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CS101 - Introduction to Computing - Lecture Handout 15

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More on Interactive Forms

(Web Development Lecture 5)

Focus of the last lecture was on Interactive Forms

We looked at the utility of forms on Web pages
We found out about the various components that are used in a form
We became able to build a simple, interactive form

In Today’s Lecture …

We will learn ways of adding more interactivity to forms
We will get 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
Last time we mentioned server-side scripts; today we will write (simple) client-side scripts in JavaScript

Single-Line Text Input Field


Password Input Field


Hidden Input

type=“hidden” name=“name”

Checkbox Input Element


Radio Button Input Element


File Upload Input Element



Reset Button Input Element


Submit Button Input

type=“submit” name=“name”
8 Possible Values for the “type” Attribute of <INPUT> tag

Multi-Line Text Input Area

initial default value

Select from a (Drop Down) List

<OPTION value=“value1”> text1
<OPTION value=“value2” selected> text2
<OPTION value=“value3”> text2


Drop Down

End of the Review of Tags Used in Forms
Now let’s take a look at a form that we constructed last time, and see how we can make it better

Send e Mail

Let’s now review what happens when I enter all the required info and press the “Send eMail” button?

form is filled correctly

This is what happens when the form is filled correctly. What if the form is filled incorrectly?
What if the users leaves one of the essential fields, blank?
What if the user enters an illegal eMail address? Examples:
This email address is being protected from spambots. You need JavaScript enabled to view it.

A Reasonable Scenario

When the “Send eMail” button is clicked, the browser sends the data collected through the form to a script running on the Web server
That server-side script:
receives that data
analyzes it
discovers the missing or incorrect data
sends a message back to the user’s browser stating the problem and asks the user to resend

This process …

That is the process of user:
Filling the incomplete/incorrect data
Sending it to the server
Receiving the response back from the server
Correcting and resending
is quite time-consuming and uses the server’s resources to help the user correct his mistakes
It can really bog down the server if a large number of users are using that Web server

Client-Side Scripting is a viable alternate

In this technique, one uses the user’s browser to checking the form data
If data is missing or is incorrect, the browser can prompt the user to take corrective action
This way, the form data is sent to the server-side script only after it has been established that the collected data is complete and correct

Server-Side Scripts: Review

Are programs that reside on Web servers
Receive info that a user enters in a form
Process that info and take appropriate action
CGI scripts on Unix servers
ASP scripts on Windows servers

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

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


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


Disadvantages 1

This is one way of including JavaScript code in an HTML document – that is, including a short JavaScript segment as part of an HTML tag
There are a few others as well. Let’s now find out about another.
But before we do that …
… let’s just make clear why we are interested in including JavaScript in our Web pages

Why JavaScript?

HTML is great for static Web pages; however, supports only rudimentary interactivity through forms and hyperlinks
JavaScript can be used (along with HTML) to develop interactive content for the Web

What is 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
Is an interpreted language
Supports event-driven programming
Is object-based

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.
Properties are modified with methods – e.g. a resize a window with resizeTo(150, 200)

Object Based

value=“Send eMail”
“if (document.sendEmail.sender.value.length < 1)
window.alert(‘Empty From field! Please correct’)”
value=“Send eMail”
value=“Send eMail”
“if (document.sendEmail.sender.value.length < 1)
window.alert(‘Empty From field! Please correct’)”


JavaScript understands onMouseOver – it is one of the pre-defined keywords in JavaScript
However, the case for checkForm() is different
A checkForm() function does not exist in JavaScript. Therefore, we will have to define it ourselves
It can either be defined in the HEAD portion or BODY portion. We will do it in the HEAD.


We have looked at 2 techniques for embedding JavaScript code in a Web page:

  1. Put the code in the tag for the “Send eMail” button - as was shown to you earlier
  2. Put the checkForm() code in the HEAD portion & put the onMouseOver=“checkForm()” attribute in the tag for the “Send eMail” button

Both perform the required function satisfactorily.

Q: Which of two techniques is better?

The “put all code in the tag” technique seems to require less code
For very short scripts, “all code in the tag” works well. However, this technique does not work when one needs to put multiple script statements in the same tag
The “code in the HEAD portion” is more general-purpose, and the right choice for developing larger JavaScript scripts
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:
Today we checked if the “From” field of the form was empty
How can we modify the JavaScript code for checking if the “To” field is empty as well?
How about checking all four fields?
How about checking if the addresses given in the “From” and “To” fields are legal eMail addresses?
Please try thinking about those possibilities?

In Today’s Lecture …

We learnt ways of constructing forms that were a bit more interactive
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
Last time we mentioned server-side scripts; today we wrote (simple) client-side scripts in JavaScript

Next (the 6th) Web Dev Lecture:

JavaScript Object, Properties, Methods

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
We will become able to perform simple tasks through the application of methods