A streaming parser for HTML form data for node.js

Usage no npm install needed!

<script type="module">
  import busboy from 'https://cdn.skypack.dev/busboy';



A node.js module for parsing incoming HTML form data.

Changes (breaking or otherwise) in v1.0.0 can be found here.



npm install busboy


  • Parsing (multipart) with default options:
const http = require('http');

const busboy = require('busboy');

http.createServer((req, res) => {
  if (req.method === 'POST') {
    console.log('POST request');
    const bb = busboy({ headers: req.headers });
    bb.on('file', (name, file, info) => {
      const { filename, encoding, mimeType } = info;
        `File [${name}]: filename: %j, encoding: %j, mimeType: %j`,
      file.on('data', (data) => {
        console.log(`File [${name}] got ${data.length} bytes`);
      }).on('close', () => {
        console.log(`File [${name}] done`);
    bb.on('field', (name, val, info) => {
      console.log(`Field [${name}]: value: %j`, val);
    bb.on('close', () => {
      console.log('Done parsing form!');
      res.writeHead(303, { Connection: 'close', Location: '/' });
  } else if (req.method === 'GET') {
    res.writeHead(200, { Connection: 'close' });
          <form method="POST" enctype="multipart/form-data">
            <input type="file" name="filefield"><br />
            <input type="text" name="textfield"><br />
            <input type="submit">
}).listen(8000, () => {
  console.log('Listening for requests');

// Example output:
// Listening for requests
//   < ... form submitted ... >
// POST request
// File [filefield]: filename: "logo.jpg", encoding: "binary", mime: "image/jpeg"
// File [filefield] got 11912 bytes
// Field [textfield]: value: "testing! :-)"
// File [filefield] done
// Done parsing form!
  • Save all incoming files to disk:
const { randomFillSync } = require('crypto');
const fs = require('fs');
const http = require('http');
const os = require('os');
const path = require('path');

const busboy = require('busboy');

const random = (() => {
  const buf = Buffer.alloc(16);
  return () => randomFillSync(buf).toString('hex');

http.createServer((req, res) => {
  if (req.method === 'POST') {
    const bb = busboy({ headers: req.headers });
    bb.on('file', (name, file, info) => {
      const saveTo = path.join(os.tmpDir(), `busboy-upload-${random()}`);
    bb.on('close', () => {
      res.writeHead(200, { 'Connection': 'close' });
      res.end(`That's all folks!`);
}).listen(8000, () => {
  console.log('Listening for requests');



busboy exports a single function:

( function )(< object >config) - Creates and returns a new Writable form parser stream.

  • Valid config properties:

    • headers - object - These are the HTTP headers of the incoming request, which are used by individual parsers.

    • highWaterMark - integer - highWaterMark to use for the parser stream. Default: node's stream.Writable default.

    • fileHwm - integer - highWaterMark to use for individual file streams. Default: node's stream.Readable default.

    • defCharset - string - Default character set to use when one isn't defined. Default: 'utf8'.

    • preservePath - boolean - If paths in filenames from file parts in a 'multipart/form-data' request shall be preserved. Default: false.

    • limits - object - Various limits on incoming data. Valid properties are:

      • fieldNameSize - integer - Max field name size (in bytes). Default: 100.

      • fieldSize - integer - Max field value size (in bytes). Default: 1048576 (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 module).

This function can throw exceptions if there is something wrong with the values in config. For example, if the Content-Type in headers is missing entirely, is not a supported type, or is missing the boundary for 'multipart/form-data' requests.

(Special) Parser stream events

  • file(< string >name, < Readable >stream, < object >info) - Emitted for each new file found. name contains the form field name. stream is a Readable stream containing the file's data. No transformations/conversions (e.g. base64 to raw binary) are done on the file's data. info contains the following properties:

    • filename - string - If supplied, this contains the file's filename. WARNING: You should almost never use this value as-is (especially if you are using preservePath: true in your config) as it could contain malicious input. You are better off generating your own (safe) filenames, or at the very least using a hash of the filename.

    • encoding - string - The file's 'Content-Transfer-Encoding' value.

    • mimeType - string - The file's 'Content-Type' value.

    Note: If you listen for this event, you should always consume the stream whether you care about its contents or not (you can simply do stream.resume(); if you want to discard/skip the contents), otherwise the 'finish'/'close' event will never fire on the busboy parser stream. However, if you aren't accepting files, you can either simply not listen for the 'file' event at all or set limits.files to 0, and any/all files will be automatically skipped (these skipped files will still count towards any configured limits.files and limits.parts limits though).

    Note: If a configured limits.fileSize limit was reached for a file, stream will both have a boolean property truncated set to true (best checked at the end of the stream) and emit a 'limit' event to notify you when this happens.

  • field(< string >name, < string >value, < object >info) - Emitted for each new non-file field found. name contains the form field name. value contains the string value of the field. info contains the following properties:

    • nameTruncated - boolean - Whether name was truncated or not (due to a configured limits.fieldNameSize limit)

    • valueTruncated - boolean - Whether value was truncated or not (due to a configured limits.fieldSize limit)

    • encoding - string - The field's 'Content-Transfer-Encoding' value.

    • mimeType - string - The field's 'Content-Type' value.

  • partsLimit() - Emitted when the configured limits.parts limit has been reached. No more 'file' or 'field' events will be emitted.

  • filesLimit() - Emitted when the configured limits.files limit has been reached. No more 'file' events will be emitted.

  • fieldsLimit() - Emitted when the configured limits.fields limit has been reached. No more 'field' events will be emitted.