groupMessagesByUser.js

onst getLast = (arr, def) => (arr[arr.length - 1] || def)

const getLastUser = arr => {
  const last = getLast(arr, [])
  return getLast(last, {})
}

const pushIntoLast = (arr, value) => {
  const last = getLast(arr, [])
  last.push(value)
  return arr
}

const groupMessagesByUser = messages => {
  console.log(messages)
  return messages.reduce((acc, current) => {
    const prevUser = getLastUser(acc)

    if (prevUser.users_id === current.users_id) {
      return pushIntoLast(acc, current)
    }

    return [ ...acc, [ current ] ]
  }, [])
}