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Related Content: CS101 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Introduction to Computing

Event Handling

(Web Development Lecture 11)

During the last lecture 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

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)
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
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

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

<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

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 lecture 18

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

<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?

<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 Lecture 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 Lecture …:

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