限制textarea字数

<dd><textarea rows="4" cols="78" name="fbmessage" id="fbmessage" οnkeyup="countChar();">${requestScope.fb == null ? '请输入公告' : requestScope.fb.fbmessage }</textarea>(建议<span id="t_count">43</span>个字符，还剩<span id="surplus">43</span>个字符)</dd>

js，用prototype写的

function countChar(textareaName,spanName) {
    var sup = $('t_count').innerHTML;
    var count = sup - $('fbmessage').value.length;
    if(count <= 0) {
      var text = $('fbmessage').value.substring(0, 43);
      $('fbmessage').setValue(text);
      count = sup - $('fbmessage').value.length;
    } 
    $('surplus').update(count);
  }
  var sup = $('t_count').innerHTML;
  var count = sup - $('fbmessage').value.length;
  $('surplus').update(count);