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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).

Translations

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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:

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 cpUpload = upload.fields([{ name: 'avatar', maxCount: 1 }, { name: 'gallery', maxCount: 8 }])
app.post('/cool-profile', cpUpload, 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 an 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)
});

API

File information

Each file contains the following information:

Key
Description
Note

fieldname
Field name specified in the form
 

originalname
Name of the file on the user’s computer
 

encoding
Encoding type of the file
 

mimetype
Mime type of the file
 

size
Size of the file in bytes
 

destination
The folder to which the file has been saved
DiskStorage
filename
The name of the file within the destination
DiskStorage
path
The full path to the uploaded file
DiskStorage
buffer
A Buffer of the entire file
MemoryStorage

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.

Key
Description

dest or storage
Where to store the files

fileFilter
Function to control which files are accepted

limits
Limits of the uploaded data

preservePath
Keep the full path of files instead of just the base name

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

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

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.

.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 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.

storage

DiskStorage

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

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, '/tmp/my-uploads')
  },
  filename: function (req, file, cb) {
    const uniqueSuffix = Date.now() + '-' + Math.round(Math.random() * 1E9)
    cb(null, file.fieldname + '-' + uniqueSuffix)
  }
})

const upload = multer({ storage: storage })

There are two options available, destination and filename. They are both
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 an 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

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.

The following integer values are available:

Key
Description
Default

fieldNameSize
Max field name size
100 bytes

fieldSize
Max field value size (in bytes)
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.

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!'))

}

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).

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