该站点对于浏览jQuery源代码非常有用.

上面链接的来源死了：

function (url,options) {

    // If url is an object,simulate pre-1.5 signature
    if (typeof url === "object") {
        options = url;
        url = undefined;
    }

    // Force options to be an object
    options = options || {};

    var // Create the final options object
    s = jQuery.ajaxSetup({},options),// Callbacks context
    callbackContext = s.context || s,// Context for global events
    // It's the callbackContext if one was provided in the options
    // and if it's a DOM node or a jQuery collection
    globalEventContext = callbackContext !== s && (callbackContext.nodeType || callbackContext instanceof jQuery) ? jQuery(callbackContext) : jQuery.event,// Deferreds
    deferred = jQuery.Deferred(),completeDeferred = jQuery._Deferred(),// Status-dependent callbacks
    statusCode = s.statusCode || {},// ifModified key
    ifModifiedKey,// Headers (they are sent all at once)
    requestHeaders = {},requestHeadersNames = {},// Response headers
    responseHeadeRSString,responseHeaders,// transport
    transport,// timeout handle
    timeoutTimer,// Cross-domain detection vars
    parts,// The jqXHR state
    state = 0,// To know if global events are to be dispatched
    fireGlobals,// Loop variable
    i,// Fake xhr
    jqXHR = {

        readyState: 0,// Caches the header
        setRequestHeader: function (name,value) {
            if (!state) {
                var lname = name.toLowerCase();
                name = requestHeadersNames[lname] = requestHeadersNames[lname] || name;
                requestHeaders[name] = value;
            }
            return this;
        },// Raw string
        getAllResponseHeaders: function () {
            return state === 2 ? responseHeadeRSString : null;
        },// Builds headers hashtable if needed
        getResponseHeader: function (key) {
            var match;
            if (state === 2) {
                if (!responseHeaders) {
                    responseHeaders = {};
                    while ((match = rheaders.exec(responseHeadeRSString))) {
                        responseHeaders[match[1].toLowerCase()] = match[2];
                    }
                }
                match = responseHeaders[key.toLowerCase()];
            }
            return match === undefined ? null : match;
        },// Overrides response content-type header
        overrideMimeType: function (type) {
            if (!state) {
                s.mimeType = type;
            }
            return this;
        },// Cancel the request
        abort: function (statusText) {
            statusText = statusText || "abort";
            if (transport) {
                transport.abort(statusText);
            }
            done(0,statusText);
            return this;
        }
    };

    // Callback for when everything is done
    // It is defined here because jslint complains if it is declared
    // at the end of the function (which would be more logical and readable)


    function done(status,statusText,responses,headers) {

        // Called once
        if (state === 2) {
            return;
        }

        // State is "done" now
        state = 2;

        // Clear timeout if it exists
        if (timeoutTimer) {
            clearTimeout(timeoutTimer);
        }

        // Dereference transport for early garbage collection
        // (no matter how long the jqXHR object will be used)
        transport = undefined;

        // Cache response headers
        responseHeadeRSString = headers || "";

        // Set readyState
        jqXHR.readyState = status ? 4 : 0;

        var isSuccess,success,error,response = responses ? ajaxHandleResponses(s,jqXHR,responses) : undefined,lastModified,etag;

        // If successful,handle type chaining
        if (status >= 200 && status < 300 || status === 304) {

            // Set the If-Modified-Since and/or If-None-Match header,if in ifModified mode.
            if (s.ifModified) {

                if ((lastModified = jqXHR.getResponseHeader("Last-Modified"))) {
                    jQuery.lastModified[ifModifiedKey] = lastModified;
                }
                if ((etag = jqXHR.getResponseHeader("Etag"))) {
                    jQuery.etag[ifModifiedKey] = etag;
                }
            }

            // If not modified
            if (status === 304) {

                statusText = "notmodified";
                isSuccess = true;

                // If we have data
            } else {

                try {
                    success = ajaxConvert(s,response);
                    statusText = "success";
                    isSuccess = true;
                } catch(e) {
                    // We have a parsererror
                    statusText = "parsererror";
                    error = e;
                }
            }
        } else {
            // We extract error from statusText
            // then normalize statusText and status for non-aborts
            error = statusText;
            if (!statusText || status) {
                statusText = "error";
                if (status < 0) {
                    status = 0;
                }
            }
        }

        // Set data for the fake xhr object
        jqXHR.status = status;
        jqXHR.statusText = statusText;

        // Success/Error
        if (isSuccess) {
            deferred.resolveWith(callbackContext,[success,jqXHR]);
        } else {
            deferred.rejectWith(callbackContext,[jqXHR,error]);
        }

        // Status-dependent callbacks
        jqXHR.statusCode(statusCode);
        statusCode = undefined;

        if (fireGlobals) {
            globalEventContext.trigger("ajax" + (isSuccess ? "Success" : "Error"),s,isSuccess ? success : error]);
        }

        // Complete
        completeDeferred.resolveWith(callbackContext,statusText]);

        if (fireGlobals) {
            globalEventContext.trigger("ajaxComplete",s]);
            // Handle the global AJAX counter
            if (! (--jQuery.active)) {
                jQuery.event.trigger("ajaxStop");
            }
        }
    }

