using mock-fs to create a text file

mock-fs-example.ts

it('throws error if class-validator return error', async () => {
      mockfs({
        'test_dir/assets': {
          'sales_sample.txt': mockfs.file({
            content:
              '2022-01-15T19:20:30-03:00CURSO DE BEM-ESTAR            0000012750JOSE CARLOS\n12021-12-03T11:46:02-03:00DOMINANDO INVESTIMENTOS       0000050000MARIA CANDIDA',
          }),
        },
      });
      const file = fs.readFileSync('test_dir/assets/sales_sample.txt');

      mockfs.restore();
      expect(
        service.importTransactions({ buffer: file } as Express.Multer.File),
      ).