function countCharacters(textArea, counterField, maxLength){

  if(textArea.value != null) {

    if (textArea.value.length > maxLength){

      alert("Your message may not exceed " +  maxLength +" characters in length.");

      textArea.value = textArea.value.substring( 0, maxLength);

    } else {

      counterField.value = maxLength - textArea.value.length;

    }

  }

}