The multer npm package is a middleware used for handling multipart/form-data, which is primarily used for file uploads in Node.js applications. It allows you to upload files to your server with ease, making it one of the most popular file upload libraries in the Node.js ecosystem.

Key Features of multer:

1. File Parsing: It automatically parses multipart/form-data requests (usually form submissions containing files), making it easy to handle file uploads in your server.
2. File Storage: multer allows you to specify where and how files should be stored, either in memory or on the disk.
3. Customizable: You can control file size limits, file types, and filenames through its configuration options.
4. Error Handling: It includes built-in error handling, making it easy to manage issues like file size exceeding limits or invalid file types.

Why You Might Use multer:

• Uploading Files: If your application needs to upload images, videos, or any other files from a client to your server, multer simplifies that process.
• Form Data Parsing: It works seamlessly with form data that contains both text fields and files.
• Disk Storage or Memory Storage: You can decide whether to temporarily store files on the server's disk or hold them in memory, depending on your needs.

import multer from 'multer';
import { v4 as uuid } from 'uuid'; // for unique identification

• multer: This is a middleware for handling multipart/form-data, which is commonly used for uploading files in Node.js and Express applications.
• uuid: The uuid library is used to generate unique identifiers. v4 creates a version 4 UUID, which is a universally unique identifier.

1. Multer Storage Configuration

const storage = multer.diskStorage({
// Define the destination folder for uploaded files
destination(req, file, callback) {
callback(null, 'uploads');
},
// Define the filename for the uploaded file
filename(req, file, callback) {
const id = uuid(); // Generate a unique ID for each file using UUID v4
const extName = file.originalname.split('.').pop(); // Extract the file extension from the original name
callback(null, `${id}.${extName}`); // Construct the new filename with the UUID and original file extension
},
});

• multer.diskStorage: This method allows you to configure how files are stored on disk.
  • destination:
    • Purpose: Specifies the folder where uploaded files should be saved.
    • Parameters:
      • req: The request object.
      • file: The file object representing the uploaded file.
      • callback: A callback function that takes two arguments: error and destination. In this case, callback(null, 'uploads') specifies that files will be saved in the uploads directory. If there were an error, you would pass an error object instead of null.
  • filename:
    • Purpose: Specifies the filename of the uploaded file on disk.
    • Parameters:
      • req: The request object.
      • file: The file object representing the uploaded file.
      • callback: A callback function that takes two arguments: error and filename.
      • const id = uuid();: Generates a unique identifier for the file using UUID v4. This ensures that each file has a unique name.
      • const extName = file.originalname.split('.').pop();: Extracts the file extension from the original filename. For instance, if the original file is named image.jpg, extName will be jpg.
      • callback(null, ${id}.${extName});: Constructs the new filename using the unique ID and the original file extension. For example, if uuid() generates 123e4567-e89b-12d3-a456-426614174000 and the original file extension is jpg, the new filename will be 123e4567-e89b-12d3-a456-426614174000.jpg.