Answers for "how to select the checked radio button in js"

javascript uncheck radio button

document.getElementById("radioButtonID").checked = false; //uncheck a radio button
document.getElementById("radioButtonID").checked = true; //check a radio button

get value of selected radio button using javascript

document.getElementById('submit').onclick = function() {
    var radios = document.getElementsByName('contact');
    for (var radio of radios)
    {
        if (radio.checked) {
            alert(radio.value);
        }
    }
}