Fast streaming multipart parsing for JavaScript. multipart-parser processes multipart bodies incrementally so large uploads can be handled without buffering the entire multipart payload in memory.
- File Upload Parsing - Parse file uploads (
multipart/form-data) with automatic field and file detection - Full Multipart Support - Support for all
multipart/*content types (mixed, alternative, related, etc.) - Convenient API -
MultipartPartAPI witharrayBuffer,bytes,text,size, and metadata access - File Size Limiting - Built-in file size limiting to prevent abuse
- Node.js Support - First-class Node.js support with native
http.IncomingMessagecompatibility - Runtime Demos - Demos for every major runtime
npm i remixThe most common use case for multipart-parser is handling file uploads when you're building a web server. For this case, the parseMultipartRequest function is your friend. It automatically validates the request is multipart/form-data, extracts the multipart boundary from the Content-Type header, parses all fields and files in the request.body stream, and gives each one to you as a MultipartPart object with a rich API for accessing its metadata and content.
import { MultipartParseError, parseMultipartRequest } from 'remix/multipart-parser'
async function handleRequest(request: Request): void {
try {
for await (let part of parseMultipartRequest(request)) {
if (part.isFile) {
// Access file data in multiple formats
let buffer = part.arrayBuffer // ArrayBuffer
console.log(`File received: ${part.filename} (${buffer.byteLength} bytes)`)
console.log(`Content type: ${part.mediaType}`)
console.log(`Field name: ${part.name}`)
// Save to disk, upload to cloud storage, etc.
await saveFile(part.filename, part.bytes)
} else {
let text = part.text // string
console.log(`Field received: ${part.name} = ${JSON.stringify(text)}`)
}
}
} catch (error) {
if (error instanceof MultipartParseError) {
console.error('Failed to parse multipart request:', error.message)
} else {
console.error('An unexpected error occurred:', error)
}
}
}A common use case when handling file uploads is limiting the size of uploaded files to prevent malicious users from sending very large files that may overload your server's memory and/or storage capacity. You can set a file upload size limit using the maxFileSize option, and return a 413 "Payload Too Large" response when you receive a request that exceeds the limit.
import {
MultipartParseError,
MaxFileSizeExceededError,
parseMultipartRequest,
} from 'remix/multipart-parser/node'
const oneMb = Math.pow(2, 20)
const maxFileSize = 10 * oneMb
async function handleRequest(request: Request): Promise<Response> {
try {
for await (let part of parseMultipartRequest(request, { maxFileSize })) {
// ...
}
} catch (error) {
if (error instanceof MaxFileSizeExceededError) {
return new Response('File size limit exceeded', { status: 413 })
} else if (error instanceof MultipartParseError) {
return new Response('Failed to parse multipart request', { status: 400 })
} else {
console.error(error)
return new Response('Internal Server Error', { status: 500 })
}
}
}The main module (import {} from 'remix/multipart-parser') assumes you're working with the fetch API (Request, ReadableStream, etc). Support for these interfaces was added to Node.js by the undici project in version 16.5.0.
If however you're building a server for Node.js that relies on node-specific APIs like http.IncomingMessage, stream.Readable, and buffer.Buffer (ala Express or http.createServer), multipart-parser ships with an additional module that works directly with these APIs.
import * as http from 'node:http'
import { MultipartParseError, parseMultipartRequest } from 'remix/multipart-parser/node'
let server = http.createServer(async (req, res) => {
try {
for await (let part of parseMultipartRequest(req)) {
// ...
}
} catch (error) {
if (error instanceof MultipartParseError) {
console.error('Failed to parse multipart request:', error.message)
} else {
console.error('An unexpected error occurred:', error)
}
}
})
server.listen(8080)If you're working directly with multipart boundaries and buffers/streams of multipart data that are not necessarily part of a request, multipart-parser provides a low-level parseMultipart() API that you can use directly:
import { parseMultipart } from 'remix/multipart-parser'
let message = new Uint8Array(/* ... */)
let boundary = '----WebKitFormBoundary56eac3x'
for (let part of parseMultipart(message, { boundary })) {
// ...
}In addition, the parseMultipartStream function provides an async generator interface for multipart data in a ReadableStream:
import { parseMultipartStream } from 'remix/multipart-parser'
let message = new ReadableStream(/* ... */)
let boundary = '----WebKitFormBoundary56eac3x'
for await (let part of parseMultipartStream(message, { boundary })) {
// ...
}The demos directory contains a few working demos of how you can use this library:
demos/bun- using multipart-parser in Bundemos/cf-workers- using multipart-parser in a Cloudflare Worker and storing file uploads in R2demos/deno- using multipart-parser in Denodemos/node- using multipart-parser in Node.js
multipart-parser is designed to be as efficient as possible, operating on streams of data and rarely buffering in common usage. This design yields exceptional performance when handling multipart payloads of any size. In benchmarks, multipart-parser is as fast or faster than busboy.
