index.js

'use strict'

require('now-env')
const Twit = require('twit')
const Task = require('data.task')
const bot = new Twit({
  consumer_key: process.env.CONSUMER_KEY,
  consumer_secret: process.env.CONSUMER_SECRET,
  access_token: process.env.ACCESS_TOKEN,
  access_token_secret: process.env.ACCESS_TOKEN_SECRET,
  timeout_ms: 60 * 1000
})

module.exports = () => {
  const diff = xs => xy => xs.filter(x => !xy.includes(x))
  const sleep = ms => new Task((rej, res) => setTimeout(res, ms))

  const get = (url, opts = { count: 200 }) =>
    new Task((rej, res) =>
      bot.get(url, opts, (err, data, response) => (err ? rej(err) : res(data)))
    )

  const notify = users =>
    users.map(user =>
      bot.post('direct_messages/new', {
        screen_name: process.env.TWITTER_HANDLER,
        text: `User @${user} stopped following you!`
      })
    )

  const getFollowers = (all = [], cursor = 0) =>
    get('followers/list', { count: 200, cursor }).chain(
      ({ users, next_cursor_str: nextCursor }) =>
        nextCursor && nextCursor === '0'
          ? Task.of(all.concat(users).map(user => user.screen_name))
          : getFollowers(all.concat(users), nextCursor)
    )

  const main = () =>
    Task.of(diff)
      .ap(getFollowers())
      .ap(sleep(15 * 60 * 1000).chain(getFollowers))
      .fork(console.error, notify)

  setInterval(main, 15 * 60 * 1000)
}