实时的监控用户输入多少字节的信息

具体的思路是，获得用户输入信息的容器对象，并且在该对象的onkeydown和onkeyup事件中调用相关函数，来计算用户还可以输入多少字节。
<script language="javascript">
//定义临时变量，来存储对象的值
var mem=0;
//将中文字符的长度转换为英文字符的长度
function utf16to8(str) {
        var out, i, len, c;
        out = "";
        len = str.length;
        for(i = 0; i < len; i++) {
            c = str.charCodeAt(i);
            if ((c >= 0x0001) && (c <= 0x007F)) {
                out += str.charAt(i);
                } else if (c > 0x07FF) {
                out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));
                out += String.fromCharCode(0x80 | ((c >>  6) & 0x3F));
                out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));
            } else {
                out += String.fromCharCode(0xC0 | ((c >>  6) & 0x1F));
                out += String.fromCharCode(0x80 | ((c >>  0) & 0x3F));
            }
        }
        return out;
}
//计算字符串整体长度
function Desp_value_count(obj)
{
    var temp = document.getElementById(obj);
    var strValue=temp.value;
    //去除左空格和右空格
    strValue=strValue.replace(/\r/g," ");
    strValue=strValue.replace(/\n/g," ");
    return     utf16to8(strValue).length;
}
//把对象的值暂存在一个变量中，以便执行完utf16to8函数后，把改过的对象值还原成原来的值
function substr_desp(obj)
{
    if(mem==0)
    {
        var test=document.getElementById(obj);
        mem=test.value.length;
    }
}
//带输入字符长度限制的 多加入变量 num
function LimitDesp_inputN(obj,span_obj,msg1,msg2,msg3,num)
{
   
    var test=document.getElementById(obj);
    var test_show=document.getElementById(span_obj);
    if(test)
    {
        var input_num=parseInt(num);
        test_show.className="span_match_class";
       
        if(test.value.length==0)
        {
            Desp_Char_Num=(input_num+1)-Desp_value_count(obj);
            mem=0;
        }
        if(Desp_value_count(obj)==input_num)
        {
            substr_desp(obj);
            test_show.innerHTML=msg2+" "+0+" "+msg3;
            //test.value=test.value.substr(0,mem);
            return false;
        }
        else if(Desp_value_count(obj)>(input_num))
        {
            substr_desp(obj);
            test_show.innerHTML=msg1;
            //test.value=test.value.substr(0,mem);
            return false;
        }
        else
        {
            Desp_Char_Num=(input_num+1)-Desp_value_count(obj);
            Desp_Char_Num--;
            test_show.innerHTML=msg2+" "+Desp_Char_Num+" "+msg3;
            mem=0;
        }
        setTimeout(" LimitDesp_inputN()",1000);
    }
}
</script>

HTML调用代码如下：
<table border="0" cellpadding="0" cellspacing="0">
                <tr>
                    <td width="200" align="right">用户信息表：</td>
                    <td width="71%" align="left"><input
                    name="szTableName"
                    type="text"
                    id="szTableName"
                    class="input"
                    maxlength="31"  size="45"
                    onkeydown="LimitDesp_inputN('szTableName','span_SzTableName','您所输入的内容超过长度限制','您还可以输入','个字符',31)" 
                    onkeyup="LimitDesp_inputN('szTableName','span_SzTableName','您所输入的内容超过长度限制','您还可以输入','个字符',31)"  />
                      <span id="span_SzTableName"></span></td></tr>
        </table>