Sort input from CSV file by header in TypeScript

sort CSV columns by header.ts

export class Challenge {
  public static sortCsvColumns (csv_data: string): string {
    const rows = csv_data.split('\n').map(row => row.split(','));
    const data: CsvData = {};
    for (const row of rows) {
      for (let i = 0; i < row.length; i++) {
        const header = rows[0][i].toLowerCase();
        if (!data[header]) {
          data[header] = [];
        } else {
          data[header].push(row[i]);
        }
      }
    }

    const headers = Object.keys(data)
      .map(key => key.toLowerCase())
      .sort(Challenge.compare);

    let values: Array<Array<string>> = [];
    headers.forEach(header => {
      values.push(data[header]);
    });
    values = values[0].map((col, i) => values.map(row => row[i])); // inversion
    return [rows[0].sort(Challenge.compare)]
      .concat(values).map(r => r.join(',')).join('\n');
  }

  private static compare(a: string, b: string) {
    return a.localeCompare(b);
  }
}