JavaScript实现Select标签左右双向移动功能

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    #box_L,#choice,#box_R{
      display: inline-block;
    }

  </style>
</head>
<body>
<div id="box_L">
  <select id="left" multiple size="10">
    <option>one</option>
    <option>two</option>
    <option>three</option>
    <option>four</option>
    <option>five</option>
    <option>six</option>
  </select>
</div>
<div id="choice">
  <input type="button" width="5px" value="--->" onclick="add()"><br>
  <input type="button" width="5px" value="====>" onclick="addall()"><br>
  <input type="butoon" width="5px" value="<---" onclick="sub()"><br>
  <input type="butoon" width="5px" value="<====" onclick="suball()"><br>
</div>
<div id="box_R">
  <select id="right" size="10" multiple>
    <option>seven</option>
  </select>
</div>

<script>
  var left=document.getElementById("left");
  var right=document.getElementById("right");
  function add(){
    var options=left.children;
    for (var i=0;i<options.length;i++){
      if (options[i].selected==true){
        options[i].selected=false;
        right.appendChild(options[i]);
        i--;
      }
    }
  }

  function addall(){
    var options=left.children;
    for (var i=0;i<options.length;i++){
      right.appendChild(options[i]);
      i--;
    }
  }
  function sub(){
    var options=right.children;
    for (var i=0;i<options.length;i++){
      if (options[i].selected==true){
        options[i].selected=false;
        left.appendChild(options[i]);
        i--;
      }
    }
  }
  function suball(){
    var options=right.children;
    for (var i=0;i<options.length;i++){
      left.appendChild(options[i]);
      i--;
    }
  }
</script>
</body>
</html>