ランキングの順位を表示するように設定

app.js

@@ -1 +1,46 @@
 'use strict';
+const fs = require('node:fs');
+const readline = require('node:readline');
+const rs = fs.createReadStream('./popu-pref.csv');
+const rl = readline.createInterface({ input: rs });
+const prefectureDataMap = new Map(); // キー：都道府県に設定 値：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[1];
+  const popu = parseInt(columns[3]);
+
+  if (year === 2016 || year === 2021) {
+    let value = null;
+    if (prefectureDataMap.has(prefecture)) {
+      value = prefectureDataMap.get(prefecture);
+    } else {
+      value = {
+        before: 0,
+        after: 0,
+        change: null
+      }
+    }
+    if (year === 2016) {
+      value.before = popu;
+    } else {
+      value.after = popu;
+    }
+    prefectureDataMap.set(prefecture, value);
+  }
+});
+
+rl.on('close', () => {
+  for (const [key, value] of prefectureDataMap) {
+    value.change = value.after / value.before
+  }
+  const rankingArray = Array.from(prefectureDataMap).sort((pair2, pair1) => {
+    return pair2[1].change - pair1[1].change;
+  });
+  const rankingString = rankingArray.map(([key, value], index) => {
+    const rank = index + 1;
+    return `${rank}位 ${key}: ${value.before}=>${value.after} 変化率: ${value.change}`;
+  });
+  console.log(rankingString);
+});