指定のフォルダ内のファイル数を一定に保つ

```js deleteExtraFile(dirPath, limit=100){ const files = fs.readdirSync(dirPath).filter(function(file){ // .XXX は除外 return !/^..+$/.test(file); });

let fileCreatedAts = [];
// fileの生成時間を抽出
for (let i = 0; i < files.length; i++) {
  const filename = files[i];

  const fileStat = fs.statSync(path.join(dirPath, filename));

  const filedata = {
    filename: filename,
    birthtimeMs: fileStat['birthtimeMs'],
  };
  fileCreatedAts.push(filedata);
}

// 日付を降順でソート
fileCreatedAts.sort(function(a, b) {
  return b['birthtimeMs'] - a['birthtimeMs'];
});

const deleteFiles = fileCreatedAts.slice(limit);

// 余分なファイルを削除
for (let i = 0; i < deleteFiles.length; i++) {
  const deleteFile = deleteFiles[i];
  fs.unlink(path.join(dirPath, deleteFile['filename']), function (err) {});
}   }
```