从多个textarea中随机选取一个内容

    <div id="IMContentTest">
        <textarea name="IMContent" class="IMClass" style="color:#ff0000;width:510px; height: 50px;" id="Textarea0"></textarea>
        <textarea name="IMContent" class="IMClass" style="color:#ff0000;width:510px; height: 50px;" id="Textarea1"></textarea><br/><br/>
        <textarea name="IMContent" class="IMClass" style="color:#ff0000;width:510px; height: 50px;" id="Textarea2"></textarea><br/><br/>
        <textarea name="IMContent" class="IMClass" style="color:#ff0000;width:510px; height: 50px;" id="Textarea3"></textarea><br/><br/>
        <textarea name="IMContent" class="IMClass" style="color:#ff0000;width:510px; height: 50px;" id="Textarea4"></textarea>
    </div>

       //变量定义
        var testflag = 0, t, sipaccount, fruency, count = 0, se1, se2, m = 0, h = 0, s = 0, ss = 1;
        var testcontent
        //时、分、秒、毫秒的计算
        function second(showtimeTag) {

            if ((ss % 100) == 0) {
                s += 1; ss = 1;
            }
            if (s > 0 && (s % 60) == 0) {
                m += 1; s = 0;
            }
            if (m > 0 && (m % 60) == 0) {
                h += 1; m = 0;
            }
            document.getElementById(showtimeTag).value = h + "时" + m + "分" + s + "秒";
            ss += 1;
        }

        var ImContentList = "";
       //获得一个0到4的随机整数
        function getNumber() {
            var random = 4 * Math.random();
            if (!isNaN(random) && isFinite(random)) {
                return parseInt(random);
            }
        }
        //发送消息动作
        function sendmessage() {
            var IMContents = $(".IMClass");

            $(IMContents).each(function () {
                if ($(this).attr("value") != undefined)
                    ImContentList += $(this).attr("value") + ",";
            });
            ImContentList = ImContentList.substr(0, ImContentList.length - 1);
            var contentlist = ImContentList.split(',');
            var numb = getNumber();
            var content = contentlist[numb];
            //document.forms[0].forms[0].elements[parseInt(5 * Math.random())].value
            testcontent = "[" + sip + "]" + " [ " + count + " ]" + "[ " + h + "时" + m + "分" + s + "秒" + " ] " + "  " + content;          

            count++;
        }

        //开始测试
        function starttest(showtimeTag) {
            var time = parseInt(document.getElementById("secondText").value) * 1000;
            var temp1 = "second(\"" + showtimeTag + "\")";
            var temp2 = "sendmessage()";
            se1 = self.setInterval(temp1, 10);
            se2 = self.setInterval(temp2, time);
        }
        //暂停测试
        function pausetest() {
            window.clearInterval(se1);
            window.clearInterval(se2);
        }
        //停止测试
        function stoptest() {
            window.clearInterval(se1);
            window.clearInterval(se2);
            testflag = 0;
            ss = 1;
            m = h = s = 0;
            document.getElementById("showtime").value = "0时0分0秒";
        }