JavaScript实现自动跳转文本功能

<html>
  <head>
    <title>文本跳转</title>
    <style type=text/css>
      body{
        MARGIN-TOP: 10px;
        FONT-SIZE: 10pt;
        MARGIN-LEFT: 10px;
        MARGIN-RIGHT: 0px;
        FONT-FAMILY: "微软雅黑"
      }
    </style>
    <meta http-equiv=Content-Type content="text/html; charset=gb2312">
  </head>

  <body>
    <script type="text/javascript">
      function InBox1() {
        if(document.code.Box1.value.length == 4) {
          document.code.Box2.focus();
        }
      }

      function InBox2() {
        if(document.code.Box2.value.length == 4) {
          document.code.Box3.focus();
        }
      }

      function InBox3() {
        if(document.code.Box3.value.length == 4) {
          document.code.Box4.focus();
        }
      }
    </script>
    <formname="code" action="" method="post">输入：
      <input maxLength="4" size="4" name="Box1" onkeyup="return InBox1()">-
      <input maxLength="4" size="4" name="Box2" onKeyUp="return InBox2()">-
      <input maxLength="4" size="4" name="Box3" onKeyUp="return InBox3()">-
      <input maxLength="4" size="4" name="Box4">
    </form>
  </body>
</html>