How to give a point when answer is correct with javascript

Question

I have a (small) problem, I tried looking on the internet, but can't find the answer. I'm new with javascript.

HTML:

    <form name="examen">
     <input type="radio" id="1a" name="1" value="">Answer 1<br></input>
     <input type="radio" id="1b" name="1" value="">Answer 2<br></input>
     <input type="radio" id="1c" name="1" value="">Answer 3<br></input>
     <input type="radio" id="1d" name="1" value="">Answer 4<br></input>
     <input type="button" name="knop" value="Bereken" onClick="calculatie()">
    </form>

JAVASCRIPT:

 function calculatie() {
  var calc = "Aantal vragen goed: ";
  var totaal = parseInt(0);
     if(document.examen.1c.checked){totaal += parseInt(1);} 
  calc += Math.round(totaal);
   alert(calc);
  }

Now, when a user is clicking on answer 3, the total points need to be one. But it is saying NaN, I can't get it right. The code needs to be in JAVASCRIPT/HTML. I can't use PHP.

Answer 1

try this function ...

function calculatie() { 
  var calc = "Aantal vragen goed: ";
  var totaal = 0;
     if(document.getElementById('1c').checked == true){
         totaal += 1;} 
       calc += Math.round(totaal);
       console.log(calc); //see console log for output or use it as u want
  }

HERE is the fiddle