javascript中的按钮值显示未定义

[pure javascript code without the html body]
<script language="Javascript">
    [ creating buttons with the alphabets on it and its values the same]
  for(var a=1;a<=26;a++)
  {
  document.write("<input type='button' size='1' value="+String.fromCharCode(a +64)+" id='btn"+a+"' onclick='see()' >");
  }

document.write("<input type='text' size='10' id='box1' style='background-color:red'>")               
[normal textbox which should contain the data]
    function see()
      {
      [trying to put the value of button on textbox on buttonclick but gives me  undefined]

       var text;
       text=*document.getElementsByTagName*("button").value;
       document.getElementById("box1").value=text;
      }
</script>

我已经试过了。getElementsByTagName或甚至由Id给出未定义的地方，因为我想要按钮的值在文本框上。我使用了forloop，因为我必须创建大约26个按钮