Multer

Multer is a Node.js middleware designed to handle multipart/form-data, primarily used for uploading files in web applications, especially with Express. It streamlines the process of parsing incoming form data and provides convenient access to uploaded files, while delegating the actual storage to configurable engines. Multer is widely used to add file upload functionality to server routes.

Multer offers storage engines and configuration options that control how and where files are saved. The diskStorage engine lets you customize the destination directory and the filename for saved files, enabling predictable paths and names. The memoryStorage engine keeps uploaded files in memory as Buffer objects for immediate processing without writing to disk. In addition to storage, Multer supports a fileFilter function to accept or reject files by type and a limits option to enforce constraints such as maximum file size and number of files. The middleware relies on the busboy library to parse multipart/form-data.

The primary API involves creating a Multer instance with options and then attaching specific upload handlers to routes. Typical methods bound to the instance include single(fieldname) for a single file, array(fieldname, maxCount) for multiple files with a field, fields(fieldsArray) for a mix of named fields, and any() for arbitrary files. Example usage: const upload = multer({ storage: diskStorage, limits: { fileSize: 1024 * 1024 * 5 } }); app.post('/upload', upload.single('file'), (req, res) => { // access via req.file and req.body });

After processing, uploaded files are available on req.file for single-file uploads or req.files for multiple files, with properties such as originalname, mimetype, size, and either path (on disk) or buffer (in memory). Non-file form fields populate req.body. Multer is open source and distributed via npm, commonly used to implement secure and controlled file upload workflows in Node.js applications.