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JavaScript: The Definitive Guide

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Radio Element

Name

Radio Element---a graphical radio button

Availability

Navigator 2.0, Internet Explorer 3.0; enhanced in Navigator 3.0

Synopsis

The Radio button element is always used in groups of mutually exclusive options that have the same name. To reference one Radio element within a group, use this syntax:

form.radio_name[j]
form.radio_name.length
form.elements[i][j]
form.elements[i].length

Properties

checked

A read/write Boolean value that specifies whether the button is checked or not.

defaultChecked

A read-only Boolean that specifies the initial state of the radio button. May be specified with the HTML CHECKED attribute.

form

A read-only reference to the Form object that contains the Radio element.

name

A read-only string, set by the HTML NAME attribute, that specifies the name of the Radio button.

type

A read-only string that specifies the type of this form element. For Radio elements, it has the value "radio". Available in Navigator 3.0 and later.

value

A read/write string, initially set by the HTML VALUE attribute, that specifies the value returned by the Radio button if it is selected when the form is submitted.

Methods

blur()

Removes keyboard focus from the radio button.

click()

Simulates a click on the radio button.

focus()

Gives keyboard focus to the radio button.

Event Handlers

onblur()

Invoked when the radio button loses keyboard focus.

onclick()

Invoked when the radio button is clicked.

onfocus()

Invoked when the radio button is given keyboard focus.

HTML Syntax

A Radio element is created with a standard HTML <INPUT> tag, with the addition of the new onClick attribute. Radio elements are created in groups by specifying multiple <INPUT> tags that have the same NAME attribute.

<FORM>
    ...
  <INPUT
    TYPE="radio"    specifies that this is a radio button
    [ NAME="name" ]   a name that can later be used to refer to this button...
    ...or to the group of buttons with this name
    specifies the name property
    [ VALUE="value" ] the value returned when this button is selected
    specifies the value property
    [ CHECKED ] specifies that the button is initially checked
    specifies the defaultChecked property
    [ onClick="handler" ] JavaScript statements to be executed when the button
    is clicked
  >
label the HTML text that should appear next to the button
    ...
</FORM>

Description

The Radio element represents a single graphical radio button in an HTML form. A radio button is one button in a group of buttons that represent mutually exclusive choices. When one button is selected, the previously selected button becomes deselected.

The onClick event handler allows you to specify JavaScript code to be executed when the button is checked or unchecked.

You can examine the checked property to determine the state of the button, and you can also set this property to select or deselect the button. Note that setting checked changes the graphical appearance of the button, but does not invoke the onClick event handler. The initial value of the checked property, and the value of the defaultChecked property are determined by the CHECKED attribute. Only one Radio element in a group may contain this attribute--it sets the checked and defaultChecked properties true for that element and false for all other Radio buttons in the group. If none of the elements have the CHECKED attribute, then the first one in the group will be checked (and defaultChecked) by default.

Note that the text that appears next to a Radio button is not part of the Radio element itself, and must be specified externally to the Radio's HTML <INPUT> tag.

It is good programming style to specify the NAME attribute for a button, and is mandatory if the radio button is part of a form that will submit data to a CGI script running on a web server. Specifying a NAME attribute sets the name property, and also allows you to refer to the button by name (instead of as a member of the form elements array) in your JavaScript code, which makes the code more modular and portable.

Radio elements are used in groups of mutually exclusive options, and each member of the group is given the same NAME attribute (the shared name defines the members of the group). So if the shared name of a group of Radio elements in form f is "opts", then f.opts is an array of Radio elements, and f.opts.length is the number of elements in the array.

Unfortunately, in Navigator 2.0, there is a bug in how Radio elements in a group are assigned to an array. See the "Bugs" section for details.

You can set the VALUE attribute or the value property of a Radio element to specify the string that will be passed to the server if the Radio element is checked when the form is submitted. Each Radio element in a group should specify a distinct value so that a script on the server can determine which one was checked when the form was submitted.

Usage

Radio elements can be used to present the user with a list of multiple, mutually exclusive, options. Use the Checkbox element to present a single option or to present a list of options that are not mutually exclusive.

Bugs

As described above, when a group of Radio elements share the same NAME attribute, JavaScript assigns them to an array that bears that name. Unfortunately, there is a bug in this process in Navigator 2.0: if the radio elements do not have event handlers specified with the onClick attribute, then they are assigned to the array in reverse order. This is counter-intuitive, and is an incompatibility with Navigator 3.0 in which the bug is fixed.

The workaround is to always assign an event handler, if only a dummy one, to all of your Radio elements that will be manipulated with JavaScript. You can do this by including onClick="0" in the <INPUT> tag for each Radio element you define. With this workaround, you can ensure that the elements will be assigned to the array in the same order in Navigator 2.0 and Navigator 3.0.

At least some versions of Navigator do not correctly select the first radio button in a group when no buttons in the group have the CHECKED attribute specified. Therefore you should always explicitly specify the CHECKED attribute for one default selection in every radio button group.

See Also

"Checkbox", "Element", "Form"


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