Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

15.1 The Form Object

The JavaScript
Form
object represents an HTML form. Forms are always found as elements of
the forms[] array, which
is a property of the Document object. Forms appear in this array in
the order in which they appear within the document. Thus,
document.forms[0] refers to the first form in a
document. You can refer to the last form in a document with the
following:

document.forms[document.forms.length-1]

The most interesting property of the Form object is the
elements[] array,
which contains JavaScript objects (of various types) that represent
the various input elements of the form. Again, elements appear in
this array in the same order they appear in the document. So you can
refer to the third element of the second form in the document of the
current window like this:

document.forms[1].elements[2]

The remaining properties of the Form
object are of less importance. The
action ,
encoding, method, and
target properties correspond directly to the
action, encoding,
method, and target attributes
of the <form> tag. These properties and
attributes are all used to control how form data is submitted to the
web server and where the results are displayed; they are therefore
useful only when the form is actually submitted to a server-side
program. See the client-side reference section for an explanation of
the properties, or see a book on HTML or CGI programming[2] for a thorough discussion of the attributes. What is
worth noting here is that these Form properties are all read/write
strings, so a JavaScript program can dynamically set their values in
order to change the way the form is submitted.

[2] Such as

CGI Programming on the World Wide
Web , by Shishir Gundavaram (O'Reilly).

In the days before JavaScript, a form was
submitted with a special-purpose

Submit button, and form elements had their
values reset with a special-purpose

Reset button. The JavaScript Form object
supports two methods, submit(
) and
(as of JavaScript 1.1) reset( ), that serve the
same purpose. Invoking the submit( ) method of a
Form submits the form, and invoking reset( )
resets the form elements.

To
accompany the submit( ) and reset(
) methods, the Form object provides the
onsubmit event handler to detect form
submission and (as of JavaScript 1.1) the
onreset event handler to detect form resets.
The onsubmit handler is invoked just before the
form is submitted; it can cancel the submission by returning
false. This provides an opportunity for a
JavaScript program to check the user's input for errors in
order to avoid submitting incomplete or invalid data over the network
to a server-side program. We'll see an example of such error
checking at the end of this chapter. Note that the
onsubmit handler is triggered only by a genuine
click on a

Submit button. Calling
the submit( ) method of a form does not trigger
the onsubmit handler.

The onreset event handler
is similar to the onsubmit handler. It is invoked
just before the form is reset, and it can prevent the form elements
from being reset by returning false. This allows a
JavaScript program to ask for confirmation of the reset, which can be
a good idea when the form is long or detailed. You might request this
sort of confirmation with an event handler like the following:

<form...
onreset="return confirm('Really erase ALL data and start over?')"
> 

Like the onsubmit handler,
onreset is triggered only by a genuine

Reset button. Calling the reset(
) method of a form does not trigger
onreset.