Introduction to JavaScript + More on Interactive Forms

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

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 ” button?

Info contained in the form Acknowledgement Message to the receiver’s 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 address? Examples: –imran2umt.edu.p

A Reasonable Scenario When the “Send ” 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

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

Additional JavaScript code for the smart “Send ” button that would not allow itself to be clicked if the “From” text field is left blank

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)

Back to our example …

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.

Send an function checkForm() { if ( document.send .sender.value.length < 1) { window.alert( “Empty From field! Please correct” ); } } … body content … JavaScript code enclosed in the new, 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 ” 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 ” 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

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 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