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 = Ext.extend(Ext.data.DataWriter, {
        /**
         * @cfg {Boolean} encode <p><tt>true</tt> to {@link Ext.util.JSON#encode JSON encode} the
         * {@link Ext.data.DataWriter#toHash hashed data} into a standard HTTP parameter named after this
         * Reader's <code>meta.root</code> property which, by default is imported from the associated Reader. Defaults to <tt>true</tt>.</p>
         * <p>If set to <code>false</code>, the hashed data is {@link Ext.util.JSON#encode JSON encoded}, along with
         * the associated {@link Ext.data.Store}'s {@link Ext.data.Store#baseParams baseParams}, into the POST body.</p>
         * <p>When using {@link Ext.data.DirectProxy}, set this to <tt>false</tt> since Ext.Direct.JsonProvider will perform
         * its own json-encoding.  In addition, if you're using {@link Ext.data.HttpProxy}, setting to <tt>false</tt>
         * will cause HttpProxy to transmit data using the <b>jsonData</b> configuration-params of {@link Ext.Ajax#request}
         * instead of <b>params</b>.</p>
         * <p>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 <b>encode: <tt>false</tt></b>, as in
         * let the lower-level connection object (eg: Ext.Ajax) do the encoding.</p>
         */
        encode : true,
        /**
         * @cfg {Boolean} encodeDelete False to send only the id to the server on delete, true to encode it in an object
         * literal, eg: <pre><code>
    {id: 1}
     * </code></pre> Defaults to <tt>false</tt>
         */
        encodeDelete: false,

        constructor : function(config){
            Ext.data.JsonWriter.superclass.constructor.call(this, config);
        },

        /**
         * <p>This method should not need to be called by application code, however it may be useful on occasion to
         * override it, or augment it with an {@link Function#createInterceptor interceptor} or {@link Function#createSequence sequence}.</p>
         * <p>The provided implementation encodes the serialized data representing the Store's modified Records into the Ajax request's
         * <code>params</code> according to the <code>{@link #encode}</code> setting.</p>
         * @param {Object} Ajax request params object to write into.
         * @param {Object} baseParams as defined by {@link Ext.data.Store#baseParams}.  The baseParms must be encoded by the extending class, eg: {@link Ext.data.JsonWriter}, {@link Ext.data.XmlWriter}.
         * @param {Object/Object[]} data Data object representing the serialized modified records from the Store. May be either a single object,
         * or an Array of objects - user implementations must handle both cases.
         */
        render : function(params, baseParams, data) {
            if (this.encode === true) {
                // Encode here now.
                Ext.apply(params, baseParams);
                params[this.meta.root] = Ext.encode(data);
            } else {
                // defer encoding for some other layer, probably in {@link Ext.Ajax#request}.  Place everything into "jsonData" key.
                var jdata = Ext.apply({}, baseParams);
                jdata[this.meta.root] = data;
                params.jsonData = jdata;
            }
        },
        /**
         * Implements abstract Ext.data.DataWriter#createRecord
         * @protected
         * @param {Ext.data.Record} rec
         * @return {Object}
         */
        createRecord : function(rec) {
           return this.toHash(rec);
        },
        /**
         * Implements abstract Ext.data.DataWriter#updateRecord
         * @protected
         * @param {Ext.data.Record} rec
         * @return {Object}
         */
        updateRecord : function(rec) {
            return this.toHash(rec);

        },
        /**
         * Implements abstract Ext.data.DataWriter#destroyRecord
         * @protected
         * @param {Ext.data.Record} rec
         * @return {Object}
         */
        destroyRecord : function(rec){
            if(this.encodeDelete){
                var data = {};
                data[this.meta.idProperty] = rec.id;
                return data;
            }else{
                return rec.id;
            }
        }
    });