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.

It is written on top of busboy for maximum efficiency.

API

Installation

$ npm install multer

Usage

var express = require('express')
var multer  = require('multer')

var app = express()
app.use(multer({ dest: './uploads/'}))

You can access the fields and files in the request object:

console.log(req.body)
console.log(req.files)

IMPORTANT: Multer will not process any form which is not multipart/form-data submitted via the POST or PUT methods.

Multer file object

A multer file object is a JSON object with the following properties.

1. fieldname - Field name specified in the form
2. originalname - Name of the file on the user's computer
3. name - Renamed file name
4. encoding - Encoding type of the file
5. mimetype - Mime type of the file
6. path - Location of the uploaded file
7. extension - Extension of the file
8. size - Size of the file in bytes
9. truncated - If the file was truncated due to size limitation

Options

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 file will be renamed and uploaded to the temporary directory of the system.

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

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

• dest
• limits
• rename(fieldname, filename)
• onFileUploadStart(file)
• onFileUploadData(file, data)
• onFileUploadComplete(file)
• onParseStart()
• onParseEnd(req, next)
• onError()
• onFilesLimit()
• onFieldsLimit()
• onPartsLimit()

Apart from these, Multer also supports more advanced busboy options like highWaterMark, fileHwm, and defCharset.

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

app.use(multer({
  dest: './uploads/',
  rename: function (fieldname, filename) {
    return filename.replace(/\W+/g, '-').toLowerCase() + Date.now()
  }
}))

The details of the properties of the options object is explained in the following sections.

dest

The destination directory for the uploaded files.

dest: './uploads/'

limits

An object specifying the size limits of the following optional properties. This object is passed to busboy directly, and the details of properties can be found on busboy's page

• fieldNameSize - integer - Max field name size (Default: 100 bytes)
• fieldSize - integer - Max field value size (Default: 1MB)
• fields - integer - Max number of non-file fields (Default: Infinity)
• fileSize - integer - For multipart forms, the max file size (in bytes) (Default: Infinity)
• files - integer - For multipart forms, the max number of file fields (Default: Infinity)
• parts - integer - For multipart forms, the max number of parts (fields + files) (Default: Infinity)
• headerPairs - integer - For multipart forms, the max number of header key=>value pairs to parse Default: 2000 (same as node's http).

limits: {
  fieldNameSize: 100,
  files: 2,
  fields: 5
}

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

rename(fieldname, filename)

Function to rename the uploaded files. Whatever the function returns will become the new name of the uploaded file (extension is not included). The fieldname and filename of the file will be available in this function, use them if you need to.

rename: function (fieldname, filename) {
  return fieldname + filename + Date.now()
}

onFileUploadStart(file)

Event handler triggered when a file starts to be uploaded. A file object with the following properties are available to this function: fieldname, originalname, name, encoding, mimetype, path, extension.

onFileUploadStart: function (file) {
  console.log(file.fieldname + ' is starting ...')
}

You can even stop a file from being uploaded - just return false from the event handler. The file won't be processed or reach the file system.

onFileUploadStart: function (file) {
  if (file.originalname == 'virus.exe') return false;
}

onFileUploadData(file, data)

Event handler triggered when a chunk of buffer is received. A buffer object along with a file object is available to the function.

onFileUploadData: function (file, data) {
  console.log(data.length + ' of ' + file.fieldname + ' arrived')
}

onFileUploadComplete(file)

Event handler trigger when a file is completely uploaded. A file object is available to the function.

onFileUploadComplete: function (file) {
  console.log(file.fieldname + ' uploaded to  ' + file.path)
}

onParseStart()

Event handler triggered when the form parsing starts.

onParseStart: function () {
  console.log('Form parsing started at: ', new Date())
}

onParseEnd(req, next)

Event handler triggered when the form parsing completes. The request object and the next objects are are passed to the function.

onParseEnd: function (req, next) {
  console.log('Form parsing completed at: ', new Date());

  // usage example: custom body parse
  req.body = require('qs').parse(req.body);

  // call the next middleware
  next();
}

Note: If you have created a onParseEnd event listener, you must manually call the next() function, else the request will be left hanging.

onError()

Event handler for any errors encountering while processing the form. The error object and the next object is available to the function. If you are handling errors yourself, make sure to terminate the request or call the next() function, else the request will be left hanging.

onError: function (error, next) {
  console.log(error)
  next(error)
}

onFilesLimit()

Event handler triggered when the number of files exceed the specification in the limit object. No more files will be parsed after the limit is reached.

onFilesLimit: function () {
  console.log('Crossed file limit!')
}

onFieldsLimit()

Event handler triggered when the number of fields exceed the specification in the limit object. No more fields will be parsed after the limit is reached.

onFieldsLimit: function () {
  console.log('Crossed fields limit!')
}

onPartsLimit()

Event handler triggered when the number of parts exceed the specification in the limit object. No more files or fields will be parsed after the limit is reached.

onPartsLimit: function () {
  console.log('Crossed parts limit!')
}

License (MIT)

Copyright (c) 2014 Hage Yaapa <http://www.hacksparrow.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.