How to properly filter data in MongoDB?

Question

JavaScript

Hello everyone, please tell me how to do the following filtering in MongoDB?
I have three documents.

{
    "_id": {
        "$oid": "5d7f70d46b647c1a74d9b8aa"
    },
    "firstName": "user",
    "lastName": "one",
    "phoneNumber": "[phone]",
    "city": "Москва",
    "userId": {
        "$oid": "5d5ea4596886b1107c2be849"
    },
    "__v": 0
}


{
    "_id": {
        "$oid": "5d7f70d46b647c1a74d9b8aa"
    },
    "firstName": "user",
    "lastName": "two",
    "phoneNumber": "[phone]",
    "city": "Сочи",
    "userId": {
        "$oid": "5d5ea4596886b1107c2be849"
    },
    "__v": 0
}


{
    "_id": {
        "$oid": "5d7f70d46b647c1a74d9b8aa"
    },
    "firstName": "user",
    "lastName": "two",
    "phoneNumber": "[phone]",
    "city": "Москва",
    "userId": {
        "$oid": "5d5ea4596886b1107c2be849"
    },
    "__v": 0
}

For example, the following array comes to my input: The
['ne', 'чи']
search result is expected for all documents and the following fields in these documents: ['firstName', 'lastName', 'phoneNumber', 'city']
Obviously, these fields may change in the future: either search only for 'city', or only for 'firstName' and 'lastName'
Result:

{
    "_id": {
        "$oid": "5d7f70d46b647c1a74d9b8aa"
    },
    "firstName": "user",
    "lastName": "one",
    "phoneNumber": "[phone]",
    "city": "Москва",
    "userId": {
        "$oid": "5d5ea4596886b1107c2be849"
    },
    "__v": 0
}


{
    "_id": {
        "$oid": "5d7f70d46b647c1a74d9b8aa"
    },
    "firstName": "user",
    "lastName": "two",
    "phoneNumber": "[phone]",
    "city": "Сочи",
    "userId": {
        "$oid": "5d5ea4596886b1107c2be849"
    },
    "__v": 0
}

The model looks like this:

const { Schema, model } = require('mongoose')

const bookSchema = new Schema({
  firstName: {
    type: String,
    required: true
  },
  lastName: {
    type: String,
    required: true
  },
  city: {
    type: String,
    required: true
  },
  phoneNumber: {
    type: String,
    required: true
  },
  userId: {
    type: Schema.Types.ObjectId,
    ref: 'User'
  }
})

module.exports = model('Book', bookSchema)

controller:

exports.searchBook = (req, res, next) => {
  console.log(req.body) // ['ne', 'чи']

  // just an example of a simple search
  // Book.find({
  //   lastName: 'one'
  // })
  //   .then(book => {
  //     console.log('book', book)
  //   })
  //   .catch(err => {
  //     if (!err.statusCode) {
  //       err.statusCode = 500
  //     }
  //     next(err)
  //   })
}

My attempt to accomplish this task. The problem is that it will search correctly only if the input contains a word without spaces: 'co'.
The task is to work with something like this 'with tw'

exports.searchBook = (req, res, next) => {
  console.log(req.body) // ['ne', 'чи']

  const term = req.body.join(' ')
  console.log(term)

  Book.aggregate([
    {
      $match: {
        $or: [
          {
            firstName: {
              '$regex': term, //  'чи'
              '$options': 'i'
            }
          },
          {
            lastName: {
              '$regex': term,
              '$options': 'i'
            }
          },
          {
            city: {
              '$regex': term,
              '$options': 'i'
            }
          },
          {
            phoneNumber: {
              '$regex': term,
              '$options': 'i'
            }
          }
        ]
      }
    }
  ])
    .then(book => {
      console.log('book', book)
    })
    .catch(err => {
      if (!err.statusCode) {
        err.statusCode = 500
      }
      next(err)
    })
}