JavaScript查找排序字符串中前7位相同的重复行

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
  <title></title>
  <style type="text/css">
    div{ float:left; }
    #divCenter{ padding-top:100px;margin:0 50px; }
    .txt{width:200px;height:200px;}
    #txtOldData{background-color:#A6CAF0;}
    #txtAnswer{background-color:#EBA9A6;}
  </style>
  <script type="text/javascript">
    function test() {
      var arr = document.getElementById("txtOldData").value.replace(/ +/g, '').split("\n");
      var tempStr = arr[0].substring(0, 7);
      var compareLen = 7, equalNum = 0;
      var answer = "";
      for (var i = 1; i < arr.length; i++) {
        if (arr[i].substring(0, 7) == tempStr) {
          if (equalNum == 0)
            answer += arr[i - 1] + "\n";
          answer += arr[i] + "\n";
          equalNum++;
        } else {
          tempStr = arr[i].substring(0, 7);
          equalNum = 0;
        }
      }
      document.getElementById("txtAnswer").value = (answer);
    }
  </script>
</head>
<body>
  <div>
    请输入数值：<br />
    <textarea id="txtOldData" class="txt">
1234567890
1234567891
1234567892
1234534124
1234614124
4321412414
5636373573
    </textarea>
  </div>
  <div style="padding-top:90px;padding" >
    <input type="button" value="测试==>" onclick="test()" />
  </div>
  <div>
    结果：<br />
    <textarea id="txtAnswer" class="txt"></textarea>
  </div>
</body>
</html>