In order to get the value of a radio button using JavaScript, we need to loop through all the radio buttons in that set and then find out the value of the radio button which is checked.
For eg:

<form name="frmRadio" method="post">
<input name="choice" value="1" type="radio" />1
<input name="choice" value="2" type="radio" />2
<input name="choice" value="3" type="radio" />3
<input [...]