attempting to use jParser to extract dimensions of JPG image file

gistfile1.js

var fs = require("fs");
var jParser = require("jParser");
var eof = false;

fs.readFile("sample.jpg",function(err,data) {
        if (!err) {
                var view = new jDataView(data, undefined, undefined, false);
                var parser = new jParser(view,{

// JPG
                        hex: function() {
                                return "0x" + this.parse('uint8').toString(16);
                        },
                        marker: ['array', 'hex', 2],
                        variable_length_segment: {
                                segment_length: 'uint16',
                                data_precision: 'uint8',
                                width: 'uint16',
                                height: 'uint16',
                                data: ['array', 'uint8', function(){
                                        return this.current.segment_length - 7;
                                }]
                        },
                        segment: function(){
                                var marker = this.parse('marker'), segment, next_marker;
                                // end-of-file?
                                if (marker[0] == "0xff" && marker[1] == "0xd9") {
                                        eof = true;
                                        return;
                                }

                                // SOF segments (which should hold dimensions!)
                                if (marker[1] == "0xc0" || marker[1] == "0xc2") {
                                        segment = this.parse('variable_length_segment');
                                        segment.marker = marker;
                                        return segment;
                                }
                                else {
                                        do {
                                                next_marker = this.parse('marker');
                                        } while (!((next_marker[0] == '0xff' && next_marker[1] != '0x0') || (next_marker[1] == '0xff')));
                                        this.seek(this.tell()-2);

                                        // just return an empty marker segment
                                        return { marker: marker };
                                }
                        },
                        header: ['array', 'hex', 2]
                });


                var header, segments = [];
                try {
                        header = parser.parse("header");
                        do {
                                segments.push(parser.parse("segment"));
                        } while (!eof);
                } catch (err) { console.log(err); }

                console.log(JSON.stringify(segments));
        }
        else console.log(err);
});