# Multer — File Upload Middleware for Node.js

> Multer is a Node.js middleware for handling multipart/form-data, primarily used for uploading files in Express and compatible frameworks with configurable storage engines and file filtering.

## Install

Save in your project root:

# Multer — File Upload Middleware for Node.js

## Quick Use
```javascript
const express = require('express');
const multer = require('multer');
const upload = multer({ dest: 'uploads/' });
const app = express();

app.post('/upload', upload.single('file'), (req, res) => {
  res.json({ filename: req.file.originalname, size: req.file.size });
});
app.listen(3000);
```

## Introduction
Multer handles `multipart/form-data` requests in Node.js, parsing file uploads and making them available via `req.file` or `req.files`. It plugs into Express as middleware and supports disk storage, memory storage, and custom storage engines.

## What Multer Does
- Parses multipart/form-data requests and extracts uploaded files
- Stores files to disk or memory with configurable storage engines
- Limits file size, field count, and number of files per request
- Filters files by MIME type or custom validation before saving
- Populates `req.file` (single) or `req.files` (multiple) with upload metadata

## Architecture Overview
Multer wraps the `busboy` streaming multipart parser. When a request hits the middleware, busboy parses the multipart boundary, streaming each file part through the configured storage engine. The disk storage engine writes to a destination directory with a generated filename, while memory storage buffers the file in a `Buffer` object. Text fields are collected into `req.body`. Multer operates as standard Express middleware and does not buffer the entire request before processing.

## Self-Hosting & Configuration
- Install via npm: `npm install multer`
- Use `multer({ dest: './uploads' })` for quick disk storage with auto-generated filenames
- Use `multer.diskStorage({ destination, filename })` for full control over paths and names
- Use `multer.memoryStorage()` to keep files in memory as `Buffer` objects for processing
- Set `limits: { fileSize: 5 * 1024 * 1024 }` to cap uploads at 5 MB

## Key Features
- Streaming parser handles large files without loading everything into memory at once
- Multiple upload modes: `.single()`, `.array()`, `.fields()`, and `.any()`
- Custom storage engines allow writing to S3, GCS, or any backend via the storage API
- File filter callback lets you accept or reject files based on type or other criteria
- Non-file fields are still parsed and available in `req.body`

## Comparison with Similar Tools
- **Formidable** — standalone multipart parser with no Express dependency; Multer is Express middleware
- **Busboy** — lower-level streaming parser that Multer wraps with a higher-level API
- **express-fileupload** — simpler API but buffers files in memory by default
- **Multiparty** — similar to Formidable; Multer offers tighter Express integration

## FAQ
**Q: Does Multer handle non-file form fields?**
A: Yes. Text fields in the multipart request are parsed and placed in `req.body`.

**Q: Can I upload to cloud storage like S3?**
A: Yes. Use community storage engines like `multer-s3` or write a custom storage engine implementing `_handleFile` and `_removeFile`.

**Q: How do I validate file types?**
A: Pass a `fileFilter` function to the Multer options that checks `file.mimetype` and calls the callback with `true` or `false`.

**Q: What happens if the upload exceeds the size limit?**
A: Multer emits a `LIMIT_FILE_SIZE` error that you can catch in Express error-handling middleware.

## Sources
- https://github.com/expressjs/multer
- https://www.npmjs.com/package/multer


---
Source: https://tokrepo.com/en/workflows/asset-ad37eb81
Author: AI Open Source