en anden måde
 
<!DOCTYPE html>
<html>
<body>
<form id="frm" action="">
  <input type="radio" name="gender" value="male">     Male<br>
  <input type="radio" name="gender" value="female"> Female<br>
  <input type="radio" name="gender" value="other"> Other
  
  <input type="hidden" name="gender_text" id="gender_text"> 
  
</form>
<script>
var element = document.getElementById("frm");
element.addEventListener("change", myFunction);
function myFunction() {
    var elm = document.getElementById("gender_text");
    var val = document.querySelector('input[name="gender"]:checked').value;
    var input_type = (val == "other")? "text" : "hidden";
    elm.type=input_type;    
}
</script>
</body>
</html>