The results of running the benchmarks on my laptop:
> @remix-run/multipart-parser@0.10.1 bench:node /Users/michael/Projects/remix-the-web/packages/multipart-parser
> node --disable-warning=ExperimentalWarning ./bench/runner.ts
Platform: Darwin (24.5.0)
CPU: Apple M1 Pro
Date: 6/13/2025, 12:27:09 PM
Node.js v24.0.2
┌──────────────────┬──────────────────┬──────────────────┬──────────────────┬───────────────────┐
│ (index) │ 1 small file │ 1 large file │ 100 small files │ 5 large files │
├──────────────────┼──────────────────┼──────────────────┼──────────────────┼───────────────────┤
│ multipart-parser │ '0.01 ms ± 0.03' │ '1.08 ms ± 0.08' │ '0.04 ms ± 0.01' │ '10.50 ms ± 0.38' │
│ multipasta │ '0.02 ms ± 0.06' │ '1.07 ms ± 0.02' │ '0.15 ms ± 0.02' │ '10.46 ms ± 0.11' │
│ busboy │ '0.06 ms ± 0.17' │ '3.07 ms ± 0.24' │ '0.24 ms ± 0.05' │ '29.85 ms ± 0.18' │
│ @fastify/busboy │ '0.05 ms ± 0.13' │ '1.23 ms ± 0.09' │ '0.45 ms ± 0.22' │ '11.81 ms ± 0.11' │
└──────────────────┴──────────────────┴──────────────────┴──────────────────┴───────────────────┘
> @remix-run/multipart-parser@0.10.1 bench:bun /Users/michael/Projects/remix-the-web/packages/multipart-parser
> bun run ./bench/runner.ts
Platform: Darwin (24.5.0)
CPU: Apple M1 Pro
Date: 6/13/2025, 12:27:31 PM
Bun 1.2.13
┌──────────────────┬────────────────┬────────────────┬─────────────────┬─────────────────┐
│ │ 1 small file │ 1 large file │ 100 small files │ 5 large files │
├──────────────────┼────────────────┼────────────────┼─────────────────┼─────────────────┤
│ multipart-parser │ 0.01 ms ± 0.04 │ 0.86 ms ± 0.09 │ 0.04 ms ± 0.01 │ 8.32 ms ± 0.26 │
│ multipasta │ 0.02 ms ± 0.07 │ 0.87 ms ± 0.03 │ 0.25 ms ± 0.21 │ 8.27 ms ± 0.09 │
│ busboy │ 0.05 ms ± 0.17 │ 3.54 ms ± 0.10 │ 0.30 ms ± 0.03 │ 34.79 ms ± 0.38 │
│ @fastify/busboy │ 0.06 ms ± 0.18 │ 4.04 ms ± 0.08 │ 0.48 ms ± 0.06 │ 39.91 ms ± 0.37 │
└──────────────────┴────────────────┴────────────────┴─────────────────┴─────────────────┘
> @remix-run/multipart-parser@0.10.1 bench:deno /Users/michael/Projects/remix-the-web/packages/multipart-parser
> deno run --allow-sys ./bench/runner.ts
Platform: Darwin (24.5.0)
CPU: Apple M1 Pro
Date: 6/13/2025, 12:28:12 PM
Deno 2.3.6
┌──────────────────┬──────────────────┬────────────────────┬──────────────────┬─────────────────────┐
│ (idx) │ 1 small file │ 1 large file │ 100 small files │ 5 large files │
├──────────────────┼──────────────────┼────────────────────┼──────────────────┼─────────────────────┤
│ multipart-parser │ "0.01 ms ± 0.03" │ "1.03 ms ± 0.04" │ "0.05 ms ± 0.01" │ "10.05 ms ± 0.20" │
│ multipasta │ "0.02 ms ± 0.07" │ "1.04 ms ± 0.03" │ "0.16 ms ± 0.02" │ "10.10 ms ± 0.08" │
│ busboy │ "0.05 ms ± 0.19" │ "3.06 ms ± 0.15" │ "0.32 ms ± 0.05" │ "29.92 ms ± 0.24" │
│ @fastify/busboy │ "0.06 ms ± 0.14" │ "14.72 ms ± 11.42" │ "0.81 ms ± 0.20" │ "127.63 ms ± 35.77" │
└──────────────────┴──────────────────┴────────────────────┴──────────────────┴─────────────────────┘
form-data-parser- Usesmultipart-parserinternally to parse multipart requests and generateFileUploads for storageheaders- Used internally to parse HTTP headers and get metadata (filename, content type) for eachMultipartPart
Thanks to Jacob Ebey who gave me several code reviews on this project prior to publishing.
See LICENSE