Multer is a multipart form parser and file upload manager designed as middleware for Node.js and Express.js applications. It extracts text fields and uploaded files from incoming requests to populate the request body and file objects for server-side use.
The project utilizes pluggable storage engines to abstract file persistence, allowing developers to switch between saving files to a physical disk or holding them as buffers in system memory. It supports customizable file naming strategies and a custom storage engine interface for defining how files are saved and deleted.
The middleware includes resource protection utilities that enforce maximum thresholds on file sizes and field counts. It also provides a request validator that evaluates uploaded files against custom predicate functions to accept or reject specific files before they are stored.
