判断文本框还能输入多少个字！

 
 js代码
 
 ////判断文本框还能输入多少个字，动态变化判断
    function txtanum()   //统计txta的输入字数
    {
        var maxl = 81;
        var num = 80;
        var content = $("#txtContent").val();
        content.slice(0, maxl);
        var nowlength = content.length;
        if (nowlength >= 0) {
            if (nowlength < num) {
                $("#spid").text(num - nowlength);
            } else {
                $("#spid").text(0);
            }
        } else {
            $("#txtContent").val(content.substring(0, maxl - 1));
        }
        if (nowlength == 0) {
            $("#spid").text(num);
        }
        if (nowlength > num) {
            $("#txtContent").val(content.substring(0, num))
        }

    }

html代码
 你还可以输入<span id="spid">80</span>字</div>
<textarea id="txtContent" onkeypress=""  class="zhongj_sou_as" tabindex="3" name="txtContent" onkeyup="txtanum()"
                            runat="server"></textarea></div>