app.js

'use strict';
const fs = require('fs');
const readline = require('readline');
const rs = fs.createReadStream('./popu-pref.csv');
const rl = readline.createInterface({ input: rs });
const prefectureDataMap = new Map(); // key: 都道府県 value: 集計データのオブジェクト
rl.on('line', lineString => {
    const columns = lineString.split(','); //データをカンマ区切りで配列にする
    const year = parseInt(columns[0]);
    const prefecture = columns[1];
    const popu = parseInt(columns[3]);
    if (year === 2010 || year === 2015){
        let value = null;
        if (prefectureDataMap.has(prefecture)) {
          value = prefectureDataMap.get(prefecture);
        } else {
          value = {
            popu10: 0,
            popu15: 0,
            change: null
          };
        }
        if (year === 2010) {
          value.popu10 = popu;
        }
        if (year === 2015) {
          value.popu15 = popu;
        }
        prefectureDataMap.set(prefecture, value);
    }
});
rl.on('close', () => {
    for (const [key, value] of prefectureDataMap) {
        value.change = value.popu15 / value.popu10;
    }
      const rankingArray = Array.from(prefectureDataMap).sort((pair1, pair2) => {
        return pair2[1].change - pair1[1].change;
      });
      const rankingStrings = rankingArray.map(([key, value]) =>{
        return `${key}:${value.popu10} => ${value.popu15} 変化率 :${value.change}`;
      })
      console.log(rankingStrings);
  });