javascript限定输入textarea输入长度

<textarea  id="content" onkeyup="limitLength(this.value,'512','content')"></textarea>

//限定输入textarea输入长度,一个汉字作为三个字节
function limitLength(value, byteLength, selectorId){
    var newvalue = value.replace(/[^\x00-\xff]/g, "**");
    var length = newvalue.length;
    //当填写的字节数小于设置的字节数
    if(length * 1 <=byteLength * 1){
        return false;
    }
    var limitDate = newvalue.substr(0, byteLength);
    var count = 0;
    var limitvalue = "";
    for(var i = 0; i < limitDate.length; i++){
        var flat = limitDate.substr(i, 1);
        if(flat == "*"){
            count++;
        }
    }
    var size = 0;
    var istar = newvalue.substr(byteLength * 1 - 1, 1);//校验点是否为“×”
    //if 基点是×; 判断在基点内有×为偶数还是奇数
    if(count % 2 == 0){
        //当为偶数时
        size = count / 2 + (byteLength * 1 - count);
        limitvalue = value.substr(0, size);
    }else{
        //当为奇数时
        size = (count - 1) /2 + (byteLength * 1 - count);
        limitvalue = value.substr(0, size);
    }
    document.getElementById(selectorId).value = limitvalue;
    return false;
}