Suppose you have the following radio controls:

<input name="Book" type="radio" value="asp" checked="true">ASP .NET</input>
<input name="Book" type="radio" value="wcf">Windows Presentation Foundation</input>
<input name="Book" type="radio" value="slg">Silverlight 2.0</input>

How do you get the selected value via JavaScript?
As you can see in the example, radio controls are declared with a name attribute (not an id). The browser treats each radio input as a different element, but the common name value groups them logically.

The getElementsByName Method

W3C Documentation — This method allows you to obtain a collection of controls from the DOM by specifying the common name value.

Here is the code to get or set the checked value:

function getRadio(name) {
    var radios = document.getElementsByName(name);
    for (var i = 0; i < radios.length; i++) {
        if (radios[i].checked) {
            return radios[i].value;
        }
    }
    return null;
}

function setRadio(name, val) {
    var radios = document.getElementsByName(name);
    for (var i = 0; i < radios.length; i++) {
        radios[i].checked = (radios[i].value == val);
    }
}