限制textarea字数

<dd><textarea rows="4" cols="78" name="fbmessage" id="fbmessage" onkeyup="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);