shapefile to geojson in node.js with mapnik

shp2geojson.js

var geojsonStream = require('geojson-stream');
var mapnik = require('mapnik');
var fs = require('fs');

var input = 'test/data/world_merc.shp';
var output = './out.geojson';

// set up geojson output stream
var fileOut = fs.createWriteStream(output);
var geojsonOut = geojsonStream.stringify();
geojsonOut.pipe(fileOut);

// set up mapnik to read shapefile
var ds = new mapnik.Datasource({type:'shape',file:input});
var featureset = ds.featureset()
var feat = featureset.next();

// read all features
while (feat) {
    geojsonOut.write(JSON.parse(feat.toJSON()));
    feat = featureset.next();
}

geojsonOut.end();