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Introduction to Computing

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Introduction to Computing ­ CS101
VU
ESSON 18
OBJECTS, PROPERTIES, METHODS
(Web Development Lesson 6)
During the last Lesson we continued our discussion on Interactive Forms
We got our first taste of JavaScript ­ the object-based language that we will be employing throughout
the rest of the Web development part of this course
We developed a (simple) client-side script in JavaScript
During Today's Lesson ...
We will have a more formal introduction to JavaScript and client-side scripting
We will become able to appreciate the concept of objects in JavaScript
We will learn about the properties of those objects, and about how to read & modify them
We will become able to perform simple tasks through the application of methods
Last time we looked at two distinct ways of performing the "form" field checking function.
From now onwards, we will be employing the 2nd way more often than not
In that 2nd way, we referred to a function in the HTML BODY, and but defined that function in the
HTML HEAD
The main code segment that goes between the
<SCRIPT>, </SCRIPT> tags in the HEAD:
function checkForm() {
if ( document.sendEmail.sender.value.length < 1)
{
window.alert( "Empty From field! Please correct"
);
}
}
The JavaScript code included as an attribute of
the "Send eMail" button:
onMouseOver="checkForm()"
<HTML>
<HEAD>
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<TITLE>Send an eMail</TITLE>
<SCRIPT>
function checkForm(){
if (document.sendEmail.sender.value.length < 1) {
window.alert('Empty From field! Please correct');
}
}
</SCRIPT>
</HEAD>
<BODY bgcolor="#FFFFCC">
<H1>Send an eMail</H1>
<FORM name="sendEmail" method="post" action=sendMailScriptURL>
<TABLE><TR> <TD>From: </TD>
<TD><INPUT type="text" name="sender" size="50" ></TD>
</TR><TR> <TD>To: </TD>
<TD><INPUT type="text" name="receiver" size="50"></TD>
</TR><TR><TD>Subject: </TD>
<TD><INPUT type="text" name="subject" size="50"></TD>
</TR><TR><TD valign="top">Message: </TD>
<TD><TEXTAREA name="message" cols="38" rows="6"></TEXTAREA></TD>
</TR><TR><TD colspan="2" align="right">
<INPUT type="reset" name="reset" value="Reset All Fields">
<INPUT type="submit" name="sendEmail" value="Send eMail" onMouseOver="checkForm()">
</TD></TR></TABLE></FORM>
</BODY>
</HTML>
18.1 New Concept: Client-Side Scripts
Small programs that are a part of the Web page and run on the user's (client's) computer
They interact with the user to collect info or to accomplish other tasks
Once it has been collected, they may help pass the collected info on to a server-side script
We'll use JavaScript to do client-side scripting. However, there are many other languages that can be
used for that purpose, e.g. VBScript
18.2 Advantages of Client-Side Scripting
Reduced server load as it does not have to send messages to the user's browser about missing or
incorrect data
Reduced network traffic as the form's data is sent only once instead of many to's and fro's
18.3 Disadvantages
Client-side scripts do not work with all browsers
Some user intentionally turn scripting off on their browsers
This increases the complexity of the Web page, as it now has to support both situations: browsers with
scripting capability, and those not having that capability
18.4 JavaScript
A programming language specifically designed to work with Web browsers
It is designed to be used for developing small programs ­ called scripts ­ that can be embedded in
HTML Web pages
JavaScript:
Is an interpreted language
Supports event-driven programming
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Is object-based
Some of things that JavaScript cannot do!
The following file operations on the client computer:
Read
-- Modify
Rename -- Delete
Create
Create graphics (although, it does have the ability to format pages through HTML - including the
placement of graphics)
Any network programming bar one function: the ability to download a file to the browser specified
through an arbitrary URL
Some of the things that JavaScript can do!
