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Chapter 15. Forms and Form Elements

As we've seen in examples throughout this book, the use of HTML forms is basic to almost all JavaScript programs. This chapter explains the details of programming with forms in JavaScript. It is assumed that you are already somewhat familiar with the creation of HTML forms and with the input elements that they contain. If not, you may want to refer to a good book on HTML.[50] The client-side reference section of this book lists the HTML syntax along with the JavaScript syntax for forms and form elements; you may find these useful for quick reference.

[50]Such as HTML: The Definitive Guide, by Chuck Musciano and Bill Kennedy (O'Reilly).

If you are already familiar with server-side programming using HTML forms, you may find that things are done differently when forms are used with JavaScript. In the server-side model, a form with the input data it contains is submitted to the web server all at once. The emphasis is on processing a complete batch of input data and dynamically producing a new web page in response. With JavaScript, the programming model is quite different. In JavaScript programs, the emphasis is not on form submission and processing but instead on event handling. A form and all input elements in it have event handlers that JavaScript can use to respond to user interactions within the form. If the user clicks on a checkbox, for example, a JavaScript program can receive notification through an event handler and might respond by changing the value displayed in some other element of the form.

With server-side programs, an HTML form isn't useful unless it has a Submit button (or unless it has only a single text input field and allows the user to press the Return key as a shortcut for submission). With JavaScript, on the other hand, a Submit button is never necessary (unless the JavaScript program is working with a cooperating server-side program, of course). With JavaScript, a form can have any number of push buttons with event handlers that perform any number of actions when clicked. In previous chapters, we've seen some of the possible actions that such buttons can trigger: replacing one image with another, using the location property to load and display a new web page, opening a new browser window, and dynamically generating a new HTML document in another window or frame. As we'll see later in this chapter, a JavaScript event handler can even trigger a form to be submitted.

As we've seen in examples throughout this book, event handlers are almost always the central element of any interesting JavaScript program. And the most commonly used event handlers (excluding the event handlers of the Link object) are those used with forms or form elements. This chapter introduces the JavaScript Form object and the various JavaScript objects that represent form elements. It concludes with an example that illustrates how you can use JavaScript to validate user input on the client before submitting it to a server-side program running on the web server.

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[51] 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.

[51]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.



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