form 안의 text 상자를 화살표키로 이동하게 하기

<script language="JavaScript">
document.onkeydown = checkKeycode
function checkKeycode(e) {
 var keycode;
 var tar = event.srcElement.form;
 var Tindex = "";

 if (window.event && window.event.keyCode == 40){
  for(i=0;i<tar.elements.length;i++){
   if(tar.elements[i] == event.srcElement) Tindex = i;
   if(Tindex !== i && Tindex !== "" && tar.elements[i].type == "text"){
    tar.elements[i].focus();
    break;
   }
  }
 }else if (window.event && window.event.keyCode == 38){

  for(i= (tar.elements.length - 1);i>=0;i--){

   if(tar.elements[i] == event.srcElement) Tindex = i;
   if(Tindex !== i && Tindex !== "" && tar.elements[i].type == "text"){
    tar.elements[i].focus();
    break;
   }
  }
 }
}
</script>