bd.js

class BD {

  constructor(db) {
    this.db = db;
  }

  /**
  * Finds items based on a query given as a JS object
  *
  * @param {object} query The query to match against (i.e. {name: 'paris'})
  * @param {function} callback   The callback to fire when the query has
  * completed running
  **/
  find(query, callback) {
    callback(this.db.filter(function (city) {
      for (const [key, value] of Object.entries(query)) {
        if (city[key] !== value) {
          return false
        }
      }
      return true
    }))
  }
}

// cities
const db = new BD([{ name: "Vincennes", population: 40000 },
{ name: "Paris", population: 2161000 }, { name: "roubais", population: 45000 }]);
//console.log(s.db);

db.find({ population: 45000 }, function (cities) {
  for (let city of cities) {
    city["population"] = 50000;
  }
});

db.find({ name: "Paris" }, function (cities) {
  for (let city of cities) {
    city["capital"] = true;
  }
});

console.log(db);