Ajax 异步传输

function getData(){
		var url = "Show.do?dh=1";
		createXmlHttpReq();
		xmlHttp.onreadystatechange = function(){
			if(xmlHttp.readyState == 4 && xmlHttp.status == 200){
				var resp = xmlHttp.responseText;
				Convert(resp);
			}
		}
		xmlHttp.open("get",url,true);
		xmlHttp.send(null);
	}
	
	function Convert(resp){
		var mtb = document.getElementById("my_tb");
		var par = resp.parseJSON();
		var my_tb = document.getElementById("my_tb");
		for(var i = 0 ; i < par.length ; i ++){
			//alert(par[i].cate_id+"\t"+par[i].cate_name+"\t"+par[i].ori_price);
			var tbl = document.createElement("table");
			var tbody = document.createElement("tbody");
			var tr = document.createElement("tr");
			var td = document.createElement("td");
			td.setAttribute("vAlign","top");
			td.setAttribute("width","90");
			td.setAttribute("height","80");
			td.innerHTML = "<A href="+par[i].img_path+" target=_blank><IMG height=80 alt=点击图片查看内容 src="+par[i].img_path+" width=80 border=0></A>";
			var td2 = document.createElement("td");
			td2.setAttribute("vAlign","top");
			var tb = document.createElement("table");
			tb.setAttribute("cellSpacing","1");
			tb.setAttribute("cellPadding","0");
			tb.setAttribute("width","100%");
			tb.setAttribute("align","center");
			tb.setAttribute("border","0");
			var body = document.createElement("tbody");
			var tr2 = document.createElement("tr");
			var td3 = document.createElement("td");
			td3.innerHTML = "<A href=# target=_blank><STRONG>"+par[i].cate_name+"</STRONG></A>";
			var tr3 = document.createElement("tr");
			var td4 = document.createElement("td");
			td4.setAttribute("height","21");
			td4.innerHTML = "<FONT color=#ff0000>现价：人民币"+par[i].cur_price+"元</FONT><BR><a href=#>"+par[i].descript+"</a>！";
			
			
			tr2.appendChild(td3);
			tr3.appendChild(td4);
			body.appendChild(tr2);
			body.appendChild(tr3);
			tb.appendChild(body);
			td2.appendChild(tb);
			tr.appendChild(td);
			tr.appendChild(td2);
			tbody.appendChild(tr);
			tbl.appendChild(tbody);
			mtb.appendChild(tbl);
	    }
	    alert("完成");
	}

<TABLE cellSpacing=0 cellPadding=0 width=776 align=center border=0 id = "my_tb">
	
</TABLE>

var par = resp.parseJSON();

var par = eval("(" + resp + ")")//如果resp是数组就不用+()了