整理了一下javascript 原生ajax的兼容写法，超级简洁的代码！

function ajax(url,callback) {        
	var xmlHttp;
	if (window.ActiveXObject){
		var arr=["MSXML2.XMLHttp.6.0","MSXML2.XMLHttp.5.0", "MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0","MSXML2.XMLHttp","Microsoft.XMLHttp"];
		for(var i=0;i<arr.length;i++) {
			try {
				xmlHttp = new ActiveXObject(arr[i]);
				if(xmlHttp){break}
			}
			catch(error) { }
		}
	} else {
		try {
			xmlHttp=new XMLHttpRequest();
		}
		catch(otherError) { }
	} 

	
	xmlHttp.open('GET',url,true);
	xmlHttp.onreadystatechange = function() {
		if(xmlHttp.readyState == 4 && xmlHttp.status == 200) {
            var result = xmlHttp.responseText;
			callback(result);
         }
	}
	xmlHttp.send();
}