jqXHR = {// 准备状态readyState: 0,// Builds headers hashtable if needed// 如果需要，创建一个响应头参数的表getResponseHeader: function(key) {var match;// 如果状态为2，状态2表示ajax完成if (state === 2) {// 如果没有相应头if (!responseHeaders) {// 相应头设空responseHeaders = {};// 组装相应头while ((match = rheaders.exec(responseHeadersString))) {responseHeaders[match[1].toLowerCase()] = match[2];}}// 响应头对应的key的值match = responseHeaders[key.toLowerCase()];}return match == null ? null : match;},// Raw string// 返回响应头字符串getAllResponseHeaders: function() {// 看看是否接收到了，接收到直接返回，否则为nullreturn state === 2 ? responseHeadersString : null;},// Caches the header// 设置请求头setRequestHeader: function(name, value) {var lname = name.toLowerCase();if (!state) {// 如果requestHeadersNames[ lname ]不为空，// 则requestHeadersNames[ lname ]不变，name设置为该值// 否则，requestHeadersNames[ lname ]不空，// 则requestHeadersNames[ lname ]设置为name，// 该映射关系用于避免用户大小写书写错误之类的问题，容错处理name = requestHeadersNames[lname] = requestHeadersNames[lname] || name;//现在的name是对的，或者是第一次设置这个name，不需要容错//设置请求头对应值requestHeaders[name] = value;}return this;},// Overrides response content-type header// 重写相应头content-typeoverrideMimeType: function(type) {if (!state) {s.mimeType = type;}return this;},// Status-dependent callbacks// 对应状态的回调函数集statusCode: function(map) {var code;if (map) {//如果状态小于2，表示旧的回调可能还没有用到if (state < 2) {for (code in map) {// Lazy-add the new callback in a way that preserves old ones//  用类似链表的方式添加，以保证旧的回调依然存在statusCode[code] = [statusCode[code], map[code]];}} else {// Execute the appropriate callbacks// 无论Deferred成功还是失败都执行当前状态回调
                jqXHR.always(map[jqXHR.status]);}}return this},// Cancel the requestabort: function(statusText) {var finalText = statusText || strAbort;if (transport) {transport.abort(finalText);}done(0, finalText);return this;}
};

$.ajax({url: "php.html",context: document.body,complete: function() {console.log(this)}
}).done(function() {console.log(this)
});

deferred.promise(jqXHR).complete = completeDeferred.add;jqXHR.success = jqXHR.done;jqXHR.error   = jqXHR.fail;

for (i in {success  : 1,error    : 1,complete : 1
}) {jqXHR[i](s[i]);
}

if (fireGlobals && jQuery.active++ === 0) {// 则通过jQuery.event.trigger模拟触发jQuery.event.trigger("ajaxStart");
}

/*** 全局事件ajaxSend* 如果需要，对特定对象触发全局事件ajaxSend*/
if (fireGlobals) {globalEventContext.trigger("ajaxSend", [jqXHR, s]);
}

if (fireGlobals) {globalEventContext.trigger(isSuccess ? "ajaxSuccess" : "ajaxError", [jqXHR, s, isSuccess ? success : error]);}// Complete
            completeDeferred.fireWith(callbackContext, [jqXHR, statusText]);if (fireGlobals) {globalEventContext.trigger("ajaxComplete", [jqXHR, s]);// Handle the global AJAX counterif (!(--jQuery.active)) {jQuery.event.trigger("ajaxStop");}}

if (s.cache === false) {s.url = rts.test(cacheURL) ?// If there is already a '_' parameter, set its valuecacheURL.replace(rts, "$1_=" + ajax_nonce++) :// Otherwise add one to the endcacheURL + (ajax_rquery.test(cacheURL) ? "&" : "?") + "_=" + ajax_nonce++;
}

$.ajax({url: "http://fiddle.jshell.net/favicon.png",beforeSend: function ( xhr ) {xhr.overrideMimeType("text/plain; charset=x-user-defined");}
}).done(function ( data ) {if( console && console.log ) {console.log("Sample of data:", data.slice(0, 100));}
});