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.^[1] 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.

^[1] 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.