The source code

    /**
     * @class Ext.data.Field
     * This class encapsulates the field definition information specified in the field definition objects
     * passed to {@link Ext.data.Record#create}.
     * Developers do not need to instantiate this class. Instances are created by {@link Ext.data.Record.create}
     * and cached in the {@link Ext.data.Record#fields fields} property of the created Record constructor's prototype.
     */
    Ext.data.Field = function(config){
        if(typeof config == "string"){
            config = {name: config};
        }
        Ext.apply(this, config);

        if(!this.type){
            this.type = "auto";
        }

        var st = Ext.data.SortTypes;
        // named sortTypes are supported, here we look them up
        if(typeof this.sortType == "string"){
            this.sortType = st[this.sortType];
        }

        // set default sortType for strings and dates
        if(!this.sortType){
            switch(this.type){
                case "string":
                    this.sortType = st.asUCString;
                    break;
                case "date":
                    this.sortType = st.asDate;
                    break;
                default:
                    this.sortType = st.none;
            }
        }

        // define once
        var stripRe = /[\$,%]/g;

        // prebuilt conversion function for this field, instead of
        // switching every time we're reading a value
        if(!this.convert){
            var cv, dateFormat = this.dateFormat;
            switch(this.type){
                case "":
                case "auto":
                case undefined:
                    cv = function(v){ return v; };
                    break;
                case "string":
                    cv = function(v){ return (v === undefined || v === null) ? '' : String(v); };
                    break;
                case "int":
                    cv = function(v){
                        return v !== undefined && v !== null && v !== '' ?
                               parseInt(String(v).replace(stripRe, ""), 10) : '';
                        };
                    break;
                case "float":
                    cv = function(v){
                        return v !== undefined && v !== null && v !== '' ?
                               parseFloat(String(v).replace(stripRe, ""), 10) : '';
                        };
                    break;
                case "bool":
                case "boolean":
                    cv = function(v){ return v === true || v === "true" || v == 1; };
                    break;
                case "date":
                    cv = function(v){
                        if(!v){
                            return '';
                        }
                        if(Ext.isDate(v)){
                            return v;
                        }
                        if(dateFormat){
                            if(dateFormat == "timestamp"){
                                return new Date(v*1000);
                            }
                            if(dateFormat == "time"){
                                return new Date(parseInt(v, 10));
                            }
                            return Date.parseDate(v, dateFormat);
                        }
                        var parsed = Date.parse(v);
                        return parsed ? new Date(parsed) : null;
                    };
                 break;

            }
            this.convert = cv;
        }
    };

    Ext.data.Field.prototype = {
        /**
         * @cfg {String} name
         * The name by which the field is referenced within the Record. This is referenced by, for example,
         * the dataIndex property in column definition objects passed to {@link Ext.grid.ColumnModel}.
         * Note: In the simplest case, if no properties other than name are required, a field
         * definition may consist of just a String for the field name.
         */
        /**
         * @cfg {String} type
         * (Optional) The data type for conversion to displayable value if {@link Ext.data.Field#convert convert}
         * has not been specified. Possible values are
         *
         * auto (Default, implies no conversion)
         * string
         * int
         * float
         * boolean
         * date
         */
        /**
         * @cfg {Function} convert
         * (Optional) A function which converts the value provided by the Reader into an object that will be stored
         * in the Record. It is passed the following parameters:
         * v : MixedThe data value as read by the Reader, if undefined will use
         * the configured {@link Ext.data.Field#defaultValue defaultValue}.
         * rec : MixedThe data object containing the row as read by the Reader.
         * Depending on the Reader type, this could be an Array ({@link Ext.data.ArrayReader ArrayReader}), an object
         *  ({@link Ext.data.JsonReader JsonReader}), or an XML element ({@link Ext.data.XMLReader XMLReader}).
         *
         *
    // example of convert function
    function fullName(v, record){
        return record.name.last + ', ' + record.name.first;
    }

    function location(v, record){
        return !record.city ? '' : (record.city + ', ' + record.state);
    }

    var Dude = Ext.data.Record.create([
        {name: 'fullname',  convert: fullName},
        {name: 'firstname', mapping: 'name.first'},
        {name: 'lastname',  mapping: 'name.last'},
        {name: 'city', defaultValue: 'homeless'},
        'state',
        {name: 'location',  convert: location}
    ]);

