que system node js

child.js

module.exports = function (inp, callback) {
    setTimeout(() => {
        callback(null, inp);
    }, 3000);
}

main.js

var workerFarm = require('worker-farm');

const FARM_OPTIONS = {
    maxConcurrentWorkers: 2,
    maxConcurrentCallsPerWorker: 1
}

var task = workerFarm(FARM_OPTIONS, require.resolve('./child'));
var workers = 0;

for (var i = 0; i < 10; i++) {
    task('#' + i + ' FOO', function (err, outp) {
        console.log(outp)
        if (++workers == 10)
            workerFarm.end(task)
    })
}