Javascript function called on checkbox change

Stuck at calling a simple JS function. Developer tools at chrome, console tab, thows

(index):591 Uncaught ReferenceError: checkbox is not defined at myFunction ((index):591) at HTMLInputElement.onclick

Code is

<span>Notificaciones</span>
    <input type="checkbox" id="myCheck" onclick="myFunction()">         
    <script type="text/javascript">
        function myFunction() {
              var checkBox = document.getElementById("myCheck");
              if(checkbox.checked){
                alert("suscribe");
              }  else{
                  alert("descuscribe");
              };
            };
    </script>