The source code

    /**
     * @class Ext.data.JsonWriter
     * @extends Ext.data.DataWriter
     * DataWriter extension for writing an array or single {@link Ext.data.Record} object(s) in preparation for executing a remote CRUD action.
     */
    Ext.data.JsonWriter = function(config) {
        Ext.data.JsonWriter.superclass.constructor.call(this, config);

        // careful to respect "returnJson", renamed to "encode"
        // TODO: remove after v3 final release
        if (this.returnJson != undefined) {
            this.encode = this.returnJson;
        }
    }
    Ext.extend(Ext.data.JsonWriter, Ext.data.DataWriter, {
        /**
         * @cfg {Boolean} returnJson Deprecated.  Use {@link Ext.data.JsonWriter#encode} instead.
         */
        returnJson : undefined,
        /**
         * @cfg {Boolean} encode true to {@link Ext.util.JSON#encode encode} the
         * {@link Ext.data.DataWriter#toHash hashed data}. Defaults to true.  When using
         * {@link Ext.data.DirectProxy}, set this to false since Ext.Direct.JsonProvider will perform
         * its own json-encoding.  In addition, if you're using {@link Ext.data.HttpProxy}, setting to false
         * will cause HttpProxy to transmit data using the jsonData configuration-params of {@link Ext.Ajax#request}
         * instead of params.  When using a {@link Ext.data.Store#restful} Store, some serverside frameworks are
         * tuned to expect data through the jsonData mechanism.  In those cases, one will want to set encode: false
         */
        encode : true,

        /**
         * Final action of a write event.  Apply the written data-object to params.
         * @param {String} action [Ext.data.Api.actions.create|read|update|destroy]
         * @param {Record[]} rs
         * @param {Object} http params
         * @param {Object} data object populated according to DataReader meta-data "root" and "idProperty"
         */
        render : function(action, rs, params, data) {
            Ext.apply(params, data);

            if (this.encode === true) { // <-- @deprecated returnJson
                if (Ext.isArray(rs) && data[this.meta.idProperty]) {
                    params[this.meta.idProperty] = Ext.encode(params[this.meta.idProperty]);
                }
                params[this.meta.root] = Ext.encode(params[this.meta.root]);
            }
        },
        /**
         * createRecord
         * @protected
         * @param {Ext.data.Record} rec
         */
        createRecord : function(rec) {
           var data = this.toHash(rec);
           delete data[this.meta.idProperty];
           return data;
        },
        /**
         * updateRecord
         * @protected
         * @param {Ext.data.Record} rec
         */
        updateRecord : function(rec) {
            return this.toHash(rec);

        },
        /**
         * destroyRecord
         * @protected
         * @param {Ext.data.Record} rec
         */
        destroyRecord : function(rec) {
            return rec.id;
        }
    });