The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @class Ext.data.ScriptTagProxy
     * @extends Ext.data.DataProxy
     * An implementation of Ext.data.DataProxy that reads a data object from a URL which may be in a domain
     * other than the originating domain of the running page.

     *
     * Note that if you are retrieving data from a page that is in a domain that is NOT the same as the originating domain
     * of the running page, you must use this class, rather than HttpProxy.

     *
     * The content passed back from a server resource requested by a ScriptTagProxy must be executable JavaScript
     * source code because it is used as the source inside a <script> tag.

     *
     * In order for the browser to process the returned data, the server must wrap the data object
     * with a call to a callback function, the name of which is passed as a parameter by the ScriptTagProxy.
     * Below is a Java example for a servlet which returns data for either a ScriptTagProxy, or an HttpProxy
     * depending on whether the callback name was passed:
     *
     *
    boolean scriptTag = false;
    String cb = request.getParameter("callback");
    if (cb != null) {
        scriptTag = true;
        response.setContentType("text/javascript");
    } else {
        response.setContentType("application/x-json");
    }
    Writer out = response.getWriter();
    if (scriptTag) {
        out.write(cb + "(");
    }
    out.print(dataBlock.toJsonString());
    if (scriptTag) {
        out.write(");");
    }

     * Below is a PHP example to do the same thing:
    $callback = $_REQUEST['callback'];

    // Create the output object.
    $output = array('a' => 'Apple', 'b' => 'Banana');

    //start output
    if ($callback) {
        header('Content-Type: text/javascript');
        echo $callback . '(' . json_encode($output) . ');';
    } else {
        header('Content-Type: application/x-json');
        echo json_encode($output);
    }

     * Below is the ASP.Net code to do the same thing:
    String jsonString = "{success: true}";
    String cb = Request.Params.Get("callback");
    String responseString = "";
    if (!String.IsNullOrEmpty(cb)) {
        responseString = cb + "(" + jsonString + ")";
    } else {
        responseString = jsonString;
    }
    Response.Write(responseString);

     *
     * @constructor
     * @param {Object} config A configuration object.
     */
    Ext.data.ScriptTagProxy = function(config){
        Ext.apply(this, config);

        Ext.data.ScriptTagProxy.superclass.constructor.call(this, config);

        this.head = document.getElementsByTagName("head")[0];

        /**
         * @event loadexception
         * Deprecated in favor of 'exception' event.
         * Fires if an exception occurs in the Proxy during data loading.  This event can be fired for one of two reasons:
         * The load call timed out.  This means the load callback did not execute within the time limit
         * specified by {@link #timeout}.  In this case, this event will be raised and the
         * fourth parameter (read error) will be null.
         * The load succeeded but the reader could not read the response.  This means the server returned
         * data, but the configured Reader threw an error while reading the data.  In this case, this event will be
         * raised and the caught error will be passed along as the fourth parameter of this event.
         * Note that this event is also relayed through {@link Ext.data.Store}, so you can listen for it directly
         * on any Store instance.
         * @param {Object} this
         * @param {Object} options The loading options that were specified (see {@link #load} for details).  If the load
         * call timed out, this parameter will be null.
         * @param {Object} arg The callback's arg object passed to the {@link #load} function
         * @param {Error} e The JavaScript Error object caught if the configured Reader could not read the data.
         * If the remote request returns success: false, this parameter will be null.
         */
    };

    Ext.data.ScriptTagProxy.TRANS_ID = 1000;

    Ext.extend(Ext.data.ScriptTagProxy, Ext.data.DataProxy, {
        /**
         * @cfg {String} url The URL from which to request the data object.
         */
        /**
         * @cfg {Number} timeout (optional) The number of milliseconds to wait for a response. Defaults to 30 seconds.
         */
        timeout : 30000,
        /**
         * @cfg {String} callbackParam (Optional) The name of the parameter to pass to the server which tells
         * the server the name of the callback function set up by the load call to process the returned data object.
         * Defaults to "callback".The server-side processing must read this parameter value, and generate
         * javascript output which calls this named function passing the data object as its only parameter.
         */
        callbackParam : "callback",
        /**
         *  @cfg {Boolean} nocache (optional) Defaults to true. Disable caching by adding a unique parameter
         * name to the request.
         */
        nocache : true,

