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CS101 Introduction to Computing

Lecture 18

Objects, Properties, Methods

(Web Development Lecture 6)

During the last lecture 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 Lecture …

• 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 (^3)

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 2 nd^ 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

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()”

}

Send an eMail

From:
To:
Subject:
Message:

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 (^8)

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

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

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

Some of things that JavaScript cannot do!

• The following file ops. 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 (^) Docsity.com 12

Some of the things that JavaScript can do!

1. Control the appearance of the browser
2. Control the content and appearance of the document displayed in the browser
3. Interact with the user through event handlers
4. Arbitrary calculations, including floating-point ones
5. Store & modify a limited amount of data about the user in the form of client-side “cookies” (^13)

Client-Side JavaScript

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: - -- - --
• 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

• Everything that JavaScript manipulates, it 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
• 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)

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prop 1 prop 2

prop 5

name prop 3

prop 4

A collection of properties & methods

All objects have the “name” property: it holds the name of the object (collection)

method 1^ method 3

method 2

Example: A Bicycle

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color

height

direction

name pressure

inflate() speed turn()

accelerate()

park()

Example: JavaScript’s “window” Object

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width height

status

name document

close() location alert()

open()

Properties

• Objects may have a single or several properties
• A property may have one of the following values:
  • Number -- Text -- Boolean
  • Array -- Functions
  • 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

objectName.propertyName

Examples:

window.width button.value

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dot