Problem: array of streams. There are two input CSV files streamed to one output JSON file stream. THe code works, but what about scaling the number of inputs? Solution: reduce an array of streams.

generate_taxonomy.js

#!/usr/bin/env node

const fs = require('fs')
const path = require('path')

/**
 * Converts a tree path to number
 *
 * BE.2.8 -> 2 + 100 * 8 + 100^2 * 0 + 100^3 * 0
 *
 * @param {String} treePath
 *
 * @returns {Number}
 */

function treePathToNumber (treePath) {
  const fields = treePath.split('\.')

  const num1 = parseInt(fields[1]) || 0
  const num2 = parseInt(fields[2]) || 0
  const num3 = parseInt(fields[3]) || 0
  const num4 = parseInt(fields[4]) || 0

  const base = 100

  return num1 + base * num2 + Math.pow(base, 2) * num3 + Math.pow(base, 3) * num4
}

const taxonomyItFile = path.join(__dirname, 'taxonomy_it.csv')
const taxonomyEsFile = path.join(__dirname, 'taxonomy_es.csv')
const taxonomyFile = path.join(__dirname, 'taxonomy.json')

const taxonomyStream = fs.createWriteStream(taxonomyFile, {
  flags: 'w',
  defaultEncoding: 'utf8',
  autoClose: true
})

const taxonomyItStream = fs.createReadStream(taxonomyItFile)

/**
 * Converts a taxonmy line to JSON string
 *
 * @param {String} line
 *
 * @returns {String}
 */

function lineToJSON (line) {
  const fields = line.split('\t')

  const name = fields[0]
  const treePath = fields[1]

  const id = treePathToNumber(treePath)

  return JSON.stringify({
    id,
    name,
    price: 0.1
  })
}

var isFirstLine = true
var lastChunk = ''

taxonomyItStream.on('data', function (data) {
  if (isFirstLine) {
    taxonomyStream.write('{\n"IT":[\n')
    isFirstLine = false
  } else {
    taxonomyStream.write('\n,')
  }

  var chunks = data.toString('utf8').split('\n')

  var numChunks = chunks.length

  // Concat previous last chunk and first one.
  taxonomyStream.write(lineToJSON(lastChunk + chunks[0]))

  lastChunk = chunks[numChunks - 1]

  for (var i = 1; i < numChunks - 1; i++) {
    if (!isFirstLine) taxonomyStream.write('\n,')

    taxonomyStream.write(lineToJSON(chunks[i]))
  }
})

taxonomyItStream.on('close', function () {
  taxonomyStream.write('\n],')

  // Reset vars.
  isFirstLine = true
  lastChunk = ''

  const taxonomyEsStream = fs.createReadStream(taxonomyEsFile)

  taxonomyEsStream.on('data', function (data) {
    if (isFirstLine) {
      taxonomyStream.write('\n"ES":[\n')
      isFirstLine = false
    } else {
      taxonomyStream.write('\n,')
    }

    var chunks = data.toString('utf8').split('\n')

    var numChunks = chunks.length

    // Concat previous last chunk and first one.
    taxonomyStream.write(lineToJSON(lastChunk + chunks[0]))

    lastChunk = chunks[numChunks - 1]

    for (var i = 1; i < numChunks - 1; i++) {
      if (!isFirstLine) taxonomyStream.write('\n,')

      taxonomyStream.write(lineToJSON(chunks[i]))
    }
  })

  taxonomyEsStream.on('close', function () {
    taxonomyStream.write('\n]\n}')
    taxonomyStream.end()
  })
})

taxonomy_es.csv

MOBILE_AUDIENCE	BE.1	Mobile Audience				This category contains people whose behaviour was determinated from their Mobile device
AUTOMOTIVE	BE.1.1	Mobile Audience	Automotive			This category contains people who are interested in automotive
CAR_DEALER	BE.1.1.1	Mobile Audience	Automotive	In-Market Auto Buyer		In-market auto shoppers who have visited car dealerships for a new vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
CHRYSLER	BE.1.1.1.10	Mobile Audience	Automotive	In-Market Auto Buyer	Chrysler	In-market auto shoppers who have visited car dealerships for a new Chrysler vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
RETAIL	BE.1.7	Mobile Audience	Retail			Shoppers who often visited different kind of retailers (e.g. electronic retailers, grocery shops, big box stores, apparel shops etc.)
APPAREL_RETAIL	BE.1.7.1	Mobile Audience	Retail	Apparel Shoppers		Shoppers who have visited Apparel Shops. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
PIMKIE	BE.1.7.1.106	Mobile Audience	Retail	Apparel Shoppers	Pimkie	Shoppers who have visited Pimkie. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
PRIMARK	BE.1.7.1.109	Mobile Audience	Retail	Apparel Shoppers	Primark	Shoppers who have visited Primark. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
PROMOD	BE.1.7.1.110	Mobile Audience	Retail	Apparel Shoppers	Promod	Shoppers who have visited Promod. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
PULL_AND_BEAR	BE.1.7.1.111	Mobile Audience	Retail	Apparel Shoppers	Pull and Bear	Shoppers who have visited Pull and Bear. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
PUMA_ITALIA	BE.1.7.1.112	Mobile Audience	Retail	Apparel Shoppers	Puma	Shoppers who have visited Puma. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
STRADIVARIUS	BE.1.7.1.124	Mobile Audience	Retail	Apparel Shoppers	Stradivarius	Shoppers who have visited Stradivarius. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
SUPERDRY	BE.1.7.1.125	Mobile Audience	Retail	Apparel Shoppers	Superdry	Shoppers who have visited Superdry. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.
THE_NORTH_FACE	BE.1.7.1.131	Mobile Audience	Retail	Apparel Shoppers	The North Face	Shoppers who have visited The North Face. Members of this category on average visit an apparel shop at least once per month, spending more than 10 minutes at each visit.

