JavaScript Events
Introduction to JavaScript events
An event is an action that occurs in the web browser, which the web browser feedbacks to you so that you can respond to it.
For example, when users click a button on a webpage, you may want to respond to this click
event by displaying a dialog box.
Each event may have an event handler which is a block of code that will execute when the event occurs.
An event handler is also known as an event listener. It listens to the event and executes when the event occurs.
Suppose you have a button with the id btn
:
To define the code that will be executed when the button is clicked, you need to register an event handler using the addEventListener()
method:
How it works.
First, select the button with the id
btn
by using thequerySelector()
method.Then, define a function called
display()
as an event handler.Finally, register an event handler using the
addEventListener()
so that when users click the button, thedisplay()
function will be executed.
A shorter way to register an event handler is to place all code in an anonymous function, like this:
Event flow
Assuming that you have the following HTML document:
When you click the button, you’re clicking not only the button but also the button’s container, the div
, and the whole webpage.
Event flow explains the order in which events are received on the page from the element where the event occurs and propagated through the DOM tree.
There are two main event models: event bubbling and event capturing.
Event bubbling
In the event bubbling model, an event starts at the most specific element and then flows upward toward the least specific element (the document
or even window
).
When you click the button, the click
event occurs in the following order:
button
div with the id container
body
html
document
The click
event first occurs on the button which is the element that was clicked. Then the click
event goes up the DOM tree, firing on each node along its way until it reaches the document
object.
The following picture illustrates the event bubbling effect when users click the button:
Note that modern web browsers bubble the event up to the window
object.
Event capturing
In the event capturing model, an event starts at the least specific element and flows downward toward the most specific element.
When you click the button, the click
event occurs in the following order:
document
html
body
div with the id container
button
The following picture illustrates the event capturing effect:
DOM Level 2 Event flow
DOM level 2 events specify that event flow has three phases:
First, event capturing occurs, which provides the opportunity to intercept the event.
Then, the actual target receives the event.
Finally, event bubbling occurs, which allows a final response to the event.
The following picture illustrates the DOM Level 2 event model when users click the button:
Event object
When the event occurs, the web browser passed an Event
object to the event handler:
Output:
The following table shows the most commonly-used properties and methods of the event
object:
Property / Method | Description |
---|---|
bubbles | true if the event bubbles |
cancelable | true if the default behavior of the event can be canceled |
currentTarget | the current element on which the event is firing |
defaultPrevented | return true if the preventDefault() has been called. |
detail | more informatio nabout the event |
eventPhase | 1 for capturing phase, 2 for target, 3 for bubbling |
preventDefault() | cancel the default behavior for the event. This method is only effective if the |
stopPropagation() | cancel any further event capturing or bubbling. This method only can be used if the |
target | the target element of the event |
type | the type of event that was fired |
Note that the event
object is only accessible inside the event handler. Once all the event handlers have been executed, the event object is automatically destroyed.
preventDefault()
To prevent the default behavior of an event, you use the preventDefault()
method.
For example, when you click a link, the browser navigates you to the URL specified in the href
attribute:
However, you can prevent this behavior by using the preventDefault()
method of the event
object:
Note that the preventDefault()
method does not stop the event from bubbling up the DOM. And an event can be canceled when its cancelable
property is true
.
stopPropagation()
The stopPropagation()
method immediately stops the flow of an event through the DOM tree. However, it does not stop the browers default behavior.
See the following example:
Without the stopPropagation()
method, you would see two messages on the Console window.
However, the click
event never reaches the body
because the stopPropagation()
was called on the click
event handler of the button.
Summary
An event is an action that occurs in the web browser e.g., a mouse click.
Event flow has two main models: event bubbling and event capturing.
DOM Level 2 Event specifies that the event flow has three phases: event bubbling, the event occurs at the exact element, and event capturing.
Use
addEventListener()
to register an event that connects an event to an event listenerThe
event
object is accessible only within the event listener.Use
preventDefault()
method to prevent the default behavior of an event, but does not stop the event flow.Use
stopPropagation()
method to stop the flow of an event through the DOM tree, but does not cancel the browser default behavior
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