// get the total of a complex structure
const cities = [
  {
    city: "Chongqing",
    population: 30165500
  },
  {
    city: "Shanghai",
    population: 24183300
  },
  {
    city: "Beijing",
    population: 21707000
  },
  {
    city: "Lagos",
    population: 16060303
  },
]

const totalPopulation = cities.reduce(
  (sum, city) => sum + city.population,
  0
);