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();  // 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(',');
    const year = parseInt(columns[0]);
    const prefecture = columns[2];
    const popu = parseInt(columns[7]);
    if (year === 2010 || year === 2015) {
        let value = map.get(prefecture) || {
            popu10: 0,
            popu15: 0,
            change: null
        };
        if (year === 2010) {
            value.popu10 += popu;
        } else if (year === 2015) {
            value.popu15 += popu;
        }
        map.set(prefecture, value);
    }
});

rl.on('close', () => {
    for (let pair of map) {
        const value = pair[1];
        value.change = value.popu15 / value.popu10;
    }
    const rankingArray = Array.from(map).sort(reverse(comparingInt(pair => pair[1].change)));
    const rakingStrings = rankingArray.map(
        pair => `${pair[0]}: ${pair[1].popu10}=>${pair[1].popu15} 変化率: ${pair[1].change}`);
    console.log(rakingStrings);
});

rl.resume();

const reverse = comparator => (source1, source2) => {
    return -1 * comparator(source1, source2);
};

const comparingInt = toIntFunction => {
    return (source1, source2) => {
        return toIntFunction(source1) - toIntFunction(source2);
    };
};