    // create the data store
    var store = new Ext.data.Store({
        reader: new Ext.data.JsonReader(
            {
                idProperty: 'key',
                root: 'daRoot',
                totalProperty: 'total'
            },
            Dude  // recordType
        )
    });

    var myData = [
        { key: 1,
          name: { first: 'Fat',    last:  'Albert' }
          // notice no city, state provided in data object
        },
        { key: 2,
          name: { first: 'Barney', last:  'Rubble' },
          city: 'Bedrock', state: 'Stoneridge'
        },
        { key: 3,
          name: { first: 'Cliff',  last:  'Claven' },
          city: 'Boston',  state: 'MA'
        }
    ];
         *
         */
        /**
         * @cfg {String} dateFormat
         * (Optional) A format string for the {@link Date#parseDate Date.parseDate} function, or "timestamp" if the
         * value provided by the Reader is a UNIX timestamp, or "time" if the value provided by the Reader is a
         * javascript millisecond timestamp.
         */
        dateFormat: null,
        /**
         * @cfg {Mixed} defaultValue
         * (Optional) The default value used when a Record is being created by a {@link Ext.data.Reader Reader}
         * when the item referenced by the {@link Ext.data.Field#mapping mapping} does not exist in the data
         * object (i.e. undefined). (defaults to "")
         */
        defaultValue: "",
        /**
         * @cfg {String/Number} mapping
         * (Optional) A path expression for use by the {@link Ext.data.DataReader} implementation
         * that is creating the {@link Ext.data.Record Record} to extract the Field value from the data object.
         * If the path expression is the same as the field name, the mapping may be omitted.
         * The form of the mapping expression depends on the Reader being used.
         *
         * {@link Ext.data.JsonReader}The mapping is a string containing the javascript
         * expression to reference the data from an element of the data item's {@link Ext.data.JsonReader#root root} Array. Defaults to the field name.
         * {@link Ext.data.XmlReader}The mapping is an {@link Ext.DomQuery} path to the data
         * item relative to the DOM element that represents the {@link Ext.data.XmlReader#record record}. Defaults to the field name.
         * {@link Ext.data.ArrayReader}The mapping is a number indicating the Array index
         * of the field's value. Defaults to the field specification's Array position.
         *
         * If a more complex value extraction strategy is required, then configure the Field with a {@link #convert}
         * function. This is passed the whole row object, and may interrogate it in whatever way is necessary in order to
         * return the desired data.
         */
        mapping: null,
        /**
         * @cfg {Function} sortType
         * (Optional) A function which converts a Field's value to a comparable value in order to ensure
         * correct sort ordering. Predefined functions are provided in {@link Ext.data.SortTypes}. A custom
         * sort example:
    // current sort     after sort we want
    // +-+------+          +-+------+
    // |1|First |          |1|First |
    // |2|Last  |          |3|Second|
    // |3|Second|          |2|Last  |
    // +-+------+          +-+------+

    sortType: function(value) {
       switch (value.toLowerCase()) // native toLowerCase():
       {
          case 'first': return 1;
          case 'second': return 2;
          default: return 3;
       }
    }
         *
         */
        sortType : null,
        /**
         * @cfg {String} sortDir
         * (Optional) Initial direction to sort ("ASC" or  "DESC").  Defaults to
         * "ASC".
         */
        sortDir : "ASC",
    	/**
    	 * @cfg {Boolean} allowBlank
    	 * (Optional) Used for validating a {@link Ext.data.Record record}, defaults to true.
    	 * An empty value here will cause {@link Ext.data.Record}.{@link Ext.data.Record#isValid isValid}
    	 * to evaluate to false.
    	 */
    	allowBlank : true
    };