City Sort (Typescript)

citySort.ts

interface ICity {
  [key: string]: number;
}

const cities: ICity[] = [
  { "San Francisco": 3000 },
  { "Los Angeles": 5000 },
  { "San Francisco": 3000 },
  { Sacramento: 2000 }
];

const uniqueCityFilter = (cityObject: ICity, index: number, array: ICity[]) => {
  return (
    index ===
    array.findIndex(s => {
      const cityName = Object.keys(s)[0];
      const currentCityName = Object.keys(cityObject)[0];
      return cityName === currentCityName;
    })
  );
};

const sumCityPopulation = (population: number, currentCity: ICity) => {
  const cityPopulation = Object.values(currentCity)[0];
  return population + cityPopulation;
};

const total = cities.filter(uniqueCityFilter).reduce(sumCityPopulation, 0);

console.log(total);