javascript表单复选框全选代码

以下是javascript表单复选框全选代码：

HTML代码：

<input type="checkbox" id="checkAll">全选
<input type="checkbox" class="checkItem">选项1
<input type="checkbox" class="checkItem">选项2
<input type="checkbox" class="checkItem">选项3

JavaScript代码：

var checkAll = document.getElementById("checkAll");
var checkItems = document.getElementsByClassName("checkItem");

checkAll.onclick = function() {
  for (var i = ; i < checkItems.length; i++) {
    checkItems[i].checked = checkAll.checked;
  }
};

for (var i = ; i < checkItems.length; i++) {
  checkItems[i].onclick = function() {
    var checkedCount = ;
    for (var j = ; j < checkItems.length; j++) {
      if (checkItems[j].checked) {
        checkedCount++;
      }
    }
    checkAll.checked = checkedCount == checkItems.length;
  };
}