how to use javascript to detect when a radio box is ticked code example

Example: javascript radio button value if checked

//alert(document.querySelector('input[name = "comp"]:checked').value);

$test=document.querySelector('input[name = "comp"]:checked').value;

if($test="silver") {
        amount=50;
    }else if($test="gold") {
      amount=90;
    }else{
      amount=30;
    }