JS 計算文本域還能輸入多少個字符

//輸入計數

//count：能輸入的數據總量
    function Calculation(v, count) {
        var span = $(v).next();
        var valLength = $(v).val().length;
        if (valLength > count) {
            $(v).val($(v).val().substr(0, count));
            valLength = count;
        }
        span.text("您已輸入" + valLength + "字元,還剩下" + (count - valLength) + "字元");
    }