JSFiddle - React, Tailwind, and code Playground

HTML

<input type="radio" value="1" name="selection"/>
<input type="radio" value="2" name="selection"/>
<input type="radio" value="3" name="selection"/>
<input type="radio" value="4" name="selection"/>
<input type="button" onclick="alert(getRadioValue('selection'));" value="Which?"/>

JavaScript

function getRadioValue(name) {
    var group = document.getElementsByName('selection');

    for (var i=0;i<group.length;i++) {
        if (group[i].checked) {
            return group[i].value;
        }
    }

    return '';
}