Batch Requests Async or Sync

batch.js

var moment = require('moment');
var _      = require('lodash');

function p(i) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve({id: i});
    }, 300)
  })
}

function Batch(opt) {
  this.fn          = opt.fn;
  this.transformFn = opt.transformFn;
  this.args        = opt.args;
  this.start       = opt.start;
  this.total       = opt.start === 0 ? opt.total : opt.total + 1;
  this.chunk       = opt.chunk;
  this.debug       = opt.debug || false;
  
  this.sendAsync = function() {
    if (this.debug) console.log(moment().format());

    var self = this;

    return new Promise((resolve, reject) => {
      var batchData = [];

      function sendRequest() {
        if (self.start >= self.total) {
          if (self.debug) console.log(moment().format());
          return resolve(batchData);
        }
        
        var promises = [];
        for (var i = self.start; i < self.start + self.chunk; i++) {
          if (i < self.total) {
            self.args[0] = i;      
            var promise = self.fn.apply(null, self.args);

            promises.push(promise);            
          }
        }
        
        Promise.all(promises)
        .then(data => {
          batchData = batchData.concat(self.transformFn(data));

          if (self.debug) console.log(batchData);

          self.start = self.start + self.chunk;
          sendRequest();
        })
        .catch(reject);
      }
      
      sendRequest();
    })
  }
  
  this.sendSync = function() {
    if (this.debug) console.log(moment().format());

    var self = this;
    
    return new Promise((resolve, reject) => {
      var batchData = [];

      function sendRequest() {
        if (self.start >= self.total) {
          if (self.debug) console.log(moment().format());

          return resolve(batchData);
        }
        
        self.args[0] = self.start;
        
        var promise = self.fn.apply(null, self.args);
        
        promise
        .then(data => {
          batchData = batchData.concat(self.transformFn(data));

          if (self.debug) console.log(batchData);

          self.start++;
          sendRequest();
        })
        .catch(reject);
      }
      
      sendRequest();
    })
  }
}

var data = {
  fulfillments: []
};

for (var i = 0; i < 50; i++) {
  data.fulfillments.push({id: i});
}

function getEvenNumbers(data) {
  return _.chain(data)
  .filter(d => d.id % 2 === 0)
  .map(d => d.id)
  .value();
}

var sync = new Batch({
  fn: p,
  transformFn: getEvenNumbers,
  args: [],
  start: 0,
  total: data.fulfillments.length,
  chunk: 5,
  debug: true
});

//sync.sendSync()
sync.sendAsync()
.then(data => {
  console.log('done', data);
})
.catch(err => {
  console.log('error', err);
});