JavaScript

During the last lecture

• 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 for Web development

• Last time we mentioned server-side scripts; today we will write (simple) client-side scripts in JavaScript

But first lets review the tags that are used in forms

<FORM

name=“name”

method=“get” or “post”

action=“URL”

>

Elements of the form

</FORM>

Single-Line Text Input Field

<INPUTtype=“text”name=“name”size=“widthInCharacters”maxlength=“limitInCharacters”value=“initialDefaultValue”

>

Password Input Field

<INPUTtype=“password”name=“name”size=“widthInCharacters”maxlength=“limitInCharacters”value=“initialDefaultValue”

>

Hidden Input

<INPUTtype=“hidden” name=“name”value=“value”

>

Checkbox Input Element

<INPUTtype=“checkbox”

name=“name”

checked

value=“checkedValue”>

Radio Button Input Element

<INPUTtype=“radio”

name=“name”

checked

value=“selectedValue”>

File Upload Input Element

<INPUTtype=“file”

name=“name”

value=“nameOfSelectedFile”

enctype=“fileEncodingType”>

Reset Button Input Element

<INPUTtype=“reset”

value=“buttonLabel”>

Submit Button Input

<INPUTtype=“submit” name=“name”value=“buttonLabel”

>

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

1. text2. password3. hidden4. checkbox5. radio6. file7. reset8. submit

Multi-Line Text Input Area

<TEXTAREAname=“areaName”cols=“width”rows=“lines”

>

initial default value

</TEXTAREA>

Select from a (Drop Down) List<SELECT

name=“name”

size=“numberOfDisplayedChoices”

multiple

>

<OPTION value=“value1”> text1

<OPTION value=“value2” selected> text2

<OPTION value=“value3”> text2…

…

</SELECT>

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

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

Info containedin the form

Acknowledgement

Message to the receiver’s eMail address

User’s Computer

Web Server

Server-Side Script

Browser

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:

– ali2umt.edu.p

– bhola@hotmail.con

– bhola@yahoo

– ali@umt.com

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 re-send

This process …• That is the process of the 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

• Examples:– 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

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

A Simple Example of Client-Side Scripting

<INPUT type=“submit” name=“sendEmail” value=“Send eMail”>

Code for the simple “Send eMail” button as was described during the last lecture

<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 would not allow itself to be clicked if the “From” text field is left blank

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

Event Handler

• 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 more 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

• JavaScript:– 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)

Scripting Languages

• Code is interpreted as it is loaded in client

What You Need to Know

• HTML

• Text editors

• Web browsers

• Different versions of JavaScript

How to Use JavaScript?

• JavaScript placed within HTML code

• Use the HTML SCRIPT tag

• JavaScript can be placed in either the ‘Head’ or ‘Body’ portion of an HTML document

SCRIPT Tag

• Comes in in pairs

<SCRIPT> Opening SCRIPT tag

</SCRIPT> Closing SCRIPT tag

• Tells browser where script begins and ends

• Identifies scripting language and version

• Specifies address for an external JavaScript file

Case Sensitivity

• SCRIPT tag is NOT case sensitive

• JavaScript inside SCRIPT tag IS case sensitive

JavaScript Rules

• Parentheses– Required by JavaScript functions– The document.write method is a function that takes an argument

contained in parentheses

• Quotation Marks– Denote a string of text in JavaScript (a string is a type of variable)

• Semicolon– Signals end of JavaScript statement (a statement is a portion of

code that does not need anything added to it to be complete in its syntax)

• Syntax– Form and order of programming code

Calling External Scripts

• External JavaScript file– Text file that contains JavaScript code– Saved with ‘.js’ extension– Saves time coding a script into each page

Hiding JavaScript from Older Browsers

• Older browsers may not recognize JavaScript– May dump entire JavaScript code as text on screen

• Tell older browsers to ignore JavaScript– Use HTML comments between ‘SCRIPT’ tags

(<!--) Opening HTML comment code

(//-->) Closing HTML comment code

JavaScript Comments

• Benefits– To add notes– To look for error in script

• Single-line comments// Your comments go here

• Multi-line comments(/*) Opening code(*/) Closing code

Back to our example …

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

<INPUT type=“submit” name=“sendEmail” value=“Send eMail” onMouseOver=“checkForm()”>

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

<INPUT type=“submit” name=“sendEmail” value=“Send eMail” onMouseOver=“checkForm()”>

checkForm()

• 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.

<HTML><HEAD><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”> … body content …</BODY></HTML>

JavaScript code enclosed in the new <SCRIPT>,</SCRIPT> HTML tags

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

Let’s again look at the JavaScript code

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()”

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 for Web development

• Last time we mentioned server-side scripts; today we wrote (simple) client-side scripts in JavaScript

Next 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