JSFiddle - React, Tailwind, and code Playground

HTML

<input type="radio" value="18" checked="true" name="ivacontract"> 18%
<input type="radio" value="16" name="ivacontract"> 16%

JavaScript

function getCheckedValues(objName)
    {   
        var arr = new Array();
        arr = document.getElementsByName(objName);
            
        for(var i = 0; i < arr.length; i++)
        {
            var obj = document.getElementsByName(objName).item(i);
            
            if(obj.checked)
            {
                alert(obj.value);
            }
        }
    }

getCheckedValues("ivacontract");