สอบถามเรื่อง function ตรวจสอบเลขบัตรประชาชน ว่าเมื่อเลขบัตรไม่มีอยู่จริงจะเคลียร์Text Field

<script type="text/javascript" >
function Numbers(e){
	var keynum;
	var keychar;
	var numcheck;
	if(window.event) {// IE
	  keynum = e.keyCode;
	}
	else if(e.which) {// Netscape/Firefox/Opera
	  keynum = e.which;
	}
	if(keynum == 13 || keynum == 8 || typeof(keynum) == "undefined"){
			return true;
	}
	keychar= String.fromCharCode(keynum);
	numcheck = /^[0-9]$/;
	return numcheck.test(keychar);
}

function keyup(obj,e){
	var keynum;
	var keychar;
    var id = '';
	if(window.event) {// IE
	  keynum = e.keyCode;
	}
	else if(e.which) {// Netscape/Firefox/Opera
	  keynum = e.which;
	}
	keychar= String.fromCharCode(keynum); 

	var tagInput = document.getElementsByTagName('input');
	for(i=0;i<=tagInput.length;i++){
		if(tagInput[i] == obj){ 
			var prevObj = tagInput[i-1];
			var nextObj = tagInput[i+1];
			break;
		}
	} 
	if(obj.value.length == 0 && keynum == 8) prevObj.focus();
	
	if(obj.value.length == obj.getAttribute('maxlength')){ 
		for(i=0;i<=tagInput.length;i++){
			if(tagInput[i].id.substring(0,5) == 'txtID'){ 
				if(tagInput[i].value.length == tagInput[i].getAttribute('maxlength')){
					id += tagInput[i].value;
					if(tagInput[i].id == 'txtID5') break;
				}
				else{
					tagInput[i].focus();
					return;
				}
			}
		} 
		if(checkID(id))
		nextObj.focus();
		else alert('รหัสประชาชนไม่ถูกต้อง');
		document.getElementById('txtID1').value = "";
		document.getElementById('txtID2').value = "";
		document.getElementById('txtID3').value = "";
		document.getElementById('txtID4').value = "";
		document.getElementById('txtID5').value = "";
		}
	
}

function checkID(id){
	if(id.length != 13) return false;
	for(i=0, sum=0; i < 12; i++)
		sum += parseFloat(id.charAt(i))*(13-i); 
	if((11-sum%11)%10!=parseFloat(id.charAt(12)))
		return false;
alert('รหัสประชาชนไม่ถูกต้อง');
return true;

}
</scrip>
 
<form action="save_register.php" method="post" enctype="multipart/form-data" name="form1" id="form1" OnSubmit="return fncSubmit();">

    <input type="text" name="txtID1" id="txtID1" style="width:12px" maxlength=1 onKeyUp="keyup(this,event)" onKeyPress="return Numbers(event)" /> 
    - 
    <input type="text" name="txtID2" id="txtID2" style="width:35px" maxlength=4 onKeyUp="keyup(this,event)" onKeyPress="return Numbers(event)" /> 
    - 
    <input type="text" name="txtID3" id="txtID3" style="width:40px" maxlength=5 onKeyUp="keyup(this,event)" onKeyPress="return Numbers(event)" /> 
    - 
    <input type="text" name="txtID4" id="txtID4" style="width:20px" maxlength=2 onKeyUp="keyup(this,event)" onKeyPress="return Numbers(event)" /> 
    - 
    <input type="text" name="txtID5" id="txtID5" style="width:12px" maxlength=1 onKeyUp="keyup(this,event)" onKeyPress="return Numbers(event)" />
</form>