Multer Storage Engine in TypeScript

multer-storage-engine.ts

import { createWriteStream, unlink } from "fs";
import type { PathLike } from "fs";
import type { Request } from "express";

type Maybe<T> = T | null | undefined;

type ResponseCallback<T> = (err: Error | null, payload?: Maybe<T>) => void;

type RequestHandler<T> = (req: Request, file: Express.Multer.File, responseCb: ResponseCallback<T>) => void;

/**
 * Custom Multer Storage Engine for uploading file to AWS S3
 * Ref: https://github.com/expressjs/multer/blob/master/StorageEngine.md
 * Example Usage:
 * ```ts
 *  const storageEngine = new MulterS3StorageEngine((_req, file, responseCb) => {
 *  responseCb(null, `/var/www/uploads/${file.originalname}`);
 *  });
 * ```
 */
export class MulterS3StorageEngine {
  /**
   * Tells the engine where to save the file.
   * @param _req
   * @param _file
   * @param responseCb
   */
  getDestination: RequestHandler<PathLike> = (_req, _file, responseCb) => {
    responseCb(null, "/dev/null");
  };

  /**
   * Stores the file and returns information on how to access the file in the future.
   * The information you provide in the callback will be merged with multer's file object, and then presented to the user via `req.files`.
   * @param req
   * @param file
   * @param responseCb
   */
  _handleFile: RequestHandler<unknown> = (
    req: Request,
    file: Express.Multer.File,
    responseCb: ResponseCallback<unknown>
  ) => {
    this.getDestination(req, file, (err, path) => {
      if (err !== null) {
        responseCb(err);
      }
      if (path !== null && path !== undefined) {
        const outStream = createWriteStream(path);
        file.stream.pipe(outStream);
        outStream.on("error", responseCb);
        outStream.on("finish", () => {
          responseCb(null, { path, size: outStream.bytesWritten });
        });
      }
    });
  };

  /**
   * Removes the file if an error is encounter during file stream handling.
   * Multer will decide which files to delete and when.
   * @param _req
   * @param file
   * @param responseCb
   */
  _removeFile: RequestHandler<unknown> = (
    _req: Request,
    file: Express.Multer.File,
    responseCb: ResponseCallback<unknown>
  ) => {
    unlink(file.path, responseCb);
  };

  constructor(destination?: RequestHandler<string>) {
    this.getDestination = destination ?? this.getDestination;
  }
}