/** Vanilla javascript **/
// Get the value of the checked radio button, undefined on not checked or not found
let radioButtonValue = (document.querySelector("input[name='radioName']:checked") || {}).value || undefined
// Check radio button with value ...
(document.querySelector("input[name='radioName'][value='A']") || {}).checked=true;
/** jQuery **/
// Get the value of the checked radio button, undefined on not checked or not found
let radioButtonValue = $("input[name='radioName']:checked").val();
// Check radio button with value ...
$("input[name='radioName'][value='A']").prop('checked',true);