NodeJS: how to run spawn in parallel properly?

I am running a for loop in which the following is executed:

let perform_vrp = function() {
    //..
    perform_tsp();
}

let perform_tsp = function() {
  //..

  const pyProg = spawn('python3', [process.env.PWD + '/server/vrp_solver/tsp_solver.py', '/../../route_data/' + depot.city + '/' + moment(route.date_driven).format('Y-MM-DD'), 'morning',route.name]);

  winston.info('Solving the TSP for %s...', route.name);

  pyProg.stdout.on('data', function (data) {

    let result_string = data.toString();
    winston.info('Route result for %s is: %s', route.name, result_string);

    let result_array = eval(result_string); 
    //...
  });
}

It basically calls a python script for each item in the for loop.

However, when one script has been completed it finalises all the others as well and continues with stdout with the same 'data' for all items in the for loop.

How can I prevent this from happening and let stdout wait for the proper child process to complete?

UPDATE:

the above is triggered from the following command:

const winston = require('../../server/winston');
const perform_vrp = require('../../server/modules/vrp');
const moment = require('moment');
const Utils = require('../modules/utils');
let utils = new Utils();

let do_vrp = async function(next_delivery_date, cities) {
  winston.info('Generating routes for %s', next_delivery_date);
  for (let index in cities) {
    if (cities.hasOwnProperty(index)) {
      let city = cities[index];
      winston.info('Generating route for %s on %s', city, next_delivery_date);
      await perform_vrp(next_delivery_date,[city]);
      await utils.sleep(60000);
    }
  }
};


let process_routes = async function() {

  //...

  let morning_cities = ['Boston','Chicago'];
  await do_vrp(next_delivery_date.format('YYYY-MM-DD'), morning_cities);
};

process_routes();

from NodeJS: how to run spawn in parallel properly?