Control the appearance of the browser
Control the content and appearance of the document displayed in the browser
Interact with the user through event handlers
Arbitrary calculations, including floating-point ones
Store & modify a limited amount of data about the user in the form of client-side "cookies"
18.5 Client-Side JavaScript
Everything that JavaScript manipulates, it Although a version of JavaScript exists that can be used to
write server-side scripts, our focus in this course will only be on client-side scripting
Case Sensitivity
HTML is not case sensitive. The following mean the same to the browser:
<HTML>
-- <html>
<Html>
-- <htMl>
JavaScript is case sensitive. Only the first of the following will result in the desired function ­ the rest
will generate an error or some other undesirable event:
onMouseClick-- OnMouseClick
onmouseclick -- ONMOUSECLICK
JavaScript
A programming language specifically designed to work with Web browsers
It is designed to be used for developing small programs ­ called scripts ­ that can be embedded in
HTML Web pages
JavaScript:
Is an interpreted language
Supports event-driven programming
Is object-based
JavaScript is Object-Based
treats as an object ­ e.g. a window or a button
An object has properties ­ e.g. a window has size, position, status, etc.
Objects are modified with methods that are associated with that object ­ e.g. a resize a window with
resizeTo(150, 200)
Not Object-Oriented!
JavaScript is not a true object-oriented language like C++ or Java
It is so because it lacks two key features:
A formal inheritance mechanism
Strong typing
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Nevertheless, it does include many key concepts that are part of almost all object-oriented languages,
and therefore is referred as an object-based language
Object: A named collection of properties
(data, state) & methods (instructions, behavior)
A collection
All objects have the
of properties
"name" property: it
& methods
holds the name of
the object
name
method 2
prop 1
prop 3
prop 5
prop 2
method 3
method 1 prop 4
Example: A Bicycle
name
accelerate()
color
pressure
direction
height
turn() park()
inflate()
speed
Example: JavaScript's "window" Object
name
open()
width
document
status
height
alert()
close()
location
18.6 Properties
Objects may have a single or several properties
A property may have one of the following values:
Number
-- Text
-- Boolean
Array
-- Functions
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Objects (Example: "document" ­ a property of the "window" object ­ is an object in itself. A
"document" in turn may contain a "form" object as a property, and then that "form" may contain a
"button" property, which, once again, is an object in itself)
Referring to a Property
dot
objectName.propertyName
Examples:
window.width
button.value
<HTML>
<HEAD>
<TITLE>Change Property Demo # 1</TITLE>
<SCRIPT>
function changeStatus() {
window.status = "Mouse has touched the button";
}
</SCRIPT>
</HEAD>
<BODY>
<H1>Change Property Demo # 1</H1>
<FORM name="dummy" method="" action="">
<INPUT type="submit" name="" value="Change Status"
onMouseOver="changeStatus()">
</FORM>
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</BODY>
</HTML>
The main code segment that goes between the
<SCRIPT>, </SCRIPT> tags in the HEAD:
function changeStatus() {
window.status="Mouse has touched the button";
}
new value
property
The JavaScript code included as an attribute of
the "Submit" button:
onMouseOver="changeStatus()"
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The main code segment that goes between the
<SCRIPT>, </SCRIPT> tags in the HEAD:
function gotoURL() {
window.location="http://www.vu.edu.pk/";
}
new value
property
The JavaScript code included as an attribute of
the "Go to VU" button:
onMouseOver="gotoURL()"
You should be connected to the Internet for the successful execution of the example that we just
discussed
A Suggestion
Please try the pieces of code that I just demonstrated to you to change the status and location properties
of the "window" object yourself
Also try changing the width, height properties of the "window" object
Types of Objects
JavaScript objects
Objects that are part of JavaScript
Examples: window, document
Browser objects
Objects that contain info not about the contents of the display, but the browser itself
Examples: history, navigator
User-defined object
One More Example:
Let us try to change the background color of the window
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The main code segment that goes between the
<SCRIPT>, </SCRIPT> tags in the HEAD:
function changeBgcolor() {
window.document.bgColor = "pink";
}
new value
property
The JavaScript code included as an attribute of
the "Change Color" button:
onMouseOver="changeBgcolor()"
In addition to "bgColor", there are many other properties of the "document" object, e.g.
cookie
fgColor
forms[ ]
linkColor
images[ ]
title
links[ ]
URL
...
referrer
...
lastModified
...
