const inviteRepo = require('../app/repos/invite')

exports.start = function (cb) {
  console.log('beginning function')
  inviteRepo.all((err, myInvites) => {
    if (err) {
      console.log(err)
      return cb(err)
    }
    if (myInvites) {
      let invite
      let n = Date.now()
      let cutoff = 7  // number of days after which invites get deleted
      n += Number(n) - cutoff
      for (invite in myInvites) {
        if (Number(invite.sentAt) < n) {
          inviteRepo.delete(invite._id, (err, results) => {
            if (err) {
              console.log(err)
              return cb(err)
            }
          })
        }
      }
    }
  })
}

this.start()  // for testing purposes

exports.all = (cb) => {
  db.invites.find().toArray((err, myInvites) => {
    if (err) return cb(err)
    if (myInvites.length === 0) {
      return cb(null, [])
    } else if (myInvites.length > 0) {
      return cb(null, myInvites)
    }
  })
}

db.invites.find().toArray((err, myInvites) => {
             ^

TypeError: Cannot read property 'find' of undefined