title: Javascript disable button until all fields are filled
body: <p>would appreciate some help solving this. 
  Need to make the event listener work for all the element. So basically when you fill out name, last-name i.e all the fields then only the button should get enabled, Even if one of the fields is empty the button should get disabled.</p>
  
  <pre><code> (function () {
   "use strict";
  
   var knapp = document.getElementById('skicka');
  
   knapp.disabled = true;
  
   var f=document.getElementById('fornamn');
   var e=document.getElementById('efternamn');
   var p=document.getElementById('passnr');
   var n=document.getElementById('nat');
  
   e.addEventListener('change',function(){
  
    if(e.value==='' ){
  
   knapp.disabled=true;
     }
  
   else{
    knapp.disabled=false;
   }
    });
  
    })();
  </code></pre>