Using JSONStream to read/filter/write a GeoJSON. Outputs only Polygon geometries.

filter-geojson-polygon-stream.js

const fs = require('fs');
const JSONStream = require('JSONStream');
const es = require('event-stream')

let stream = fs.createReadStream('./data/kensington/kensington.geojson', {encoding: 'utf8'});
let WriteStream =fs.createWriteStream('./data/kensington/kensington_clean.geojson', {encoding: 'utf8'});

let open = '{"type":"FeatureCollection","features":[',
    close = ']}';
let transformStream =JSONStream.stringify(open, '\n,\n', close);

let  filterOutMultiPolygons= type=> {
  if (type.geometry.type == 'Polygon')
        return type
}
let parser = JSONStream.parse('features.*', filterOutMultiPolygons);

stream.pipe(parser).pipe(transformStream).pipe(WriteStream);

//parser.on('data', d =>d )