확인란 모두 선택 JavaScript 작성(그래픽 튜토리얼)

<tr>
     <td>爱  好</td>
     <td>
      <label for=""><input type="checkbox" name="fav" id="" value="苹果" class="btn"/>苹果</label>
     </td>
     <td>
      <label for=""><input type="checkbox" name="fav" id="" value="香蕉" class="btn"/>香蕉</label>
     </td>
     <td>
      <label for=""><input type="checkbox" name="" id="checkAll" value="全选" class="btn"/>全选</label>
     </td>
    </tr>

var oChkAll = document.getElementById("checkAll");
    //全选
    oChkAll.onclick = function() {
     for(var i = 0; i < oFav.length; i++) {
      oFav[i].checked = this.checked;
     }
    }
    //复选框组
    for(var i = 0; i < oFav.length; i++) {
     oFav[i].onclick = function() {
      //如果全选
      if(isChkAll()) {
       oChkAll.checked = true;
      } else {
       oChkAll.checked = false;
      }
     }
    }
    //判断是否全选
    function isChkAll() {
     var all = oFav.length;
     var chk = 0;
     for(var i = 0; i < oFav.length; i++) {
      if(oFav[i].checked) {
       chk++;
      }
     }
     if(all == chk) {
      return true;
     } else {
      return false;
     }
    }