The Book of JavaScript, 2nd Edition

Radio

The radio button form element. Radio buttons given the same name are considered a set and are stored in an array with the set's name. A visitor can select only one radio button of the set at any given time. If a web page has five radio buttons named favorite_color, the second radio button in the set is referred to as:

window.document.the_form.favorite_color[1]

Properties

checked

FF, IE 3

true if a visitor has selected the radio button and false otherwise. Setting the property to true causes the radio button to act as if a visitor selected the button.

Example:

if (window.document.the_form.favorite_color[3].checked == true)
{
    alert("I like that color too!");
}

This if-then statement calls an alert box if a visitor selects the fourth radio button named ...