function checkRequired() {
  if (document.getElementById) {
    for (i = 0; i <fields.length; i++) {
      if (checkValue(fields[i]) == false) { return false; }
    }
  }
  return true;
}

function checkValue(fieldName) {
  var el = document.getElementById('fm_' + fieldName);
  if (el.value.length == 0) {
    alert('* Vul alle verplichte velden in');
    el.focus();
    return false;
  }
  return true;
}