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.JsonReader
     * @extends Ext.data.DataReader
     * Data reader class to create an Array of {@link Ext.data.Record} objects
     * from a JSON packet based on mappings in a provided {@link Ext.data.Record}
     * constructor.
     * Example code:
     *
    var myReader = new Ext.data.JsonReader({
        // metadata configuration options:
        {@link #idProperty}: 'id'
        {@link #root}: 'rows',
        {@link #totalProperty}: 'results',
        {@link Ext.data.DataReader#messageProperty}: "msg"  // The element within the response that provides a user-feedback message (optional)

        // the fields config option will internally create an {@link Ext.data.Record}
        // constructor that provides mapping for reading the record data objects
        {@link Ext.data.DataReader#fields fields}: [
            // map Record's 'firstname' field to data object's key of same name
            {name: 'name', mapping: 'firstname'},
            // map Record's 'job' field to data object's 'occupation' key
            {name: 'job', mapping: 'occupation'}
        ]
    });

     * This would consume a JSON data object of the form:
    {
        results: 2000, // Reader's configured {@link #totalProperty}
        rows: [        // Reader's configured {@link #root}
            // record data objects:
            { {@link #idProperty id}: 1, firstname: 'Bill', occupation: 'Gardener' },
            { {@link #idProperty id}: 2, firstname: 'Ben' , occupation: 'Horticulturalist' },
            ...
        ]
    }

     * Automatic configuration using metaData
     * It is possible to change a JsonReader's metadata at any time by including
     * a metaData property in the JSON data object. If the JSON data
     * object has a metaData property, a {@link Ext.data.Store Store}
     * object using this Reader will reconfigure itself to use the newly provided
     * field definition and fire its {@link Ext.data.Store#metachange metachange}
     * event. The metachange event handler may interrogate the metaData
     * property to perform any configuration required.
     * Note that reconfiguring a Store potentially invalidates objects which may
     * refer to Fields or Records which no longer exist.
     * To use this facility you would create the JsonReader like this:
    var myReader = new Ext.data.JsonReader();

     * The first data packet from the server would configure the reader by
     * containing a metaData property and the data. For
     * example, the JSON data object might take the form:
    {
        metaData: {
            "{@link #idProperty}": "id",
            "{@link #root}": "rows",
            "{@link #totalProperty}": "results"
            "{@link #successProperty}": "success",
            "{@link Ext.data.DataReader#fields fields}": [
                {"name": "name"},
                {"name": "job", "mapping": "occupation"}
            ],
            // used by store to set its sortInfo
            "sortInfo":{
               "field": "name",
               "direction": "ASC"
            },
            // {@link Ext.PagingToolbar paging data} (if applicable)
            "start": 0,
            "limit": 2,
            // custom property
            "foo": "bar"
        },
        // Reader's configured {@link #successProperty}
        "success": true,
        // Reader's configured {@link #totalProperty}
        "results": 2000,
        // Reader's configured {@link #root}
        // (this data simulates 2 results {@link Ext.PagingToolbar per page})
        "rows": [ // *Note: this must be an Array
            { "id": 1, "name": "Bill", "occupation": "Gardener" },
            { "id": 2, "name":  "Ben", "occupation": "Horticulturalist" }
        ]
    }
     *
     * The metaData property in the JSON data object should contain:
     *
     * any of the configuration options for this class
     * a {@link Ext.data.Record#fields fields} property which
     * the JsonReader will use as an argument to the
     * {@link Ext.data.Record#create data Record create method} in order to
     * configure the layout of the Records it will produce.
     * a {@link Ext.data.Store#sortInfo sortInfo} property
     * which the JsonReader will use to set the {@link Ext.data.Store}'s
     * {@link Ext.data.Store#sortInfo sortInfo} property
     * any custom properties needed
     *
     *
     * @constructor
     * Create a new JsonReader
     * @param {Object} meta Metadata configuration options.
     * @param {Array/Object} recordType
     * Either an Array of {@link Ext.data.Field Field} definition objects (which
     * will be passed to {@link Ext.data.Record#create}, or a {@link Ext.data.Record Record}
     * constructor created from {@link Ext.data.Record#create}.
     */
    Ext.data.JsonReader = function(meta, recordType){
        meta = meta || {};
        /**
         * @cfg {String} idProperty [id] Name of the property within a row object
         * that contains a record identifier value.  Defaults to id
         */
        /**
         * @cfg {String} successProperty [success] Name of the property from which to
         * retrieve the success attribute. Defaults to success.  See
         * {@link Ext.data.DataProxy}.{@link Ext.data.DataProxy#exception exception}
         * for additional information.
         */
        /**
         * @cfg {String} totalProperty [total] Name of the property from which to
         * retrieve the total number of records in the dataset. This is only needed
         * if the whole dataset is not passed in one go, but is being paged from
         * the remote server.  Defaults to total.
         */
        /**
         * @cfg {String} root [undefined] Required.  The name of the property
         * which contains the Array of row objects.  Defaults to undefined.
         * An exception will be thrown if the root property is undefined. The data
         * packet value for this property should be an empty array to clear the data
         * or show no data.
         */
        Ext.applyIf(meta, {
            idProperty: 'id',
            successProperty: 'success',
            totalProperty: 'total'
        });

