For Medium Article (Creating/Writing/Downloading Files in NestJS)

users.service.ts

// This file should exist in `src/models/users`
import { parse } from "json2csv";
import { Injectable, NotFoundException } from "@nestjs/common";
import {
  getFile,
  createFile,
  checkIfFileOrDirectoryExists,
} from "./../../common/helpers/storage.helper";

/**
 * Service dealing with User based operations.
 *
 * @class
 */
@Injectable()
export class UsersService {
  /**
   * Creates a CSV file with users data
   *
   * @returns {Promise<string>}
   */
  async exportUserDataToCSV(): Promise<string> {
    return await this.getMany() // Some function that gets users data.
      .then(async (users) => {
        const [csvData, csvFields] =
          this.transformUsersDataForCSV(users); // Some function that returns you csv data & fields.

        if (!csvData || !csvFields) {
          return Promise.reject("Unable to transform users data for CSV.");
        }

        const csv = parse(csvData, { fields: csvFields });

        const filePath = `storage/app/exports/users`;
        const fileName = `users-${new Date().toISOString()}.csv`;

        await createFile(filePath, fileName, csv);

        return Promise.resolve(fileName);
      })
      .catch((error) => Promise.reject(error));
  }

  /**
   * Gets an exported CSV file
   *
   * @param {string} filename
   *
   * @returns {Promise<string>}
   */
   async getExportedUserCSV(filename: string): Promise<string> {
    const filePath = `storage/app/exports/users/${filename}`;

    if (!checkIfFileOrDirectoryExists(filePath)) {
      throw new NotFoundException('Users export not found.');
    }

    return (await getFile(filePath, 'utf8')).toString();
  }
}