        /**
         * HttpProxy implementation of DataProxy#doRequest
         * @param {String} action
         * @param {Ext.data.Record/Ext.data.Record[]} rs If action is read, rs will be null
         * @param {Object} params An object containing properties which are to be used as HTTP parameters
         * for the request to the remote server.
         * @param {Ext.data.DataReader} reader The Reader object which converts the data
         * object into a block of Ext.data.Records.
         * @param {Function} callback The function into which to pass the block of Ext.data.Records.
         * The function must be passed
         * The Record block object
         * The "arg" argument from the load function
         * A boolean success indicator
         *
         * @param {Object} scope The scope (this reference) in which the callback function is executed. Defaults to the browser window.
         * @param {Object} arg An optional argument which is passed to the callback as its second parameter.
         */
        doRequest : function(action, rs, params, reader, callback, scope, arg) {
            var p = Ext.urlEncode(Ext.apply(params, this.extraParams));

            var url = this.buildUrl(action, rs);
            if (!url) {
                throw new Ext.data.Api.Error('invalid-url', url);
            }
            url = Ext.urlAppend(url, p);

            if(this.nocache){
                url = Ext.urlAppend(url, '_dc=' + (new Date().getTime()));
            }
            var transId = ++Ext.data.ScriptTagProxy.TRANS_ID;
            var trans = {
                id : transId,
                action: action,
                cb : "stcCallback"+transId,
                scriptId : "stcScript"+transId,
                params : params,
                arg : arg,
                url : url,
                callback : callback,
                scope : scope,
                reader : reader
            };
            window[trans.cb] = this.createCallback(action, rs, trans);
            url += String.format("&{0}={1}", this.callbackParam, trans.cb);
            if(this.autoAbort !== false){
                this.abort();
            }

            trans.timeoutId = this.handleFailure.defer(this.timeout, this, [trans]);

            var script = document.createElement("script");
            script.setAttribute("src", url);
            script.setAttribute("type", "text/javascript");
            script.setAttribute("id", trans.scriptId);
            this.head.appendChild(script);

            this.trans = trans;
        },

        // @private createCallback
        createCallback : function(action, rs, trans) {
            var self = this;
            return function(res) {
                self.trans = false;
                self.destroyTrans(trans, true);
                if (action === Ext.data.Api.actions.read) {
                    self.onRead.call(self, action, trans, res);
                } else {
                    self.onWrite.call(self, action, trans, res, rs);
                }
            };
        },
        /**
         * Callback for read actions
         * @param {String} action [Ext.data.Api.actions.create|read|update|destroy]
         * @param {Object} trans The request transaction object
         * @param {Object} res The server response
         * @protected
         */
        onRead : function(action, trans, res) {
            var result;
            try {
                result = trans.reader.readRecords(res);
            }catch(e){
                // @deprecated: fire loadexception
                this.fireEvent("loadexception", this, trans, res, e);

                this.fireEvent('exception', this, 'response', action, trans, res, e);
                trans.callback.call(trans.scope||window, null, trans.arg, false);
                return;
            }
            if (result.success === false) {
                // @deprecated: fire old loadexception for backwards-compat.
                this.fireEvent('loadexception', this, trans, res);

                this.fireEvent('exception', this, 'remote', action, trans, res, null);
            } else {
                this.fireEvent("load", this, res, trans.arg);
            }
            trans.callback.call(trans.scope||window, result, trans.arg, result.success);
        },
        /**
         * Callback for write actions
         * @param {String} action [Ext.data.Api.actions.create|read|update|destroy]
         * @param {Object} trans The request transaction object
         * @param {Object} res The server response
         * @protected
         */
        onWrite : function(action, trans, response, rs) {
            var reader = trans.reader;
            try {
                // though we already have a response object here in STP, run through readResponse to catch any meta-data exceptions.
                var res = reader.readResponse(action, response);
            } catch (e) {
                this.fireEvent('exception', this, 'response', action, trans, res, e);
                trans.callback.call(trans.scope||window, null, res, false);
                return;
            }
            if(!res.success === true){
                this.fireEvent('exception', this, 'remote', action, trans, res, rs);
                trans.callback.call(trans.scope||window, null, res, false);
                return;
            }
            this.fireEvent("write", this, action, res.data, res, rs, trans.arg );
            trans.callback.call(trans.scope||window, res.data, res, true);
        },

        // private
        isLoading : function(){
            return this.trans ? true : false;
        },

        /**
         * Abort the current server request.
         */
        abort : function(){
            if(this.isLoading()){
                this.destroyTrans(this.trans);
            }
        },

        // private
        destroyTrans : function(trans, isLoaded){
            this.head.removeChild(document.getElementById(trans.scriptId));
            clearTimeout(trans.timeoutId);
            if(isLoaded){
                window[trans.cb] = undefined;
                try{
                    delete window[trans.cb];
                }catch(e){}
            }else{
                // if hasn't been loaded, wait for load to remove it to prevent script error
                window[trans.cb] = function(){
                    window[trans.cb] = undefined;
                    try{
                        delete window[trans.cb];
                    }catch(e){}
                };
            }
        },

        // private
        handleFailure : function(trans){
            this.trans = false;
            this.destroyTrans(trans, false);
            if (trans.action === Ext.data.Api.actions.read) {
                // @deprecated firing loadexception
                this.fireEvent("loadexception", this, null, trans.arg);
            }

            this.fireEvent('exception', this, 'response', trans.action, {
                response: null,
                options: trans.arg
            });
            trans.callback.call(trans.scope||window, null, trans.arg, false);
        },

        // inherit docs
        destroy: function(){
            this.abort();
            Ext.data.ScriptTagProxy.superclass.destroy.call(this);
        }
    });