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

users.controller.ts

// This file should exist in `src/models/users`
import { UsersService } from "./users.service";
import { Response as ExpressResponse } from "express";
import { Controller, Get, Response } from "@nestjs/common";

/**
 * Controller dealing with user entity based operations.
 *
 * @class
 */
@Controller("users")
export class UsersController {
  /**
   * Create an instance of class.
   *
   * @constructs
   *
   * @param {UsersService} usersService
   */
  constructor(private readonly usersService: UsersService) {}

  /**
   * Exports a CSV & downloads it for users.
   *
   * @public
   * @async
   *
   * @returns {Promise<ExpressResponse>} Returns success response entity.
   */
  @Get("/export-csv")
  async export(@Response() res: ExpressResponse): Promise<ExpressResponse> {
    return await this.usersService.exportUserDataToCSV().then(
      async (fileName) =>
        await this.usersService.getExportedUserCSV(fileName).then((csvData) => {
          res.set("Content-Type", "text/csv");

          return res.send(csvData);
        })
    );
  }
}