Multer is a node.js middleware for handling multipart/form-data.

It is written on top of busboy for maximum efficiency.

$ npm install multer

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.

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
10. buffer - Raw data (is null unless the inMemory option is true)

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. If the inMemory option is true, no data is written to disk but data is kept in a buffer accessible in the file object.

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
• includeEmptyFields
• inMemory
• rename(fieldname, filename)
• onFileUploadStart(file)
• onFileUploadData(file, data)
• onFileUploadComplete(file)
• onParseStart()
• onParseEnd(req, next)
• onError()
• onFileSizeLimit(file)
• 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.

The destination directory for the uploaded files.

dest: './uploads/'

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.

A Boolean value to specify whether empty submitted values should be processed and applied to req.body; defaults to false;

includeEmptyFields: true

If this Boolean value is true, the file.buffer property holds the data in-memory that Multer would have written to disk. The dest option is still populated and the path property contains the proposed path to save the file. Defaults to false.

inMemory: true

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

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()
}

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;
}

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')
}

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)
}

Event handler triggered when the form parsing starts.

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

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.

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)
}

Event handler triggered when a file size exceeds the specification in the limit object. No more files will be parsed after the limit is reached.

onFileSizeLimit: function (file) {
  console.log('Failed: ', file.originalname)
  fs.unlink('./' + file.path) // delete the partially written file
}

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!')
}

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!')
}

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!')
}