Multer is a nodejs middleware for uploading files in a nodejs application (Express Multer Middleware, n.d.).
To use multer install the package using

npm install multer

Once it is installed, import the package using this command

const multer = require("multer")

Next is configuring the files' storage destination and preferred filenames. A callback function is used to create the filenames for each of the file uploads.

const storage = multer.diskStorage({
  destination: "./images",
  filename: (req, file, cb) => {
    return cb(
      null,
      `${file.fieldname}_${Date.now()}`
    );
  },
});

After configuring the storage location and the filename, add the storage object to a multer instance.

const upload = multer({ storage: storage });

The middleware is then added to the request to process any incoming file from the request body. A post request to the route "/upload" will upload a single image file from a multi-part form field named product and save it inside a folder called images.

app.post("/upload", upload.single("product"), (req, res) => {
  res.json({
  file_info: req.file,
});

To access the file details, we can use the output of the res.file object and return it in json format.

{
  "file_info": {
    "fieldname": "product",
    "originalname": "img3.jpeg",
    "encoding": "7bit",
    "mimetype": "image/jpeg",
    "destination": "./upload/images",
    "filename": "1706247839665_img3.jpeg",
    "path": "upload/images/1706247839665_img3.jpeg",
    "size": 107421
  }
}

This is an easy way to upload files in a nodejs application. With more configurations, you can specify the required file types and so on.
Credits:

• GreatStack. (2024, January 2). How to create full stack E-Commerce website using React JS, MongoDB, Express & Node JS 2024 [Video]. YouTube. https://www.youtube.com/watch?v=y99YgaQjgx4

• Express multer middleware. (n.d.). https://expressjs.com/en/resources/middleware/multer.html