We have learnt how to modify the properties of objects
But the properties are not there just so that we can modify them; we can also just read them ­ that is just
find out their current value
Let us now look at an example where we first read a property, display the current value, and then
change the property
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The main code segment that goes between the <SCRIPT>, </SCRIPT> tags in the
HEAD:
function changeBgcolor() {
oldColor = window.document.bgColor;
window.document.bgColor = "pink";
window.alert("The old color was " + oldColor);
}
The JavaScript code included as an attribute of the "Change Color" button:
onMouseOver="changeBgcolor()"
Now we have established what we mean by objects: a named collection of properties and methods
And that JavaScript treats everything that it manipulates as an object
We have also learnt how to change the properties of these objects by selecting a property and equating it
to a new value
Methods: Functions (code, instructions, behavior) associated with objects
Methods are functions associated with an object that can be used to manipulate that object
Example:
window.close()
Here "close()" is a method that has been defined for the "window" object. Its function is to close the
"window"
Referring to a Method
dot
objectName.methodName( argumnets
)
Examples:  Info is
window.close() passed on to
button.click() the method
through one
or more
arguments
A few more methods associated with the
"window" object
alert()
confirm()
prompt()
close()
open()
focus()
blur()
setTimeOut()
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The main code segment that goes between the
<SCRIPT>, </SCRIPT> tags in the HEAD:
function vuWindow() {
window.open("http://www.vu.edu.pk/");
}
argument
method
The JavaScript code included as an attribute of
the "New VU Window" button:
different event handler
onClick="vuWindow()"
18.7 Event Handlers
Objects are made up of properties and associated methods
Many objects also have "event handlers" associated with them
"Events" are actions that occur as a result of user's interaction with the browser
We use "event handlers" [e.g. onMouseOver(), onClick()] to design Web pages that can react to those
events
More on event handlers in a future lecture
During Today's Lesson ...
We had a more formal introduction to JavaScript and client-side scripting
We became able to appreciate the concept of objects in JavaScript
We learnt about the properties of those objects
We also became able to perform simple tasks through the application of methods
Next (the 7th) Web Dev Lecture:
Data Types and Operators
To find out about data types
To become familiar with JavaScript data types
To become able to use JavaScript statements and arithmetic operators
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Table of Contents:
  1. INTRODUCTION
  2. EVOLUTION OF COMPUTING
  3. World Wide Web, Web’s structure, genesis, its evolution
  4. Types of Computers, Components, Parts of Computers
  5. List of Parts of Computers
  6. Develop your Personal Web Page: HTML
  7. Microprocessor, Bus interface unit, Data & instruction cache memory, ALU
  8. Number systems, binary numbers, NOT, AND, OR and XOR logic operations
  9. structure of HTML tags, types of lists in web development
  10. COMPUTER SOFTWARE: Operating Systems, Device Drivers, Trialware
  11. Operating System: functions, components, types of operating systems
  12. Forms on Web pages, Components of Forms, building interactive Forms
  13. APPLICATION SOFTWARE: Scientific, engineering, graphics, Business, Productivity, Entertainment, Educational Software
  14. WORD PROCESSING: Common functions of word processors, desktop publishing
  15. Interactivity to Forms, JavaScript, server-side scripts
  16. ALGORITHMS
  17. ALGORITHMS: Pseudo code, Flowcharts
  18. JavaScript and client-side scripting, objects in JavaScript
  19. Low, High-Level, interpreted, compiled, structured & object-oriented programming languages
  20. Software Design and Development Methodologies
  21. DATA TYPES & OPERATORS
  22. SPREADSHEETS
  23. FLOW CONTROL & LOOPS
  24. DESIGN HEURISTICS. Rule of thumb learned through trial & error
  25. WEB DESIGN FOR USABILITY
  26. ARRAYS
  27. COMPUTER NETWORKS: types of networks, networking topologies and protocols
  28. THE INTERNET
  29. Variables: Local and Global Variables
  30. Internet Services: FTP, Telnet, Web, eMail, Instant messaging, VoIP
  31. DEVELOPING PRESENTATIONS: Effective Multimedia Presentations
  32. Event Handlers
  33. GRAPHICS & ANIMATION
  34. INTELLIGENT SYSTEMS: techniques for designing Artificial Intelligent Systems
  35. Mathematical Functions in JavaScript
  36. DATA MANAGEMENT
  37. DATABASE SOFTWARE: Data Security, Data Integrity, Integrity, Accessibility, DBMS
  38. String Manipulations:
  39. CYBER CRIME
  40. Social Implications of Computing
  41. IMAGES & ANIMATION
  42. THE COMPUTING PROFESSION
  43. THE FUTURE OF COMPUTING
  44. PROGRAMMING METHODOLOGY
  45. REVIEW & WRAP-UP of Introduction to Computing