findOne() is returning the whole model instead of a document

I am using findOne() to extract a document. On printing the returned value it prints the correct result but on looping over it, it prints the model instead of the original document. The document in the City table is stored as:

{
 _id: 62e135519567726de42421c2,
configType: 'cityConfig'
'16': {
        cityName: 'Delhi',
        metro: true,
        tier1: true
      },
'17': {
        cityName: 'Indore',
        metro: false,
        tier1: false
      }
}

The code:

const getCities = async () => {
  const cityConfig = await City.findOne({
    configType: "cityConfig"
  });

  console.log("cityConfig - ", cityConfig);
  
  let metroCities = [];

  for(var city in cityConfig) {
    console.log("city - ", city);
    if(cityConfig[city].metro == true) { // throws an error here obviously
      metroCities.push(city);
    }
  }
  return metroCities;

};

The output of cityConfig is correct, but inside the for loop it's iterating over the model instead of the document. Output:

cityPassConfig -  {
 '16': {
    cityName: 'Nagpur',
    passIssuedCities: false,
    digitalDiscountEnabledCities: false,
    bhopalIndore: false,
    mPanelCitiesForPass: false
  },
  '17': {
    cityName: 'Mumbai',
    passIssuedCities: false,
    digitalDiscountEnabledCities: false,
    bhopalIndore: false,
    mPanelCitiesForPass: false
  },
 _id: 62e288807b59432f87e32a82,
  configType: 'cityPassConfig'
}
city -  $__
city -  isNew
city -  errors
city -  $locals
city -  $op
city -  _doc
city -  $init
city -  db
city -  discriminators
city -  configType
city -  cityId
city -  configData
city -  enabled
city -  lastModifiedby
city -  _id
city -  updatedAt
city -  createdAt
.
.
.
.

I want to just iterate over 16 and 17 (the keys of the object). How to do that?

Node v14.15.4

You are using Mongoose, so if you want to return the JavaScript object, you should use lean() method.

const cityConfig = await City.findOne({ configType: "cityConfig" }).lean();

If you don't use lean(), Mongoose will hydrate the document. You can read more here.