taxonomy_it.csv

MOBILE_AUDIENCE	BE.1	Mobile Audience				This category contains people whose behaviour was determinated from their Mobile device
AUTOMOTIVE	BE.1.1	Mobile Audience	Automotive			This category contains people who are interested in automotive
CAR_DEALER	BE.1.1.1	Mobile Audience	Automotive	In-Market Auto Buyer		In-market auto shoppers who have visited car dealerships for a new vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
CITROEN	BE.1.1.1.11	Mobile Audience	Automotive	In-Market Auto Buyer	Citroen	In-market auto shoppers who have visited car dealerships for a new Citroen vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
DACIA	BE.1.1.1.12	Mobile Audience	Automotive	In-Market Auto Buyer	Dacia	In-market auto shoppers who have visited car dealerships for a new Dacia vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
DR	BE.1.1.1.14	Mobile Audience	Automotive	In-Market Auto Buyer	Dr	In-market auto shoppers who have visited car dealerships for a new Dr vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
DS	BE.1.1.1.15	Mobile Audience	Automotive	In-Market Auto Buyer	DS	In-market auto shoppers who have visited car dealerships for a new DS vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
FERRARI	BE.1.1.1.16	Mobile Audience	Automotive	In-Market Auto Buyer	Ferrari	In-market auto shoppers who have visited car dealerships for a new Ferrari vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
FIAT	BE.1.1.1.17	Mobile Audience	Automotive	In-Market Auto Buyer	Fiat-Abarth	In-market auto shoppers who have visited car dealerships for a new Fiat vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
FORD	BE.1.1.1.18	Mobile Audience	Automotive	In-Market Auto Buyer	Ford	In-market auto shoppers who have visited car dealerships for a new Ford vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
ALFA_ROMEO	BE.1.1.1.2	Mobile Audience	Automotive	In-Market Auto Buyer	Alfa romeo	In-market auto shoppers who have visited car dealerships for a new Alfa Romeo vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
HONDA	BE.1.1.1.20	Mobile Audience	Automotive	In-Market Auto Buyer	Honda	In-market auto shoppers who have visited car dealerships for a new Honda vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
HYUNDAI	BE.1.1.1.21	Mobile Audience	Automotive	In-Market Auto Buyer	Hyundai	In-market auto shoppers who have visited car dealerships for a new Hyundai vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
INFINITI	BE.1.1.1.22	Mobile Audience	Automotive	In-Market Auto Buyer	Infiniti	In-market auto shoppers who have visited car dealerships for a new Infiniti vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
JAGUAR	BE.1.1.1.23	Mobile Audience	Automotive	In-Market Auto Buyer	Jaguar	In-market auto shoppers who have visited car dealerships for a new Jaguar vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
JEEP	BE.1.1.1.24	Mobile Audience	Automotive	In-Market Auto Buyer	Jeep	In-market auto shoppers who have visited car dealerships for a new Jeep vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
KIA	BE.1.1.1.25	Mobile Audience	Automotive	In-Market Auto Buyer	Kia	In-market auto shoppers who have visited car dealerships for a new Kia vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
LAMBORGHINI	BE.1.1.1.26	Mobile Audience	Automotive	In-Market Auto Buyer	Lamborghini	In-market auto shoppers who have visited car dealerships for a new Lamborghini vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
LANCIA	BE.1.1.1.27	Mobile Audience	Automotive	In-Market Auto Buyer	Lancia	In-market auto shoppers who have visited car dealerships for a new Lancia vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .
LAND_ROVER	BE.1.1.1.28	Mobile Audience	Automotive	In-Market Auto Buyer	Land Rover	In-market auto shoppers who have visited car dealerships for a new Land Rover vehicle. Profiles in this audience visited at least one car dealer in the last month, spending more than 30 minutes per visit.  .