Building Forms with JavaScript

The ability to interact with, and obtain information from web site visitors, is a central theme behind the JavaScript language. No JavaScript book would be complete, therefore, without exploring how to handle Graphical User Interface Componments (GUI) objects such as buttons, text input fields, checkboxes, radio boxes and selections, together with the Form object which brings them all together.

The JavaScript Form object serves a number purposes. It acts a container for all of the GUI objects that make up a form. It also acts as the mechansism for grouping together the information provided by the user and sending it back to the server for processing.

The <form> takes a number of attributes of which the following are the most commonly used:

• name - the name of the form, which will be used when referencing objects within the the form four JavaScript code.

• method - the method by which data is sent to the server. This will either be "GET" ot "POST" depdniong on the server-side configuration.

• action - the URL of the CGI script to which the data entered by the user will be sent. It is also possible to use mailto: to have the data sent to an email address.

The following is an example of a Form object named registerForm which submits the user data to a cgi script called register.cgi located in the /cgi-bin directory on the server which hosts the web site. The transfer is configured to use the GET method:

<form name="registerForm" method="GET" action="/cgi-bin/register.cgi">

<-- Form Objects go here -->

</form>

As we mentioned in the previous section the Form object is really just a container which contains other objects (such as text fields and buttons). Later in the chapter we will learn how to create those objects inside the Form object. First, however, it is useful to explore how to access the oibjects in a Form.

One way to access the elements in a Form is to use the Form object's elements property. This is an array containing all the objects in a Form (see JavaScript Arrays for details). We can, therefore, access any element in the array by using the index (note that the objects in the array are in the order in whcih they are specified in the Form. Suppose we have a text object which is the first object inside a Form called myForm. We can access this object as follows:

document.myForm.elements[0]

A much easier approach is to access the object using the name that was assigned to it at creation. This is achieved using the getElementById(). Assuming in our first example the Text object was assigned a name of phoneNumber we could asssign the object to a variable as follows:

phone = document.getElementById("phoneNumber");

Now that we have covered the basics of the JavaScript Form Object it is time to look at the objects we can place into the Form container to interact with the user. The most common of these objects is the Text object. The Text object places a single line text field into tyhe form into which the user can type information (such a name, address or telephone number).

The syntax for creating a text object within a Form is as follows:

<input type="TEXT" name="objectname" value="current value" size="30" event handling>

• type - specifies that the type of object is a TEXT object.

• name - repesents the name by which this object may be reference in JavaScript.

• value - primes the text object with an initial value (optional).

• size - specifies the maximum number of characters that can be entered into the Text field.

event handling specifies what action to take when a particular event on the object is triggered. Events that can be triggered by the Text object are:

• onFocus - triggered when the text field gains focus (typically when the cursor is moved into the field to begin typing).

• onBlur - triggered when the text field loses focus (typically when the user clicks somewhere outsite the text area).

• onChange - triggered when the contents of the text area is changed by the user and focus is lost.

• onSelect - triggered when the user highlights text in the text field.