javascript InputTypeRadio Description

The radio object is a form control generated with an input element that has a type attribute set to "radio". radio objects related to each other are assigned the same name. This means all like-named radio objects become a collection (array) of radio objects. It may be necessary, therefore, to reference an individual radio button as an item in an array. The entire array, of course, has a length property you can use to assist in looping through all radio objects within the group, if necessary, to find which one is checked, and retrieve that object's value:

var radioGrp = document.forms[0].myRadio;
for (var i = 0; i< radioGrp.length; i++) {
    if (radioGrp[i].checked) {
        alert("The value of the chosen button is " + radioGrp [i].value);
    }
}