I think it is very simple way.

var x = confirm("Are you sure you want to submit?");
if(x){
    if(navigator.onLine == true) {
        return true;
    } else {
        alert('Internet connection is lost');
        return false; 
    }
}
else {
    return false;
}

I think it is a very simple way.

var x = confirm("Are you sure you want to submit?");
if (x) {
  if (navigator.onLine == true) {
    return true;
  }
  alert('Internet connection is lost');
  return false;
}
return false;