Other Form Object Properties and Methods - Page 4

The HTML elements commonly found in forms, which we will look at in more detail shortly, also have corresponding objects. One way of accessing these is through the elements[] property of the Form object. This is an array just like the forms[] array property of the document object that we have just seen. The elements[] array contains all the objects corresponding to the HTML interaction elements within the form, with the exception of the little used <INPUT TYPE=image> element. As we'll see later, this property is very useful for looping through each of the elements in a form. For example, we could loop through each element checking that it contains valid data prior to submitting the form.

Being an array, the elements[] property of the Form object has the length property, which tells us how many elements are in the form. The Form object also has the length property, which also gives us the number of elements in the form. Which of these you use is up to you since both do the same job, although writing document.myForm.length is shorter, and so quicker to type and less lengthy to look at in code, than document.myForm.elements.length.

When we submit data from a form to a server, we normally use the submit button, which we will come to shortly. However, the Form object also has the submit() method which does nearly the same thing. It differs in Netscape Navigator since it does not call the onsubmit event handler for the submit event of the Form object.

Recall that in the last chapter we saw how return values passed back from an event handler's code can affect whether the normal course of events continues or is cancelled. We saw, for example, that returning false from a hyperlink's onclick event handler causes the link's navigation to be cancelled. Well, the same principle applies to the Form object's onsubmit event handler, which fires when the user submits the form. If we return true to this event handler, then the form submission goes ahead; if we return false then the submission is cancelled. This makes the onsubmit event handler's code a great place to do form validation; checking that what the user has entered into the form is valid. For example, if we ask for their age and they enter "mind your own business" we can spot that this is text rather than a valid number, and stop them from continuing. We'll see this in action when we look at server-side scripting in Chapter 15.

As well as there being a reset button, which we will discuss later in the chapter, theForm object has the reset() method, which clears the form, or restores default values if these exist.

Creating blank forms is not exactly exciting or useful, so now let's turn our attention to the HTML elements that provide interaction functionality inside our forms.

HTML Elements in Forms

There are about ten elements commonly found within <FORM> elements. The most useful are shown below, ordered into general types. We give each its name and, in parentheses, the HTML needed to create it, though note this is not the full HTML but only a portion.

As you can see, most of the form elements are created using the <INPUT> tag. One of the <INPUT> tag's attributes is the TYPE attribute. It's this attribute that decides which of the HTML elements this will be. Examples of values for this attribute include button, to create a button, and text to create a text box.

Each form element inside the web page is made available to us as, yes, you guessed it, an object. As with all the other objects we have seen, each element's object has its own set of distinctive properties, methods and events. We'll be taking a look at each form element in turn and how to use its particular properties, methods, and events. But, before we do that, let's look at properties and methods that the objects of the form elements have in common.

Try It Out: The Forms Array - Page 3
Beginning JavaScript
Common Properties and Methods - Page 5