อยากจะสอบถามพี่ๆ เรื่องการ Check_All นะคับ ผมควรทำอย่างไร นะคับ

<input type="checkbox" name="selectAll" id="selectAll" onclick="checkAll(document.myform.list)" />

<script language="javascript">
function sendloi(obj){
	 
	var type_of_mem = document.getElementById('type').value;
						    	 var req;
						    		if(window.XMLHttpRequest){
						    			req=new XMLHttpRequest();
						    		}else if(window.ActiveXObject){
						    			req=new ActiveXObject("Microsoft.XMLHTTP");
						    		}else{
						    			alert("Browser error");
						    			return false;
						    		}
						    				req.onreadystatechange=function()
						    					{
						    						if(req.readyState==4){
						    						   var resultarea=document.getElementById("area");
						    							resultarea.innerHTML=req.responseText; 
											alert('You will then be loi.')		
						    						}else {
						    							var resultarea=document.getElementById("area");
						    							resultarea.innerHTML="กรุณารอสักครู่.....";
						    						}
						    					}
						    							str=Math.random()
						    							
						    						var url="bill2.php?pop="+str;
						    	                       url+="&id="+obj;
						    	        		req.open("GET",url,true);		
														     }
								 
function approve()
{
	alert('111111');
}


function checkAll(field)
{
if(document.myform.selectAll.checked == true)
{
	for (i = 0; i < field.length; i++)
	{
		field[i].checked = true ;
	}
}
else
{
	for (i = 0; i < field.length; i++)
	{
		field[i].checked = false ;
	}
}

}
</script>