ランキングを格納する配列の改変

app.js

@@ -1 +1,47 @@
 'use strict';
+const fs = require('fs');
+const readline = require('readline');
+const rs = fs.createReadStream('./popu-pref.csv');
+const rl = readline.createInterface({ input: rs });
+const prefDataMap = new Map();
+rl.on('line', (lineString) => {
+  const columns = lineString.split(',');
+  const year = parseInt(columns[0]);
+  const pref = columns[1];
+  const popu = parseInt(columns[3]);
+  if (year === 2010 || year === 2015) {
+    let value = null;
+    if (prefDataMap.has(pref)) {
+      value = prefDataMap.get(pref);
+    } else {
+      value = {
+        popu10: 0,
+        popu15: 0,
+        change: null,
+      };
+    }
+    if (year === 2010) {
+      value.popu10 = popu;
+    }
+    if (year === 2015) {
+      value.popu15 = popu;
+    }
+    prefDataMap.set(pref, value);
+  }
+});
+
+rl.on('close', () => {
+  for (const [key, value] of prefDataMap) {
+    value.change = value.popu15 / value.popu10;
+  }
+  //Array.from( )でMapを配列に変換
+  const rankingArr = Array.from(prefDataMap).sort((pair1, pair2) => {
+    //降順に並び変える
+    // return pair2[1].change - pair1[1].change;
+    return pair1[1].change - pair2[1].change;
+  });
+  const rankingStrings = rankingArr.map(([key, value], i) => {
+    return `第${i + 1}位　${key}　変化率: ${value.change}`;
+  });
+  console.log(rankingStrings);
+});