    // Attach deferreds
    deferred.promise(jqXHR);
    jqXHR.success = jqXHR.done;
    jqXHR.error = jqXHR.fail;
    jqXHR.complete = completeDeferred.done;

    // Status-dependent callbacks
    jqXHR.statusCode = function (map) {
        if (map) {
            var tmp;
            if (state < 2) {
                for (tmp in map) {
                    statusCode[tmp] = [statusCode[tmp],map[tmp]];
                }
            } else {
                tmp = map[jqXHR.status];
                jqXHR.then(tmp,tmp);
            }
        }
        return this;
    };

    // Remove hash character (#7531: and string promotion)
    // Add protocol if not provided (#5866: IE7 issue with protocol-less urls)
    // We also use the url parameter if available
    s.url = ((url || s.url) + "").replace(rhash,"").replace(rprotocol,ajaxLocParts[1] + "//");

    // Extract dataTypes list
    s.dataTypes = jQuery.trim(s.dataType || "*").toLowerCase().split(rspacesAjax);

    // Determine if a cross-domain request is in order
    if (s.crossDomain == null) {
        parts = rurl.exec(s.url.toLowerCase());
        s.crossDomain = !!(parts && (parts[1] != ajaxLocParts[1] || parts[2] != ajaxLocParts[2] || (parts[3] || (parts[1] === "http:" ? 80 : 443)) != (ajaxLocParts[3] || (ajaxLocParts[1] === "http:" ? 80 : 443))));
    }

    // Convert data if not already a string
    if (s.data && s.processData && typeof s.data !== "string") {
        s.data = jQuery.param(s.data,s.traditional);
    }

    // Apply prefilters
    inspectPrefiltersOrTransports(prefilters,options,jqXHR);

    // If request was aborted inside a prefiler,stop there
    if (state === 2) {
        return false;
    }

    // We can fire global events as of now if asked to
    fireGlobals = s.global;

    // Uppercase the type
    s.type = s.type.toUpperCase();

    // Determine if request has content
    s.hasContent = !rnoContent.test(s.type);

    // Watch for a new set of requests
    if (fireGlobals && jQuery.active++===0) {
        jQuery.event.trigger("ajaxStart");
    }

    // More options handling for requests with no content
    if (!s.hasContent) {

        // If data is available,append data to url
        if (s.data) {
            s.url += (rquery.test(s.url) ? "&" : "?") + s.data;
        }

        // Get ifModifiedKey before adding the anti-cache parameter
        ifModifiedKey = s.url;

        // Add anti-cache in url if needed
        if (s.cache === false) {

            var ts = jQuery.now(),// try replacing _= if it is there
            ret = s.url.replace(rts,"$1_=" + ts);

            // if nothing was replaced,add timestamp to the end
            s.url = ret + ((ret === s.url) ? (rquery.test(s.url) ? "&" : "?") + "_=" + ts : "");
        }
    }

    // Set the correct header,if data is being sent
    if (s.data && s.hasContent && s.contentType !== false || options.contentType) {
        jqXHR.setRequestHeader("Content-Type",s.contentType);
    }

    // Set the If-Modified-Since and/or If-None-Match header,if in ifModified mode.
    if (s.ifModified) {
        ifModifiedKey = ifModifiedKey || s.url;
        if (jQuery.lastModified[ifModifiedKey]) {
            jqXHR.setRequestHeader("If-Modified-Since",jQuery.lastModified[ifModifiedKey]);
        }
        if (jQuery.etag[ifModifiedKey]) {
            jqXHR.setRequestHeader("If-None-Match",jQuery.etag[ifModifiedKey]);
        }
    }

    // Set the Accepts header for the server,depending on the dataType
    jqXHR.setRequestHeader("Accept",s.dataTypes[0] && s.accepts[s.dataTypes[0]] ? s.accepts[s.dataTypes[0]] + (s.dataTypes[0] !== "*" ? ",*/*; q=0.01" : "") : s.accepts["*"]);

    // Check for headers option
    for (i in s.headers) {
        jqXHR.setRequestHeader(i,s.headers[i]);
    }

    // Allow custom headers/mimetypes and early abort
    if (s.beforeSend && (s.beforeSend.call(callbackContext,s) === false || state === 2)) {
        // Abort if not done already
        jqXHR.abort();
        return false;

    }

    // Install callbacks on deferreds
    for (i in {
        success: 1,error: 1,complete: 1
    }) {
        jqXHR[i](s[i]);
    }

    // Get transport
    transport = inspectPrefiltersOrTransports(transports,jqXHR);

    // If no transport,we auto-abort
    if (!transport) {
        done(-1,"No Transport");
    } else {
        jqXHR.readyState = 1;
        // Send global event
        if (fireGlobals) {
            globalEventContext.trigger("ajaxSend",s]);
        }
        // Timeout
        if (s.async && s.timeout > 0) {
            timeoutTimer = setTimeout(function () {
                jqXHR.abort("timeout");
            },s.timeout);
        }

        try {
            state = 1;
            transport.send(requestHeaders,done);
        } catch(e) {
            // Propagate exception as error if not done
            if (status < 2) {
                done(-1,e);
                // Simply rethrow otherwise
            } else {
                jQuery.error(e);
            }
        }
    }

    return jqXHR;
}