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)
});
API
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, or the full path when preservePath: true |
|
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 client-supplied path in file.originalname instead of just the base name |
defParamCharset |
Default character set to use for values of part header parameters (e.g. filename) that are not extended parameters (that contain an explicit charset). Default: 'latin1' |
defCharset |
Default character set to use for text field values that do not declare one. Default: 'utf8' |
highWaterMark |
highWaterMark of the multipart parser stream. Default: busboy's default |
fileHwm |
highWaterMark of each file stream (file.stream). Default: busboy's default |
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 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 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:
| Key |
Description |
Default |
fieldNameSize |
Max field name size |
Infinity |
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 |
fieldNestingDepth |
Max number of nesting levels for field names (e.g. a[b][c] has 2 levels) |
Infinity |
fieldArrayIndexLimit |
Max numeric array index accepted inside a field name (e.g. a[3] uses index 3) |
Infinity |
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!'))
}
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 casefiles -- set to the maximum number of files per requestfields -- set to the maximum number of text fields per requestfieldNestingDepth -- 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
Multer 是一个 Node.js 中间件，用于处理 multipart/form-data 格式的数据，主要在 Express 应用中用来上传文件。它基于 busboy 构建，以提高效率。