groupby.js

var months = [
  "jan", "fév", "mars", "avril", "mai", "juin", "juil", "août", "sep", "oct", "nov", "déc"
]

var data = [
  { user: 'kim', createdAt: '2019-10-01' },
  { user: 'haikel', createdAt: '2019-11-01' },
  { user: 'james', createdAt: '2019-11-01' },
  { user: 'mike', createdAt: '2019-10-01' },
  { user: 'joe', createdAt: '2019-09-01' },
]

function groupBy (data, months, field) {
  return data.reduce((a, c) =>
    (v = months[new Date(c[field]).getMonth()], a[v] ? a[v]++ : a[v] = 1, a), []);
}

let objGrouped = groupBy(data, months, 'createdAt')
console.log(objGrouped); // [ oct: 2, nov: 2, sep: 1 ]

function groupsBy (months) {
  return Object.keys(objGrouped).map(o => {
    return { n: objGrouped[o], m: o, indx: months.indexOf(o) }
  })
}

let objSorted = groupsBy(months).sort((i, j) => i.indx - j.indx)
console.log(objSorted);
//  [ { n: 1, m: 'sep', indx: 8 }, { n: 2, m: 'oct', indx: 9 }, { n: 2, m: 'nov', indx: 10 } ]