WEB PROGRAMMING UNIT-II NOTES

                    UNIT – II

              Overview :

Unit-II demonstrates on client side validations using JavaScript. It focuses on how to handle events, exceptions, etc. This unit also focuses on DHTML concepts.

               CONTENTS:

1)                JavaScript concepts

2)                Objects in java scripts

3)                DHTML with JavaScript

JavaScript (also called JScript) is a scripting language with the primary aim of giving life to our web pages.It is very powerful, flexible, and easy to learn.

     Features

Ø  Imperative and structured

Ø  Dynamic

o   dynamic typing

o   object based

o   run-time evaluation

Ø  Functional

o   first-class functions

o   nested functions

o   closures

Ø  Prototype-based

Ø  Miscellaneous

Ø  Vendor-specific extensions

There are three ways by which we can place Javascript for use in a web page.

1.  Inside the head section.

2.  Within the body section.

3.  In an external file.

            The HTML <script> tag is used to insert a Java Script into an HTML page.

Events

By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by Java Script. Every element on a dynamic or static web page has certain events which can trigger Java Script functions. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on that button. We define the events in the HTML tags. Examples of events: A mouse click A web page or an image loading Mousing over a hot spot on the web page Selecting an input box in an HTML form Submitting an HTML form A keystroke Events are normally used in combination with functions, and the function can not be executed before the event occurs.

JavaScript - Catching Errors

There are two ways of catching errors in a Webpage

Try...Catch Statement

The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.

try

{

//Run some code here

}

catch(err)

{

//Handle errors here

}

The onerror Event

The onerror event can be explained soon, but first you will learn how to use the throw statements to create an exception. The throw statements can be used together with the try...catch statement.

The Date object

The Date class is used to store and retrieve dates in JavaScript.

Array

The Array object is used to holding a set of data or values in a single variable name. var urArray=new Array()

DOM (document object model)

A DOM (document object model) is an application programming interface (API) for representing a document (such as an HTML document) and accessing and manipulating the various elements (such as HTML tags and strings of text) that make up that document. Java Script-enabled web browsers have always defined a document object model; a web-browser DOM may specify, for example: that the forms in an HTML document are accessible through the forms[] array of the Document object.

form validation

Form validation is the process of checking that a form has been filled in correctly or not before it is processed.

There are two methods

 1: Client-Side validation

In Java Script Client-side form validation is an important part of a web site where data needs to be collected from the user. Users are innately ignorant, and will mess up data entry in a web form if given the chance. It is the job of the web programmer, then, to make sure his pages which use forms include client-side form validation using JavaScript.

2: Server-side validation

In Java Script the server also benefits from client-side validation since it saves a number of round-trips between the visitor and the server owing to typos and easily spotted mistakes. This advantage does not alleviate the neccessity of doing server-side validation.