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Asynchronous Node.js file upload (multipart/form-data) handling.

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Pechkin is a modern, asynchronous, flexible and configurable Node.js library for handling file uploads (i.e. multipart/form-data requests), written in TypeScript. It's perfect for complex usecases requiring lots of flexibility with fields and multiple files mixed together.


  • Fast, based on busboy.
  • No temporary files are created, files are not loaded in memory.
  • Asynchronous, Promise- and AsyncIterator-based. Fields and each file are available as Promises as soon as they're parsed.
  • Flexible: you don't need to provide any storage engines, file handlers, etc. Pechkin only provides the parsed data in form of streams and promises, and you can do whatever you want with it.
  • Highly configurable, with possibility to override some configuration options per-field (e.g. maxFileByteLength: 1MB for all files, but 5MB for file fieldname my_custom_video_file).
  • Expressive TypeScript typings.
  • Robust error handling: you can be sure that all errors have been caught, handled, and underlying resources (streams) were properly handled/closed.
  • Only 1 dependency (busboy).

Check for tips on migration from v1.x to v2.x.



npm install pechkin

Examples / Usage


  • All fields in the FormData request should come before any files. Any fields submitted after the first file are lost.
  • parseFormData() returns a Promise that resolves when all fields are parsed, and the first file is encountered (or the request ended).
  • The promise contains a populated fields object, and a files AsyncIterator/AsyncIterable.
  • Asynchronously iterate over the files using the for-await-of loop or using the next() method.
  • File streams should always be consumed (e.g. by the code inside for-await-of loop, or before the subsequent next() call). Otherwise the request parsing will stall.



The package provides both CommonJS and ESM modules.

// ESM: index.mjs

import * as pechkin from 'pechkin';
// or
import { parseFormData } from 'pechkin';

// CommonJS: index.cjs

const pechkin = require('pechkin');
// or
const { parseFormData } = require('pechkin');

Files are processed sequentially.

// Full working example: `examples/fs.js`

http.createServer(async (req, res) => {
  const { fields, files } = await pechkin.parseFormData(req, {
    maxTotalFileFieldCount: Infinity,
    maxFileCountPerField: Infinity,
    maxTotalFileCount: Infinity

  const results = [];

  for await (const { filename: originalFilename, stream, ...file } of files) {
    const newFilename = `${Math.round(Math.random() * 1000)}-${originalFilename}`;
    const dest = path.join(os.tmpdir(), newFilename);

    // Pipe the stream to a file
    // The stream will start to be consumed after the current block of code
    // finishes executing...
    // ...which allows us to set up event handlers for the stream and wrap
    // the whole thing in a Promise, so that we can get the stream's length.
    const length = await new Promise((resolve, reject) => {
      // Since Node v15.0.0, you can use `stream.finished()`, instead of
      // manually setting up event listeners and resolving/rejecting inside
      // them.
        // `stream` is an instance of Transform, which is a Duplex stream,
        // which means you can listen to both 'end' (Readable side)
        // and 'finish' (Writable side) events.
        .on('end', () => resolve(stream.bytesWritten))
        .on('finish', () => resolve(stream.bytesWritten))
        // You can either reject the Promise and handle the Promise rejection
        // using .catch() or await + try-catch block, or you can directly
        // somehow handle the error in the 'error' event handler.
        .on('error', reject);

    results.push({ ...file, dest, originalFilename, newFilename, length});



    "fields": { [fieldname: string]: string },
    "files": [
        "field": string,
        "filename": string,
        "mimeType": string,
        "dest": string,
        "originalFilename": string,
        "newFilename": string,
        "length": number

Pechkin doesn't provide an Express middleware out-of-the-box, but it's very easy to create one yourself.

// FULL WORKING EXAMPLE: `examples/express.js`

// ... Boilerplate code ...

function pechkinFileUpload (config, fileFieldConfigOverride, busboyConfig) {
  return async (req, res, next) => {
    try {
      const { fields, files } = await parseFormData(req, config, fileFieldConfigOverride, busboyConfig);

      req.body = fields;
      req.files = files;

      return next();
    } catch (err) {
      return next(err);
  async (req, res) => {
    const files = [];

    for await (const { stream, field, filename } of req.files) {
      // Process files however you see fit...
      // Here, streams are simply skipped

      files.push({ field, filename });

    return res.json({ fields: req.body, files });

// ... Boilerplate code ...


Pechkin exposes only 1 function:



function parseFormData(
  request:                  IncomingMessage,
  config?:                  Pechkin.Config
  fileFieldConfigOverride?: Pechkin.FileFieldConfigOverride,
  busboyConfig?:            Pechkin.BusboyConfig,
): Promise<{
  fields: Pechkin.Fields,
  files:  Pechkin.Files,

Given a request (of type http.IncomingMessage, e.g. the request object in http.createServer((req, ...) => { ... })), return a Promise, containing:

  • All parsed fields,
  • An AsyncIterableIterator of files, which you can use both as an iterator (calling await, or as an iterable (for await (const file of files) { ... }).

🚧 Warning:

fields are parsed only until the first file – when constructing a FormData request, you should always put all fields before any files.

Parameter: config

All fields are optional. Numerical limits are INCLUSIVE.

Key Type Default Description
maxTotalHeaderPairs number 2000 From Busboy: the max number of header key-value pairs to parse.
Default is same as node's http module.
maxTotalPartCount number 110 (100 fields + 10 files) The max number of parts (fields + files).
maxFieldKeyByteLength number 100 bytes The max byte length (each char is 1 byte) of a field name.
maxFieldValueByteLength number 1024 * 1024 bytes, 1 MB The max byte length of a field value.
maxTotalFieldCount number 100 The max total number of all non-file fields.
maxTotalFileFieldCount number 1 The max total number of all file fields.
Each file field may contain more than 1 file, see config.maxFileCountPerField.

