window.onload = init; function init() {
  document.getElementById('countArea').innerHTML = 255-document.getElementById('tMessaggio').value.length; 
}

function updateCounter(field,maxlength,id,counter) {
  var totalLength = field.value.length; 
  if(totalLength >= maxlength) {
    field.value = field.value.substring(0, maxlength);
  }
  document.getElementById(counter).innerHTML = maxlength-field.value.length;
}