JavaScript - Chapter 11 - Events

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Events
JavaScript


JavaScript Events

Event Flow

Event Types
Table of Content

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JavaScript Events
(JavaScript)

Events
●
An event is some notable action to which a script can respond. It
may be
– Click
– Mouseover
– Keystroke etc.
●
When a function is assigned to an event handler, that function is run
when that event occurs.
●
An Event handler is JavaScript code associated with a particular
part of the document and a particular event.

Event Handler
●
An Event handler is JavaScript code associated with a
particular part of the document and a particular event.
●
For example, an event handler associated with a button
could open a new window when the button is clicked.
●
A handler to the click event is called onclick.

Event Handler
●
Events are not limited to basic user-actions associated
with the document.
●
Browser supports events such as resize, load, and
unload.

Event Flow
(JavaScript)

Event Flow
●
The order in which events are received on the web page
are described by event flow.
●
An event has three phases :
– Cycle
– Target
– Bubbling

Event Bubbling
●
Event Bubbling , the event is first captured and handled
by innermost element and then propagated to outer
element.
<html>
<body>
<ul>
<li>
<a>

Event Capturing
●
Event Capturing , the event is first captured by the outermost
element and propagated to the inner elements.
<html>
<body>
<ul>
<li>
<a>

Event Target
●
Event Capturing,it provides an opportunity to intercept
events if necessary.
●
Then the actual target receives the event.
●
Final phase is the bubbling , which allows a response to the
event.

Event Listeners
The DOM 2 level 2 define two methods:
– addEventListeners
– removeEventListeners

Event Types
(JavaScript)

Mouse Events
Event Attribute Description
onclick Occurs when the mouse button is clicked
ondblclick Occurs when the mouse button is double clicked
onmousedown Occurs when the mouse button is pressed
onmouseup Occurs when the mouse button is released
onmousemove Occurs when mouse has moved while over an element.
onmouseover Occurs when mouse has moved over an element.
onmouseout Occurs when mouse has moved away from an element.

Keyboard Events
onkeypress Occurs when a key pressed and released with focus on
element
onkeydown Occurs when a key pressed down
onkeyup Releases a key

Loading Events
onload Occurs when element has loaded
onunload Indicates that browser is leaving the current document
onabort Occurs when the user abort the loading of an image

Selection and Focus Event
onselect Occurs after some text has been selected in an element
onchange Occurs when text input has been changed
onfocus Indicates that an element has received focus
onblur Occurs when an element losses focus

Other Events
onresize User resizes a window or a frame
onsubmit Indicates form submission by clicking a submit button
onreset Indicates that form is being reset by clicking reset
button

Events Example
<body>
<button onclick="show()">Click Here</button>
<p id="ex"></p>
<script>
function show() {
document.getElementById("ex").innerHTML = "Hello World";
}
</script>
</body>

Events Example
<!DOCTYPE html>
<html>
<body onLoad="alert('Welcome to my page!');"
onUnload="alert('Goodbye! Sorry to see you go!');">
<img src="birdflying.GIF">
</body>
</html>

Events Example
<script>
function OnMouseIn (elem) {
elem.style.border = "4px solid green";
}
function OnMouseOut (elem) {
elem.style.border = "";
}
</script>
</head>
<body>
<div style="background-color:#ddf0af; width:300px;color:#800000"
onmouseover="OnMouseIn (this)" onmouseout="OnMouseOut (this)">
Move your mouse pointer into and out of this element!
</div>
</body>

Event Listeners
●
The method addEventListeners() is used to register a single event
listener on the document.
●
These methods exist on all DOM nodes. There is a slight change in event
naming convention also, compared to how they are used with button
elements (ex: onclick vs click)
●
The event type to listen for (eg: mouseout, click, error etc)
●
The event handler function to be executed when the event is occurs.
●
The third parameter is a boolean value specifying whether to use event
bubbling or event capturing. This parameter is optional.
●
The keyword this used with event handler represents a reference to the
HTML element which fired the event handler.

Event Listeners
Syntax :
element.addEventListener(event, function, useCapture);
●
We can add many event handlers to one element.
●
We can add many event handlers of the same type to one element, i.e
two "click" events.
●
We can easily remove an event listener by using the
removeEventListener() method.

Event Listeners Example

Exercise
●
Write a JavaScript program to create a paragraph and background color must
change after some mouse events:
+ onclick button → yellow
+ odblclick button → blue
+ onmouseout → green
+ onmouseover → red
●
Write a JavaScript program to create a text field and show the effect of some
events:
+ onchange
+ onfocus
+ onblur

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