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 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:

Don't forget the enctype="multipart/form-data" in your form.

<form action="/profile" method="post" enctype="multipart/form-data">
  <input type="file" name="avatar" />
</form>

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

const app = express()

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

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 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 should use the .none() method:

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

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

Here's an example on how multer is used in a HTML form. Take special note of the enctype="multipart/form-data" and name="uploaded_file" fields:

<form action="/stats" enctype="multipart/form-data" method="post">
  <div class="form-group">
    <input type="file" class="form-control-file" name="uploaded_file">
    <input type="text" class="form-control" placeholder="Number of speakers" name="nspeakers">
    <input type="submit" value="Get me the stats!" class="btn btn-default">
  </div>
</form>

Then in your javascript file you would add these lines to access both the file and the body. It is important that you use the name field value from the form in your upload function. This tells multer which field on the request it should look for the files in. If these fields aren't the same in the HTML form and on your server, your upload will fail:

const multer  = require('multer')
const upload = multer({ dest: './public/data/uploads/' })
app.post('/stats', upload.single('uploaded_file'), function (req, res) {
  // req.file is the name of your file in the form above, here 'uploaded_file'
  // req.body will hold the text fields, if there were any
  console.log(req.file, req.body)
});

Complete client-side and server-side examples (browser FormData uploads of a blob and of an image) are in the examples/ folder.

API

File information

Each file contains the following information:

multer(opts)

Multer accepts an options object, the most basic of which is the dest property, which tells Multer where to upload the files. In case you omit the options object, the files will be kept in memory and never written to disk.

By default, Multer will rename the files so as to avoid naming conflicts. The renaming function can be customized according to your needs.

The following are the options that can be passed to Multer.

In an average web app, only dest might be required, and configured as shown in the following example.

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

When preservePath is enabled, Multer passes the incoming filename through with any path segments provided by the client. This is exposed as file.originalname; it does not change the destination folder, create directories, or sanitize the path for you. file.originalname is always client-supplied and should be treated as untrusted; with preservePath it additionally contains the path segments the client sent. Normalize or validate it before using it in a custom filename or storage engine.

If you want more control over your uploads, you'll want to use the storage option instead of dest. Multer ships with storage engines DiskStorage and MemoryStorage; More engines are available from third parties.

.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.

Files skipped by fileFilter do not count towards maxCount. Use limits.files to bound how many files are read from a request.

.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.

.any()

Accepts all files that come 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.

storage

DiskStorage

The disk storage engine gives you full control on storing files to disk.

const crypto = require('crypto')

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, '/tmp/my-uploads')
  },
  filename: function (req, file, cb) {
    crypto.randomBytes(16, function (err, raw) {
      if (err) return cb(err)
      cb(null, file.fieldname + '-' + raw.toString('hex'))
    })
  }
})

const upload = multer({ storage: storage })

There are three options available: destination, filename and flush. All of them except flush are functions that determine where the file should be stored.

destination is used to determine within which folder the uploaded files should be stored. This can also be given as a string (e.g. '/tmp/uploads'). If no destination is given, the operating system's default directory for temporary files is used.

Note: You are responsible for creating the directory when providing destination as a function. When passing a string, multer will make sure that the directory is created for you.

filename is used to determine what the file should be named inside the folder. If no filename is given, each file will be given a random name that doesn't include any file extension.

Note: Multer will not append any file extension for you, your function should return a filename complete with a file extension.

Each function gets passed both the request (req) and some information about the file (file) to aid with the decision.

Note that req.body might not have been fully populated yet. It depends on the order that the client transmits fields and files to the server.

For understanding the calling convention used in the callback (needing to pass null as the first param), refer to Node.js error handling

flush is an optional boolean. When set to true, the file data is flushed to disk with an fsync call before the upload handler runs, which prevents data loss if the process or the system crashes while the data is still sitting in the operating system's page cache. It is disabled by default because forcing a disk flush on every upload has a performance cost. Note that this syncs the file contents only; if the new directory entry must survive a crash as well, fsync the destination directory yourself.

MemoryStorage

The memory storage engine stores the files in memory as Buffer objects. It doesn't have any options.

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

When using memory storage, the file info will contain a field called buffer that contains the entire file.

WARNING: Uploading very large files, or relatively small files in large numbers very quickly, can cause your application to run out of memory when memory storage is used.

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.

limits can also be a function that receives the request and returns such an object. It is called once per request, before parsing starts, so limits can depend on e.g. the authenticated user:

const upload = multer({
  limits: function (req) {
    return { fileSize: req.user.maxUploadSize }
  }
})

Limits apply to the whole request; there is no per-file limit. To restrict individual files by type or name, use fileFilter or a custom storage engine.

The following integer values are available:

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

fileFilter

Set this to a function to control which files should be uploaded and which should be skipped. The function should look like this:

function fileFilter (req, file, cb) {

  // The function should call `cb` with a boolean
  // to indicate if the file should be accepted

  // To reject this file pass `false`, like so:
  cb(null, false)

  // To accept the file pass `true`, like so:
  cb(null, true)

  // You can always pass an error if something goes wrong:
  cb(new Error('I don\'t have a clue!'))

}

streamHandler

By default Multer reads the request with req.pipe(busboy). Some platforms consume the request body before your handlers run, so there is nothing left to pipe. Google Cloud Functions and Firebase Functions, for example, expose the already-read body as req.rawBody. Provide a streamHandler to feed busboy yourself in that case:

const upload = multer({
  storage: multer.memoryStorage(),
  streamHandler: function (req, busboy) {
    if (req.rawBody) {
      busboy.end(req.rawBody)
    } else {
      req.pipe(busboy)
    }
  }
})

The function receives the request and the busboy instance and must write the whole multipart body to busboy and end it.

Security

Specifying the limits can help protect your site against denial of service (DoS) attacks. The following limits are recommended for most applications:

• fileSize -- set to the maximum expected file size for your use case
• files -- set to the maximum number of files per request
• fields -- set to the maximum number of text fields per request
• fieldNestingDepth -- set to the minimum depth your field names require (e.g. 3 for a[b][c])
• fieldArrayIndexLimit -- set to the largest array index your field names require (e.g. 100 for a[99])

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. Also, if you want to catch only the Multer errors, you can use the MulterError class that is attached to the multer object itself (e.g. err instanceof multer.MulterError).

Multer errors carry a code (e.g. LIMIT_FILE_SIZE) and, when the error concerns a specific field, its name in field. Errors about a specific file (LIMIT_FILE_SIZE, LIMIT_UNEXPECTED_FILE) also expose the client-supplied file name in filename; treat it as untrusted input, like file.originalname.

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

app.post('/profile', function (req, res) {
  upload(req, res, function (err) {
    if (err instanceof multer.MulterError) {
      // A Multer error occurred when uploading.
    } else if (err) {
      // An unknown error occurred when uploading.
    }

    // Everything went fine.
  })
})

Custom storage engine

For information on how to build your own storage engine, see Multer Storage Engine.

License

MIT