Multer

A storage engine is a class used to define how uploaded files are stored and removed. To create a custom engine, you must implement two core methods: _handleFile and _removeFile.

Core Methods

1. _handleFile(req, file, cb):

   • Responsible for storing the file data.
   • The file data is provided via file.stream (a readable stream).
   • You should pipe this stream to your storage destination.
   • Once the operation is complete, call the callback cb with an object containing information about the file (e.g., { path: '...' }). This object is merged with Multer's file object and becomes available in req.file or req.files.

2. _removeFile(req, file, cb):

   • Responsible for deleting the file if an error occurs later in the request lifecycle.
   • Multer decides when to trigger this method.
   • Call the callback cb once the removal is complete.

Implementation Guidelines

• When defining helper functions (like a destination function), always use the signature (req, file, cb) to ensure compatibility and ease of switching between engines.
• The destination function should return the path where the file should be saved via the callback.

var fs = require('fs')

function getDestination (req, file, cb) {
  cb(null, '/dev/null')
}

function MyCustomStorage (opts) {
  this.getDestination = (opts.destination || getDestination)
}

MyCustomStorage.prototype._handleFile = function _handleFile (req, file, cb) {
  this.getDestination(req, file, function (err, path) {
    if (err) return cb(err)

    var outStream = fs.createWriteStream(path)

    file.stream.pipe(outStream)
    outStream.on('error', cb)
    outStream.on('finish', function () {
      cb(null, {
        path: path,
        size: outStream.bytesWritten
      })
    })
  })
}

MyCustomStorage.prototype._removeFile = function _removeFile (req, file, cb) {
  fs.unlink(file.path, cb)
}

module.exports = function (opts) {
  return new MyCustomStorage(opts)
}

Multer is middleware for handling multipart/form-data. It populates req.body with text fields and either req.file (for a single file) or req.files (for multiple files) with file information.

Warning: Never use Multer as global middleware. Only apply it to specific routes that require file uploads to prevent malicious users from uploading files to unexpected routes.

const express = require('express')
const multer  = require('multer')
const upload = multer({ dest: 'uploads/' })

const app = express()

// Handle a single file named 'avatar'
app.post('/profile', upload.single('avatar'), function (req, res, next) {
  // req.file is the 'avatar' file
  // req.body contains text fields
})

// Handle an array of files named 'photos' (max 12)
app.post('/photos/upload', upload.array('photos', 12), function (req, res, next) {
  // req.files is the array of 'photos' files
})

// Handle mixed fields
const uploadMiddleware = upload.fields([{ name: 'avatar', maxCount: 1 }, { name: 'gallery', maxCount: 8 }])
app.post('/cool-profile', uploadMiddleware, function (req, res, next) {
  // req.files is an object (String -> Array) where keys are field names
  // e.g., req.files['avatar'][0] -> File
})