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In this video we'll use event handlers for the focus and blur events.
DOM and Event Listeners
element.addEventListener(eventType, callback);
eventType could be a keyboard key press or a mouse event—like a mouse click. 'focus' is the event when the cursor appears in a text input or text area, and when the form control loses 'focus', the 'blur' event is triggered.
An event callback looks like this:
function (event) {
// Do something
}
Or...
event => {
// Do something
}
The event object has a type property for the type of event that triggered the callback and a target property for the element from the event object.
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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 error function.
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It's a function that has one argument, the event object.
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The event update 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 appjs file, we have two elements selected,
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the name input and the message text area field.
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We'll work with the name input 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 uses 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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