javascript判断textarea输入文字的长度

<textarea rows="4" cols="30" id="test" onKeyUp="textLimitCheck(this, 250);"></textarea>
<br/>(限 250 个字符  已输入 <font color="#CC0000"><span id="messageCount">0</span></font> 个字)
<script language=javascript>    
String.prototype.getBytes = function() {   
    var cArr = this.match(/[^\x00-\xff]/ig);   
    return this.length + (cArr == null ? 0 : cArr.length);   
}
function textLimitCheck(thisArea, maxLength){
	var len = thisArea.value.getBytes();
	if (len > maxLength)
	{
	    alert(maxLength + ' 个字限制. \r超出的将自动去除.');
		var tempStr = "";
		var areaStr = thisArea.value.split("");
		var tempLen = 0;
		for(var i=0,j=areaStr.length;i<j;i++){
			tempLen += areaStr[i].getBytes();
			if(tempLen<=maxLength){
				tempStr += areaStr[i];
			}				
		}		    
		thisArea.value = tempStr
		thisArea.focus();
		}
		/*回写span的值，当前填写文字的数量*/
		messageCount.innerText = thisArea.value.length;
	}
</script>