users.js

app.route('/users')

  .post((req, res) => {
    User.count({email: req.body.email}, function (err, count) {
      if (count == 0) {
        // email doesn't exist
        const user = new User()
        user.email = req.body.email
        user.newsletter = req.body.newsletter
        user.save((err) => {
          if (err)
            res.send(err)
          res.json({message: 'User created'})
          io.emit('checkEmail', {check: 'User added'})
        })
      } else {
        // email exists
        User.find({email: req.body.email}, (err, users) => {
          if (err)
            res.send(err)
          res.json(users[0]._id)
          io.emit('checkEmail', {check: users[0]._id})
        }).limit(1)
      }
    })
  })