make a diff-list of two mailchimp exports

diff.js

'use strict';
var fsp = require('fs-promise');
var differenceBy = require('lodash/differenceBy');
var keys = require('lodash/keys');
var values = require('lodash/values');
var parseCsv = require('csv-parse');

function parse(fileContent) {
  return new Promise(function(resolve, reject) {
    parseCsv(fileContent, {delimiter: ',', quote: '"', columns: true}, function (err, data) {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

Promise.all([
  fsp.readFile('listA.csv', 'utf8').then(parse),
  fsp.readFile('listB.csv', 'utf8').then(parse)
]).then(function(res) {
  var usersNl = res[0];
  var users = res[1];
  var header = keys(users[0]).join('","');
  var rows = differenceBy(users, usersNl, 'Email Address').map(function(user) {
    return values(user).join('","');
  });
  var out = '"' + [header].concat(rows).join('"\n"') + '"';
  fsp.writeFile('listA-listB.csv', out, 'utf8');
});

• create two exports (listA.csv, listB.csv)
• run node diff.js
• diff is now in listA-listB.csv

Do not use MS Excel to modify lists before re-importing to Mailchimp. Script is working beautifully.