AJAX完整示例

var xmlhttp;
function createRequest() {
	if (window.ActiveXObject) {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    else if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();
    }
}
createRequest();
function sendRequest()
{
	if(xmlhttp)
	{
		xmlhttp.open("GET", "URL", true);
		xmlhttp.send(null);
		xmlhttp.onreadystatechange = getRequestAndManager;
	}
}

function getRequestAndManager()
{
	if (xmlhttp_request.readyState == 4) {// 收到完整的服务器响应
		if (xmlRequest.status == 200) {
			var requestReturnStr = xmlhttp.responseText;
			var json = eval("("+requestReturnStr+")");
			
		}
	} 
}

跨域请求(跨域端口也属于跨域，80和8080属于跨域)在火狐等标准浏览器中Status为0，而且responseText为""，所以要注意这个问题