減ったランキング順位を実装

app.js

@@ -1 +1,47 @@
-'use strict';
+'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 })
+// key: 都道府県 value: 集計データのオブジェクト
+const prefectureDataMap = new Map()
+
+rl.on('line', (lineString) => {
+  const columns = lineString.split(',')
+  const year = parseInt(columns[0])
+  const prefecture = columns[1]
+  const population = 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 = population
+    }
+    if (year === 2021) {
+      value.after = population
+    }
+    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((pair1, pair2) => {
+    return pair1[1].change - pair2[1].change
+  })
+  const rankigStrings = rankingArray.map(([key, value], index) => {
+    return `${index + 1}位 ${key}: ${value.before} => ${value.after} 変化率：${
+      value.change
+    }`
+  })
+  console.log(rankigStrings)
+})