multer

What is multer?

Multer is a node.js middleware for handling multipart/form-data, which is primarily used for uploading files. It is written on top of busboy for maximum efficiency.

What are multer's main functionalities?

File Uploads

This feature allows you to upload files to your server. The code sample demonstrates how to handle a single file upload with Multer.

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

app.post('/upload', upload.single('file'), function (req, res) {
  // req.file is the `file` file
  res.send('File uploaded!');
});

Multiple Files Upload

Multer also supports uploading multiple files at once. The code sample shows how to handle multiple file uploads, limiting to 12 files in this case.

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

app.post('/upload', upload.array('files', 12), function (req, res) {
  // req.files is array of `files` files
  res.send('Multiple files uploaded!');
});

Disk Storage

Multer allows you to customize the storage of files. This code sample demonstrates how to use disk storage to control the storage location and file naming.

const multer = require('multer');

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/')
  },
  filename: function (req, file, cb) {
    cb(null, file.fieldname + '-' + Date.now())
  }
});

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

Memory Storage

For temporary storage or when you want to process the file without saving it to disk, you can use memory storage. The code sample shows how to store a file in memory.

const multer = require('multer');
const upload = multer({ storage: multer.memoryStorage() });

app.post('/upload', upload.single('file'), function (req, res) {
  // req.file is the `file` file stored in memory
  res.send('File uploaded and stored in memory!');
});

File Filtering

Multer provides a way to filter out files based on conditions you set. This code sample demonstrates file filtering to only allow JPEG images.

const multer = require('multer');
const upload = multer({
  fileFilter: function (req, file, cb) {
    if (file.mimetype !== 'image/jpeg') {
      return cb(new Error('Only JPEG files are allowed!'), false);
    }
    cb(null, true);
  }
});

Other packages similar to multer

formidable

Formidable is an alternative to Multer for parsing form data, especially file uploads. It is less middleware-oriented and more flexible in terms of handling various form parsing tasks.

busboy

Busboy is a low-level Node.js module for parsing incoming HTML form data. Multer is built on top of Busboy, but provides a more convenient middleware API for integrating with Express.js applications.

multiparty

Multiparty is another module for handling multipart/form-data requests, which is the type of requests that file uploads usually come in. It is similar to Multer but has a different API and is used in a slightly different way.

README

Multer

Multer is a node.js middleware for handling multipart/form-data, which is primarily used for uploading files. It is written on top of busboy for maximum efficiency.

NOTE: Multer will not process any form which is not multipart (multipart/form-data).

Installation

npm install --save multer

Usage

Multer adds a body object and a file or files object to the request object. The body object contains the values of the text fields of the form, the file or files object contains the files uploaded via the form.

Basic usage example:

const multer = require('multer')
const express = require('express')

const app = express()
const upload = multer()

app.post('/profile', upload.single('avatar'), (req, res, next) => {
  // req.file is the `avatar` file
  // req.body will hold the text fields, if there were any
})

app.post('/photos/upload', upload.array('photos', 12), (req, res, next) => {
  // req.files is array of `photos` files
  // req.body will contain the text fields, if there were any
})

const cpUpload = upload.fields([{ name: 'avatar', maxCount: 1 }, { name: 'gallery', maxCount: 8 }])
app.post('/cool-profile', cpUpload, (req, res, next) => {
  // req.files is an object (String -> Array) where fieldname is the key, and the value is array of files
  //
  // e.g.
  //  req.files['avatar'][0] -> File
  //  req.files['gallery'] -> Array
  //
  // req.body will contain the text fields, if there were any
})

In case you need to handle a text-only multipart form, you can use the .none() method, example:

const multer = require('multer')
const express = require('express')

const app = express()
const upload = multer()

app.post('/profile', upload.none(), (req, res, next) => {
  // req.body contains the text fields
})

API

File information

Each file contains the following information:

Key	Description
fieldName	Field name specified in the form
originalName	Name of the file on the user's computer (undefined if no filename was supplied by the client)
size	Size of the file in bytes
stream	Stream of file
detectedMimeType	The detected mime-type, or null if we failed to detect
detectedFileExtension	The typical file extension for files of the detected type, or empty string if we failed to detect (with leading . to match path.extname)
clientReportedMimeType	The mime type reported by the client using the Content-Type header, or null1 if the header was absent
clientReportedFileExtension	The extension of the file uploaded (as reported by path.extname)

¹ Currently returns text/plain if header is absent, this is a bug and it will be fixed in a patch release. Do not rely on this behavior.

multer(opts)

Multer accepts an options object, the following are the options that can be passed to Multer.

Key	Description
limits	Limits of the uploaded data (full description)

.single(fieldname)

Accept a single file with the name fieldname. The single file will be stored in req.file.

.array(fieldname[, maxCount])

Accept an array of files, all with the name fieldname. Optionally error out if more than maxCount files are uploaded. The array of files will be stored in req.files.

.fields(fields)

Accept a mix of files, specified by fields. An object with arrays of files will be stored in req.files.

fields should be an array of objects with name and optionally a maxCount. Example:

[
  { name: 'avatar', maxCount: 1 },
  { name: 'gallery', maxCount: 8 }
]

.none()

Accept only text fields. If any file upload is made, error with code "LIMIT_UNEXPECTED_FILE" will be issued. This is the same as doing upload.fields([]).

.any()

Accepts all files that comes over the wire. An array of files will be stored in req.files.

WARNING: Make sure that you always handle the files that a user uploads. Never add multer as a global middleware since a malicious user could upload files to a route that you didn't anticipate. Only use this function on routes where you are handling the uploaded files.

limits

An object specifying the size limits of the following optional properties. Multer passes this object into busboy directly, and the details of the properties can be found on busboy's page.

The following integer values are available:

Key	Description	Default
fieldNameSize	Max field name size	100 bytes
fieldSize	Max field value size	1MB
fields	Max number of non-file fields	Infinity
fileSize	For multipart forms, the max file size (in bytes)	Infinity
files	For multipart forms, the max number of file fields	Infinity
parts	For multipart forms, the max number of parts (fields + files)	Infinity
headerPairs	For multipart forms, the max number of header key=>value pairs to parse	2000

Specifying the limits can help protect your site against denial of service (DoS) attacks.

Error handling

When encountering an error, multer will delegate the error to express. You can display a nice error page using the standard express way.

If you want to catch errors specifically from multer, you can call the middleware function by yourself.

const upload = multer().single('avatar')

app.post('/profile', (req, res) => {
  upload(req, res, (err) => {
    if (err) {
      // An error occurred when uploading
      return
    }

    // Everything went fine
  })
})