reduce_replace-dual-calls.js

const counties = [
  { name: 'Clackamas', state: 'Oregon', population: 380000 },
  { name: 'King', state: 'Washington', population: 2000000 },
  { name: 'Kitsap', state: 'Washington', population: 250000 },
  { name: 'Multinomah', state: 'Oregon', population: 750000 },
  { name: 'Snohomish', state: 'Washington', population: 720000 },
  { name: 'Wasco', state: 'Oregon', population: 25000 },
];

const highPopulousCounties = counties.reduce((acc, { name, state, population }) => {
  // This filters out anything smaller than 700000
  if (population <= 700000) return acc;

  // Again using a spread here. If the array is already started,
  // it will fill, otherwise it'll initialize a new array.
  const updatedAcc = [...acc]; 
  updatedAcc.push({ fullName: `${name} County, ${state}`, population });

  return updatedAcc;
}, []);

// Returns:
// [
//   { fullName: 'King County, Washington', population: 2000000 },
//   { fullName: 'Multinomah County, Oregon', population: 750000 },
//   { fullName: 'Snohomish County, Oregon', population: 720000 },
// ]