How to Delete Documents with Mongoose

You have to add a callback to the statement e.g. User.deleteOne({_id: req.params.userId}, err => console.log(err));

You can't do something like this User.deleteOne({_id: req.params.userId})

If that doesn't work try this longer way

jsTodoList.findOne({ _id: req.params.todoListId }).then((result) => {
  result.participants.push(req.user.id)
  result.save()
  res.json('worked')
})