17.1 Forms in CGI and JavaScript

If you are already familiar with CGI programming using HTML forms, you may find that things are done somewhat differently when forms are used with JavaScript. In the CGI model, a form, and the input data it contains is "submitted"--sent 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. Forms and the input elements they contain each have event handlers that JavaScript can use to respond to user interactions with a 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 CGI programs, an HTML form can't be useful unless it has a Submit button (or unless it has only a single text input field and allows the user to strike 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 CGI program, of course). With JavaScript, your forms 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 a button can trigger: replacing one image with another, using the location property to load and display a new web page, opening a new browser window, or dynamically generating a new HTML document in another window or frame. As we'll see later in this section, 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 used with forms or form elements. The following subsections introduce the JavaScript Form object, and the various JavaScript objects that represent form elements. The section concludes with an example that illustrates how you can use JavaScript to validate user input on the client before submitting it to a CGI program running on the web server.