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);