        Ext.data.JsonReader.superclass.constructor.call(this, meta, recordType || meta.fields);
    };
    Ext.extend(Ext.data.JsonReader, Ext.data.DataReader, {
        /**
         * This JsonReader's metadata as passed to the constructor, or as passed in
         * the last data packet's metaData property.
         * @type Mixed
         * @property meta
         */
        /**
         * This method is only used by a DataProxy which has retrieved data from a remote server.
         * @param {Object} response The XHR object which contains the JSON data in its responseText.
         * @return {Object} data A data block which is used by an Ext.data.Store object as
         * a cache of Ext.data.Records.
         */
        read : function(response){
            var json = response.responseText;
            var o = Ext.decode(json);
            if(!o) {
                throw {message: 'JsonReader.read: Json object not found'};
            }
            return this.readRecords(o);
        },

        /*
         * TODO: refactor code between JsonReader#readRecords, #readResponse into 1 method.
         * there's ugly duplication going on due to maintaining backwards compat. with 2.0.  It's time to do this.
         */
        /**
         * Decode a JSON response from server.
         * @param {String} action [Ext.data.Api.actions.create|read|update|destroy]
         * @param {Object} response The XHR object returned through an Ajax server request.
         * @return {Response} A {@link Ext.data.Response Response} object containing the data response, and also status information.
         */
        readResponse : function(action, response) {
            var o = (response.responseText !== undefined) ? Ext.decode(response.responseText) : response;
            if(!o) {
                throw new Ext.data.JsonReader.Error('response');
            }

            var root = this.getRoot(o),
                success = this.getSuccess(o);
            if (success && action === Ext.data.Api.actions.create) {
                var def = Ext.isDefined(root);
                if (def && Ext.isEmpty(root)) {
                    throw new Ext.data.JsonReader.Error('root-empty', this.meta.root);
                }
                else if (!def) {
                    throw new Ext.data.JsonReader.Error('root-undefined-response', this.meta.root);
                }
            }

            // instantiate response object
            var res = new Ext.data.Response({
                action: action,
                success: success,
                data: (root) ? this.extractData(root, false) : [],
                message: this.getMessage(o),
                raw: o
            });

            // blow up if no successProperty
            if (Ext.isEmpty(res.success)) {
                throw new Ext.data.JsonReader.Error('successProperty-response', this.meta.successProperty);
            }
            return res;
        },

        /**
         * Create a data block containing Ext.data.Records from a JSON object.
         * @param {Object} o An object which contains an Array of row objects in the property specified
         * in the config as 'root, and optionally a property, specified in the config as 'totalProperty'
         * which contains the total size of the dataset.
         * @return {Object} data A data block which is used by an Ext.data.Store object as
         * a cache of Ext.data.Records.
         */
        readRecords : function(o){
            /**
             * After any data loads, the raw JSON data is available for further custom processing.  If no data is
             * loaded or there is a load exception this property will be undefined.
             * @type Object
             */
            this.jsonData = o;
            if(o.metaData){
                this.onMetaChange(o.metaData);
            }
            var s = this.meta, Record = this.recordType,
                f = Record.prototype.fields, fi = f.items, fl = f.length, v;

            var root = this.getRoot(o), c = root.length, totalRecords = c, success = true;
            if(s.totalProperty){
                v = parseInt(this.getTotal(o), 10);
                if(!isNaN(v)){
                    totalRecords = v;
                }
            }
            if(s.successProperty){
                v = this.getSuccess(o);
                if(v === false || v === 'false'){
                    success = false;
                }
            }

