dom를 사용하여 복선상자 전체 선택, 취소 효과 구현

<form action="">
     ：<br/>
    <input type="checkbox" id="all"> <br/>
    <input type="checkbox" name="hobby" value=" "> 
    <input type="checkbox" name="hobby" value=" "> 
    <input type="checkbox" name="hobby" value=" "> 
    <input type="checkbox" name="hobby" value=" "> <br/>
    <button id="choice" value=" "> </button>
</form>

<script> window.onload = function(){ document.getElementById("all").addEventListener("click",function(){ var a = document.getElementById("all"); var b = document.getElementsByName("hobby"); for(var i=0;i< b.length;i++) { b[i].checked = a.checked; } },false) } </script>