commands.js

var async = require('async')
  , _ = require('lodash');

module.exports = [
  require('cqrs-domain').defineCommand({
    name: 'addInvoiceLine',
    payload: 'payload'
  }, function (data, aggregate) {
      aggregate.apply('invoiceLineAdded', data); //1 - 1
      calculateTotal(aggregate) //2 - 3
  }),
  require('cqrs-domain').defineCommand({
    name: 'changeInvoiceLine',
    payload: 'payload'
  }, function (data, aggregate) {
    aggregate.apply('invoiceLineChanged', data);
     calculateTotal(aggregate)
  }),
  require('cqrs-domain').defineCommand({
    name: 'removeInvoiceLine',
    payload: 'payload'
  }, function (data, aggregate) {
    aggregate.apply('invoiceLineRemoved', data);
    calculateTotal(aggregate)
  })
];

function calculateTotal(aggregate) {//3 - 4

  // Here for exemple with command AddInvoiceLine lines are empty.
  var lines = aggregate.get('lines');

  async.parallel({
    totalHT: function (callback) {
      var total = _.reduce(lines, function (sum, line) {
        return sum + line['quantity'] * line['article_unitPrice'];
      }, 0)

      callback(null, total);
    },

    totalTTC: function (callback) {
      var total = _.reduce(lines, function (sum, line) {
        return sum + (line['quantity'] * line['article_unitPrice']) * (1 + line['article_TVA']);
      }, 0);

      callback(null, total);
    },

    recapTVA: function (callback) {
      var total = _.reduce(lines, function (recap, line) {
        if (line['article_TVA'] !== 0) {
          recap[line['article_TVA']] = recap[line['article_TVA']] || 0;
          recap[line['article_TVA']] += line['quantity'] * line['article_unitPrice'] * line['article_TVA'];
        }
        return recap;
      }, {});

      callback(null, total);
    }
  }, function(err, result){
    aggregate.apply('invoiceTotalUpdated', result);
  );

}

invoiceLineAdded.js

module.exports = require('cqrs-domain').defineEvent({
  name: 'invoiceLineAdded',
  payload: 'payload' // if not defined it will pass the whole event...
}, function (data, aggregate) {
  var lines = aggregate.get('lines');//4 - 2
  lines.push(data);
  aggregate.set('lines', lines);
});