            // TODO return Ext.data.Response instance instead.  @see #readResponse
            return {
                success : success,
                records : this.extractData(root, true), // <-- true to return [Ext.data.Record]
                totalRecords : totalRecords
            };
        },

        // private
        buildExtractors : function() {
            if(this.ef){
                return;
            }
            var s = this.meta, Record = this.recordType,
                f = Record.prototype.fields, fi = f.items, fl = f.length;

            if(s.totalProperty) {
                this.getTotal = this.createAccessor(s.totalProperty);
            }
            if(s.successProperty) {
                this.getSuccess = this.createAccessor(s.successProperty);
            }
            if (s.messageProperty) {
                this.getMessage = this.createAccessor(s.messageProperty);
            }
            this.getRoot = s.root ? this.createAccessor(s.root) : function(p){return p;};
            if (s.id || s.idProperty) {
                var g = this.createAccessor(s.id || s.idProperty);
                this.getId = function(rec) {
                    var r = g(rec);
                    return (r === undefined || r === '') ? null : r;
                };
            } else {
                this.getId = function(){return null;};
            }
            var ef = [];
            for(var i = 0; i < fl; i++){
                f = fi[i];
                var map = (f.mapping !== undefined && f.mapping !== null) ? f.mapping : f.name;
                ef.push(this.createAccessor(map));
            }
            this.ef = ef;
        },

        /**
         * @ignore
         * TODO This isn't used anywhere??  Don't we want to use this where possible instead of complex #createAccessor?
         */
        simpleAccess : function(obj, subsc) {
            return obj[subsc];
        },

        /**
         * @ignore
         */
        createAccessor : function(){
            var re = /[\[\.]/;
            return function(expr) {
                if(Ext.isEmpty(expr)){
                    return Ext.emptyFn;
                }
                if(Ext.isFunction(expr)){
                    return expr;
                }
                var i = String(expr).search(re);
                if(i >= 0){
                    return new Function('obj', 'return obj' + (i > 0 ? '.' : '') + expr);
                }
                return function(obj){
                    return obj[expr];
                };

            };
        }(),

        /**
         * type-casts a single row of raw-data from server
         * @param {Object} data
         * @param {Array} items
         * @param {Integer} len
         * @private
         */
        extractValues : function(data, items, len) {
            var f, values = {};
            for(var j = 0; j < len; j++){
                f = items[j];
                var v = this.ef[j](data);
                values[f.name] = f.convert((v !== undefined) ? v : f.defaultValue, data);
            }
            return values;
        }
    });

    /**
     * @class Ext.data.JsonReader.Error
     * Error class for JsonReader
     */
    Ext.data.JsonReader.Error = Ext.extend(Ext.Error, {
        constructor : function(message, arg) {
            this.arg = arg;
            Ext.Error.call(this, message);
        },
        name : 'Ext.data.JsonReader'
    });
    Ext.apply(Ext.data.JsonReader.Error.prototype, {
        lang: {
            'response': 'An error occurred while json-decoding your server response',
            'successProperty-response': 'Could not locate your "successProperty" in your server response.  Please review your JsonReader config to ensure the config-property "successProperty" matches the property in your server-response.  See the JsonReader docs.',
            'root-undefined-config': 'Your JsonReader was configured without a "root" property.  Please review your JsonReader config and make sure to define the root property.  See the JsonReader docs.',
            'idProperty-undefined' : 'Your JsonReader was configured without an "idProperty"  Please review your JsonReader configuration and ensure the "idProperty" is set (e.g.: "id").  See the JsonReader docs.',
            'root-empty': 'Data was expected to be returned by the server in the "root" property of the response.  Please review your JsonReader configuration to ensure the "root" property matches that returned in the server-response.  See JsonReader docs.'
        }
    });