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Lesson#32
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Event Handeling
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(Web Development Lesson 11)
During the last Lesson we discussed Functions & Variable Scope:
We looked at functions and their
use for solving simple problems
We became familiar with a couple of JavaScript’s built-in functions
We became familiar with the concept of local and global variables
Function:
A group of statements that is put
together (or defined) once and then can be used (by reference)
repeatedly on a Web page
Also known as subprogram, procedure, subroutine
Advantages of Functions:
Number of lines of code is reduced
Code becomes easier to read & understand
Code becomes easier to maintain as changes need to be made only at a
single location instead multiple
locations
function writeList( heading, words ) {
document.write( heading + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
document.write( words[ k ] + "<BR>" ) ;
}
}
Arguments of a Function:
A comma-separated list of data
Arguments define the interface between the function and the rest of the
Web page
Arguments values are passed to the function by value (some popular
languages pass arguments ‘by
reference’ as well)
Keyword
Function definition
enclosed in a pair of
curly braces
To ensure that a function is defined before it is called up, define
all functions in the HEAD portion
of Web pages
Two Ways of Calling Functions:
function popUp( message ) {
window.alert( message ) ;
}
popUp( “Warning!” ) ;
What Would this Statement Do?
factorial( factorial ( 3 ) ) ;
This is termed as the
recursive
use of a function.
Methods:
Methods
are
functions
They are unusual
in the sense that they are stored as
properties of objects
Predefined, Top-Level or Built-In Functions:
Event handlers are not the only
functions that come predefined with JavaScript. There are many others.
Practically, there is no difference between predefined functions and
those that are defined by the
programmer (termed as user-defined or custom functions)
There are many of them, but here we discuss only two: parseInt( ),
parseFloat( )
Local Variables:
Declaring variables (using the var
keyword) within a function, makes them
local
function add( a, b ) {
c = a + b ;
return c ;
}
sum = add( 2, 4 ) ;
document.write( sum ) ;
A function call appearing as
part of a statement.
Definitions of such functions
include a ‘return’ statement
prop
1
prop
2
prop
5
nam
e
prop
3
prop
4
A collection of
properties & methods
All objects have the “name”
property: it holds the name
of the object (collection)
method
method 3
1
method
2
A function call appearing as
a complete statement
They are available only within the
function and hold no meaning outside of it.
Local –vs– Global:
Global variables can make the
logic of a Web page difficult to understand
Global variables also make the reuse and maintenance of your code much
more difficult
Event Handlers:
Special-purpose functions that
come predefined with JavaScript
They are unusual
in the sense that they are mostly called
from the HTML part of a Web page and not
the <SCRIPT> … </SCRIPT> part
Today’s Goal:
Event Handlers
To become able to appreciate the
concept of event handlers:
What are they?
What do they do?
How do we benefit from them?
To learn to write simple programs that use event handlers
32.1 What is Event Handling?
Capturing events and responding to them
The system sends events to the program and the program responds to them
as they arrive
Events can include things a user does - like clicking the mouse - or
things that the system itself does -
like updating the clock. Today we will exclusively focus on user-events
Event Driven Programs:
Programs that can capture and
respond to events are called ‘event-driven programs’
JavaScript was specifically designed for writing such programs
Almost all programs written in JavaScript are event-driven
JavaScript Handling of Events:
Events handlers are placed in the
BODY part of a Web page as attributes in HTML tags
Events can be captured and responded to directly with JavaScript
one-liners embedded in HTML tags in
the BODY portion
Alternatively, events can be captured in the HTML code, and then
directed to a JavaScript function for
an appropriate response
HEURISTIC:
If it’s possible to define a variable as
local,
do it!
<INPUT
type=“submit”
name=“sendEmail”
value=“Send eMail”
onMouseOver=
“if (document.sendEmail.sender.value.length < 1)
window.alert(‘Empty From field! Please correct’)”
>
Additional JavaScript code for the
smart
‘Send eMail’ button that does not allow
itself to be clicked if
the “From” text field is left blank
That was event handling through what we may call ‘in-line JavaScript’
That is, the event was captured and handled with a JavaScript one-liner
that was embedded in the
HTML
tag
32.2 In-Line JavaScript Event Handling :
Event handlers are placed in the
BODY portion of a Web page as attributes of HTML tags
The event handler attribute consists of 3 parts:
The identifier of the event handler
The equal sign
A string consisting of JavaScript statements enclosed in double or
single quotes
Multiple JavaScript statements (separated by semicolons) can be placed
in that string, but all have to fit
in a single line; no newline characters are allowed in that string
Due to this limitation, sophisticated event handling is not possible
with in-line event handling
Another - more sophisticated -
way of accomplishing the same task:
Usage Guideline:
For very short scripts, “all code
in the tag” works well
The “code in the HEAD portion” is the right choice for developing larger
JavaScript scripts
It makes the code easier to read
It allows the reuse of a function for multiple event handlers
Another event-handling example; this time from Lesson 18
onMouseOver=“checkForm( )”
JavaScript that goes between the <SCRIPT>, </SCRIPT> tags:
JavaScript included as an attribute of the “Send eMail” button:
function checkForm() {
if ( document.sendEmail.sender.value.length < 1) {
window.alert( “Empty From field! Please correct” );
}
}
A Few of My Favorite Event Handlers:
There are many more: there is an
expanded, but still incomplete list in your book.
