we can apply the class name
of highlight to the event target.
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Remember that target is the element
that triggered the event.
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Save the file and test it.
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When we click in the field,
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we see that the field is highlighted
with the highlight class.
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But there are other ways to
enter into a text field, for
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example, when you press the tab key and
when you jump from field to field.
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When you use the tab key the click
event doesn't get triggered.
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Let me show you.
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When I refresh the page and hit tab,
the highlight class isn't applied.
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When I click in it,
the highlight class is applied.
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When you see the cursor
appear in a text field,
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the text field is described
as being in focus.
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This is also an event that we can use,
the focus event.
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Let's update click to the focus event.
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Preview and hit tab and
the highlight class is applied.
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When a field loses focus
it triggers the blur event.
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Let's remove the class
name on the blur event.
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I'll copy and paste the focus event and
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replace focus with blur.
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And the empty string for the class name.
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Save and preview.
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And now, when I tab around the form
the highlight gets applied.
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It also works when I click.
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When it loses focus,
the highlight class is removed.
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Here's the challenge for the next video.
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Apply the highlight class on
the focus event for the text area.
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And remove the highlight
class on the blur event.
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I'll show you how I did it with some
optimizations that you can perform, too.
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