计算还可以输入多少个字

Demo

html页面

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>还可以输入多少字</title>
<meta name="keywords" content="">
<meta name="description" content="">

<script src="./jquery-1.10.2.js" type="text/javascript"></script>
<script src="./count.js" type="text/javascript"></script>

</head>

<body>
<div>
<span>
<input type="text" name="subject" id="subject" value="" onkeyup="strLenCalc(this, 'checklen', 80);" >
</span>        
<span id="subjectchk">还可输入 <strong id="checklen">80</strong> 个字符</span>
</div>
</body>
</html>

js代码--count.js

var BROWSER = {};
var USERAGENT = navigator.userAgent.toLowerCase();
browserVersion({'ie':'msie','firefox':'','chrome':'','opera':'','safari':'','mozilla':'','webkit':'','maxthon':'','qq':'qqbrowser','rv':'rv'});
if(BROWSER.safari || BROWSER.rv) {
    BROWSER.firefox = true;
}
BROWSER.opera = BROWSER.opera ? opera.version() : 0;

HTMLNODE = document.getElementsByTagName('head')[0].parentNode;
if(BROWSER.ie) {
    BROWSER.iemode = parseInt(typeof document.documentMode != 'undefined' ? document.documentMode : BROWSER.ie);
    HTMLNODE.className = 'ie_all ie' + BROWSER.iemode;
}
var charset =BROWSER.firefox ? document.characterSet : document.charset;

function browserVersion(types) 
{
    var other = 1;
    for(i in types) {
        var v = types[i] ? types[i] : i;
        if(USERAGENT.indexOf(v) != -1) {
            var re = new RegExp(v + '(\\/|\\s|:)([\\d\\.]+)', 'ig');
            var matches = re.exec(USERAGENT);
            var ver = matches != null ? matches[2] : 0;
            other = ver !== 0 && v != 'mozilla' ? 0 : other;
        }else {
            var ver = 0;
        }
        eval('BROWSER.' + i + '= ver');
    }
    BROWSER.other = other;
}

function strlen(str)
{
    return (BROWSER.ie && str.indexOf('\n') != -1) ? str.replace(/\r?\n/g, '_').length : str.length;
}

function mb_cutstr(str, maxlen, dot)
{
    var len = 0;
    var ret = '';
    var dot = !dot ? '...' : dot;
    maxlen = maxlen - dot.length;
    for(var i = 0; i < str.length; i++) {
        len += str.charCodeAt(i) < 0 || str.charCodeAt(i) > 255 ? (charset == 'utf-8' ? 3 : 2) : 1;
        if(len > maxlen) {
            ret += dot;
            break;
        }
        ret += str.substr(i, 1);
    }
    return ret;
}

function strLenCalc(obj, checklen, maxlen)
{
    var v = obj.value;
    var charlen = 0;
    var maxlen = !maxlen ? 200 : maxlen;
    var curlen = maxlen;
    var len = strlen(v);
    for(var i = 0; i < v.length; i++)
    {
        if(v.charCodeAt(i) < 0 || v.charCodeAt(i) > 255) {
            curlen -= charset == 'utf-8' ? 2 : 1;
        }
    }
    
    if(curlen >= len) {
        $("#"+checklen)[0].innerHTML = curlen - len;
    } else {
        obj.value = mb_cutstr(v, maxlen, 0);
    }
}