Select by Group in javascript / 자바스크립트로 그룹별로 선택하는 함수

<script>
  function selectivecheck(field, myselection){
      var fieldid;
      var pos;
      var criteria;
      var strng;
      strng = myselection.value;
      for (i=0; i<field.length; i++){
          if (strng=="all"){
            field[i].checked = true;
          }else{
              fieldid = field[i].id;
              pos = strng.substring(0,1);
              criteria = strng.substring(1,2);
              if (fieldid.substring(pos,pos+1)==criteria){
                  field[i].checked = true;
               }else{
                  field[i].checked = false;
                }
            }
        }
    }
</script>
<form>
  <input id=a1 type=checkbox name=list value="1">Group A sub 1<br>
  <input id=a2 type=checkbox name=list value="2">Group A sub 2<br>
  <input id=b1 type=checkbox name=list value="3">Group B sub 1<br>
  <input id=b2 type=checkbox name=list value="4">Group B sub 2<br>
  <input id=c1 type=checkbox name=list value="5">Group C sub 1<br>
  <input id=c2 type=checkbox name=list value="6">Group C sub 2<br>
<span>Select by Group : </span>
<select size="1" name="mycombobox">
  <option value="all">Select All</option>
  <option value="0a ">Group A</option>
  <option value="0b ">Group B</option>
  <option value="0c ">Group C</option>
  <option value="11 ">Sub 1</option>
  <option value="12 ">Sub 2</option>
</select>
<span><input type="button" value="Select" onclick=" selectivecheck(this.form.list, this.form.mycombobox)"></span>
</form>
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Group A sub 1
Group A sub 2
Group B sub 1
Group B sub 2
Group C sub 1
Group C sub 2
Select by Group : Select All Group A Group B Group C Sub 1 Sub 2