app.js

"use strict";
const fs = require("fs");
const readline = require("readline");
const rs = fs.ReadStream("./popu-pref.csv");
const rl = readline.createInterface({ input: rs, output: {} });
const map = new Map();
rl.on("line", lineString => {
  const columns = lineString.split(",");
  const year = parseInt(columns[0]);
  const prefecture = columns[2];
  const popu = parseInt(columns[7]);
  if (year === 2010 || year === 2015) {
    let value = map.get(prefecture);
    if (!value) {
      value = {
        popu10: 0,
        popu15: 0,
        change: null
      };
    }
    if (year === 2010) {
      value.popu10 += popu;
    }
    if (year === 2015) {
      value.popu15 += popu;
    }
    map.set(prefecture, value);
  }
});
rl.resume();
rl.on("close", () => {
  for (let pair of map) {
    const value = pair[1];
    value.change = (value.popu15 / value.popu10) * 100;
  }
  const rankingArray = Array.from(map).sort(
    (pair1, pair2) => pair1[1].change - pair2[1].change
  );
  const rankingStrings = rankingArray.map((pair, indx) => {
    return `${indx + 1}位 ${pair[0]}: ${pair[1].popu10} => ${
      pair[1].popu15
    } 変化率: ${pair[1].change}`;
  });
  console.log(rankingStrings);
});