Now let’s look at some of these error handlers in a bit more detail
onFocus & onBlur:
onFocus executes the specified
JavaScript code when a window receives focus or when a form element
receives input focus
onBlur executes the specified JavaScript code when a window loses focus
or a form element loses focus
onClick
onDblClick
onMouseOver
onMouseDown
onFocus
onBlur
onReset
onSubmit
onLoad
onUnload
onClick=“vuWindow()”
JavaScript that goes between the <SCRIPT>, </SCRIPT> tags:
JavaScript included as an attribute of the “New Window” button:
function vuWindow() {
window.open(“http://www.zainbooks.com/”) ;
}
<INPUT type="text" name="age"
onBlur="checkAge( ) "
>
<HTML><HEAD>
<TITLE>onBlur( ) Demo</TITLE>
<SCRIPT>
function checkAge() {
if( parseInt(document.form1.age.value) < 12) {
window.alert("Stop! You are younger than 12" ) ;
}
}
</SCRIPT></HEAD>
<BODY bgcolor="#66FFCC">
<FORM name="form1" method="post" action="">
<TABLE border="1">
<TR> <TD>Age</TD>
<TD><INPUT type="text" name="age" onBlur="checkAge()">
</TD></TR><TR> <TD>Phone Number</TD>
<TD><INPUT type="text" name="phNo"></TD>
</TR><TR> <TD><INPUT type="reset" value="Reset"></TD>
<TD><INPUT type="submit" value="Submit"></TD></TR>
</TABLE></FORM></BODY></HTML>
onLoad & onUnload:
onLoad executes the specified
JavaScript code when a new document is loaded into a window
onUnload executes the specified JavaScript code when a user exits a
document
What is the key difference between these 2 and the 4 event handlers
(onMouseOver, onClick, onFocus,
onBlur) that we have used so far?
JavaScript that goes between the
<SCRIPT>, </SCRIPT> tags:
JavaScript included as an attribute of the
INPUT
tag:
function checkAge( ) {
if( parseInt( document.form1.age.value ) < 12 ) {
window.alert( "Stop! You are younger than 12" ) ;
}
}
<HTML>
<HEAD>
<TITLE>onUnload Demo</TITLE>
<SCRIPT>
function annoyUser( ) {
currentUrl = window.location ;
window.alert( "You can't leave this page" ) ;
window.location = currentUrl ;}
</SCRIPT></HEAD>
<BODY onUnload="annoyUser( )">
This page uses the onUnload event handler …
</BODY></HTML>
<HTML>
<HEAD>
<TITLE>onUnload Demo</TITLE>
<SCRIPT>
function annoyUser( ) {
currentUrl = window.location ;
window.alert( "You can't leave this page" ) ;
window.location = currentUrl ;
}
</SCRIPT></HEAD>
<BODY onUnload="annoyUser( )">
This page uses the onUnload event handler …
</BODY></HTML>
More Uses for onLoad/onUnload?
onLoad can be used to open multiple Windows when a particular document
is opened
onUnload can be used to say “Thank you for the visit” when a user is
leaving a Web page
At times, a user opens multiple inter-dependent windows of a Web site
(e.g. VULMS). onUnload can
be used to warn that all child Windows will become inoperable if the
user closes the parent Window
A Note on Syntax:
Mixed-case capitalization of event
handlers (e.g. onClick) is a convention (but not a requirement) for
JavaScript event handlers defined in HTML code. Using ‘ONCLICK’ or
‘onclick’ as part of a an
HTML tag is perfectly legal as well
At times, you may wish to use event handlers in JavaScript code enclosed
in <SCRIPT>, </SCRIPT>
tags
In those cases you have to strictly follow the JavaScript rule for all
event handler identifiers: they must
all be typed in small case, e.g. ‘onclick’ or ‘onmouseover’
A misleading statement from Lesson 18:
I stated:
JavaScript is case sensitive. Only the first of the following will
result in the desired function – the
rest will generate errors or other undesirable events:
onMouseClick – OnMouseClick
onmouseclick – ONMOUSECLICK
That statement is incorrect in
two ways:
All four will work fine as part of HTML tags
Only the ‘all small case’ version will be interpreted as intended in
JavaScript code
During Today’s Lesson …:
We looked at the concept of
event-driven programs and event handlers
What are they?
What do they do?
How do we benefit from them?
We wrote simple programs to demonstrate the capabilities of a few event
handlers
Next (the 12th) Web Dev Lecture:
Mathematical Methods
We’ll look at JavaScript’s Math
object
We will produce solutions for simple problems using various methods of
the Math object |
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