To use if you have more than 1 <input type="file">.
maxTotalFileCount number 10 The max total number of all files (summed across all fields).
maxFileByteLength number 50 * 1024 * 1024 (50 MB) The max byte length of a file
maxFileCountPerField number 1 The max number of files allowed for each file field.

To use with <input type="file" multiple>.
abortOnFileByteLengthLimit boolean true If a file goes over the maxFileByteLength limit, whether to:

- Throw an error (and do cleanup, i.e. abort the entire operation), or
- To truncate the file.

Parameter: fileFieldConfigOverride

For each field, you can set the values of:

  • maxFileCountPerField
  • maxFileByteLength
  • abortOnFileByteLengthLimit

which will override the values in the general config (including the defaults). The values for numerical limits can be both smaller and larger than the ones in the general config.


Let's say you configured parseFormData() the following way:

await parseFormData(
    maxFileByteLength: 15, // 10 bytes
    exampleOverrideFile: {
      maxFileByteLength: 10, // 5 bytes
      abortOnFileByteLengthLimit: false,

Now, if you send a FormData request with following structure (represented as JSON, this is NOT a valid FormData request):

  "normalFile": {
    "type": "file",
    byte length (15) === config.maxFileByteLength,
    no error thrown,
    no truncation
    "content": "15 bytes, innit?"
  "examplePriorityFile": {
    "type": "file",
    byte length (10) > fileFieldConfigOverride["exampleOverrideFile"],
    fileFieldConfigOverride["exampleOverrideFile"].abortOnFileByteLengthLimit === false,
    FILE TRUNCATED TO 10 BYTES: "will be tr"
    "content": "will be truncated" 
  "file2": {
    "type": "file",
    byte lenght (26) > config.maxFileByteLength,
    config.abortOnFileByteLengthLimit === true (by default, as no custom value and no override was provided),

    Exceeded file byte length limit ("maxFileByteLength").
    Corresponding Busboy configuration option: Busboy.Limits["files"].
    Field: "file2".
    Configuration info: 26
    "content": "26 bytes, so will throw :("

Parameter: busboyConfig

Type: Pechkin.BusboyConfig, which equals to Busboy.Config (from busboy package) without the limits property. Limits passed to busboy are ignored, and instead the limits are set by pechkin's config & fileFieldConfigOverride parameters are used.

Return value: Files AsyncIterator / AsyncIterable


type Files = {
  next: () => Promise<{
    done: boolean
    value: Pechkin.File
  return: () => Promise<void>,
  throw: (error: Error) => Promise<void>,
  [Symbol.asyncIterator]: () => this

Files is both an AsyncIterator and an AsyncIterable, so you can use it both as an iterator (calling await and as an iterable (for await (const file of files) { ... }). It is recommended to use it only as an iterable in a for-await-of loop, as it's much easier and less error-prone to use.

❗️ Very important note on iteration:

The should always be consumed, otherwise the request parsing will hang, and you might never get access to the next file. If you don't care about a particular file, you can simply do, but the stream should always be consumed.

(Internal) Error handling inside `Pechkin::FileIterator``

This section is for those who want to know how errors are handled internally. This is not necessary to use pechkin.

  • If an error occurs inside next() (for example, a file exceeded its maxFileByteLength limit), a cleanup function is called, which unpipes the request from the parser (busboy), the iterator is stopped, and the error is thrown.

  • If an error occurs inside the body of the for-await-of loop, return() is called, a cleanup function is called, and the iterator is stopped.

  • If an error occurs anywhere else inside Pechkin, throw() method is called, which either:

    • Rejects the currently-awaited next() call,
    • Or, if there is no next() call currently awaited, sets the next next() call to reject with the error.

    Apart from that, the usual cleanup function is called, and the iterator is stopped.

Type: File

  filename: string;
  encoding: string;
  mimeType: string;
  field: string;
  stream: ByteLengthTruncateStream; // See below: "Type: ByteLengthTruncateStream"
  • filename: The client-provided filename of the file.
  • encoding: The encoding of the file. List of encodings supported by Node.js.
  • mimeType: The MIME type of the file. If the MIME type is crucial for your application, you should not trust the client-provided mimeType value – the client can easily lie about it (e.g. send an .exe file with mimeType: "image/png"). Instead, you should use a library like file-type.
  • field: The name of the field the file was sent in.
  • stream: The file Readable stream. The stream should always be consumed, otherwise the request parsing will hang, and you might never get access to the next file. If you don't care about a particular file, you can simply do, but the stream should always be consumed.

Type: ByteLengthTruncateStream

A Transform stream, which does the following to source streams piped into it:

  • Does nothing, i.e. acts as a PassThrough stream, as long as the source stream hasn't reached maxFileByteLength limit bytes.
  • As soon as the source stream reaches maxFileByteLength limit bytes:
    • Sets the truncated property to true
    • Throws if abortOnFileByteLimit = true
    • Truncates the file if abortOnFileByteLimit = false
Transform & {
  bytesRead: number;
  bytesWritten: number;
  truncated: boolean;
  • bytesRead: The number of bytes read from the source stream.
  • bytesWritten: The number of bytes written to the destination stream.
  • truncated: Whether the file was truncated or not. Truncation only happens with abortOnFileByteLimit = false. bytesRead - bytesWritten is the number of bytes truncated, and is larger than 0 only if truncated = true, and 0 if truncated = false.

All of the above properties are updated in real time, as the stream is consumed. This means that you have to wait until the stream is fully consumed (i.e. 'finish'/'end' events are emitted, after e.g. an upload to file system or S3) to get the final values of bytesRead, bytesWritten and truncated.