核心ajax（options）函数中，包含了建立xmlhttprequest,提取数据，判断是否回复成功等，基本满足了日常需求。
代码如下：

// A generic function for performming AJAX requests
// It takes one argument, which is an object that contains a set of options
// All of which are outline in the comments, below
function ajax( options ) {
// Load the options object with defaults, if no
// values were provided by the user
options = {
// The type of HTTP Request
type: options.type || "POST",
// The URL the request will be made to
url: options.url || "",
// How long to wait before considering the request to be a timeout
timeout: options.timeout || 5000,
// Functions to call when the request fails, succeeds,
// or completes (either fail or succeed)
onComplete: options.onComplete || function(){},
onError: options.onError || function(){},
onSuccess: options.onSuccess || function(){},
// The data type that'll be returned from the server
// the default is simply to determine what data was returned from the
// and act accordingly.
data: options.data || ""
};
// Create the request object
var xml = new XMLHttpRequest();
// Open the asynchronous POST request
//xml.open("GET", "/some/url.cgi", true);
xml.open("GET",options.url, true);
// We're going to wait for a request for 5 seconds, before giving up
var timeoutLength = 5000;
// Keep track of when the request has been succesfully completed
var requestDone = false;
// Initalize a callback which will fire 5 seconds from now, cancelling
// the request (if it has not already occurred).
setTimeout(function(){
requestDone = true;
}, timeoutLength);
// Watch for when the state of the document gets updated
xml.onreadystatechange = function(){
// Wait until the data is fully loaded,
// and make sure that the request hasn't already timed out
if ( xml.readyState == 4 && !requestDone ) {
// Check to see if the request was successful
if ( httpSuccess( xml ) ) {
// Execute the success callback with the data returned from the server
options.onSuccess( httpData( xml, options.type ) );
// Otherwise, an error occurred, so execute the error callback
} else {
options.onError();
}
// Call the completion callback
options.onComplete();
// Clean up after ourselves, to avoid memory leaks
xml = null;
}
};
// Establish the connection to the server
xml.send();
// Determine the success of the HTTP response
function httpSuccess(r) {
try {
// If no server status is provided, and we're actually
// requesting a local file, then it was successful
return !r.status && location.protocol == "file:" ||
// Any status in the 200 range is good
( r.status = 200 && r.status 300 ) ||
// Successful if the document has not been modified
r.status == 304 ||
// Safari returns an empty status if the file has not been modified
navigator.userAgent.indexOf("Safari") = 0 && typeof r.status == "undefined";
} catch(e){}
// If checking the status failed, then assume that the request failed too
return false;
}
// Extract the correct data from the HTTP response
function httpData(r,type) {
// Get the content-type header
var ct = r.getResponseHeader("content-type");
// If no default type was provided, determine if some
// form of XML was returned from the server
var data = !type && ct && ct.indexOf("xml") = 0;
// Get the XML Document object if XML was returned from
// the server, otherwise return the text contents returned by the server
data = type == "xml" || data ? r.responseXML : r.responseText;
// If the specified type is "script", execute the returned text
// response as if it was JavaScript
if ( type == "script" )
eval.call( window, data );
// Return the response data (either an XML Document or a text string)
return data;
}
}

在同等目录中，我们可以建立一个rss.xml文件，用这个函数来访问。
rss.xml如下：
代码如下：

titles
title
缘份
/title
title
月亮
/title
title
缘份月亮
/title
/titles

再建立一个html文档，调用它，就能看到rss.xml中的内容就能被访问到。
整体看看，其实真的比较简洁和简单。不仅是能访问xml格式文件，html，.js格式的文件都可以调用的；
这些都可以在本地建立对应的文件，进行调用，都可以实现。