I thought I needed this but turns out I didn't. None of the existing solutions worked well for me so I built this. Maybe it'll be useful someday...

parse-multipart-form-data.ts

import {typedBoolean} from './misc'

function headerValuesAsObject(headerValue: string) {
  const valuesAsObject: Record<string, string> = Object.fromEntries(
    headerValue
      .split(', ')
      .filter(h => h.includes('='))
      .map(h => {
        const [key, valueString] = h.split('=')
        if (!valueString) return []
        const unwrappedValue = valueString.replace(/^"(.*)"$/, '$1')
        return [key, unwrappedValue]
      }),
  )
  return valuesAsObject
}

function parse(body: string, contentType: string) {
  if (!contentType.startsWith('multipart/form-data;')) {
    throw new Error(`Unsupported content type: ${contentType}`)
  }
  const contentTypeHeaderValue = contentType
    .replace('multipart/form-data;', '')
    .trim()
  const {boundary} = headerValuesAsObject(contentTypeHeaderValue)
  if (!boundary) {
    throw new Error('No boundary found in content-type header')
  }

  const trailerlessBody = body.split(`--${boundary}--`)[0]?.trim() // get rid of the trailer
  if (!trailerlessBody) {
    throw new Error('No multipart body found')
  }

  const rawParts = trailerlessBody
    .split(`--${boundary}`)
    .map(l => l.trim())
    .filter(typedBoolean)
  const parts = rawParts.map(rawPart => {
    const match = rawPart.match(
      /^(?<rawHeaderLines>.*?)(?<separator>(\r\n|\n){2})(?<partBody>.*?)$/s,
    )
    if (
      !match?.groups ||
      !match.groups.rawHeaderLines ||
      !match.groups.partBody
    ) {
      throw new Error('No header or body found in part')
    }
    const {rawHeaderLines, partBody} = match.groups
    const headers = new Headers()
    for (const rawHeaderLine of rawHeaderLines.split('\n')) {
      const [key, values] = rawHeaderLine.trim().split(':')
      if (key) {
        for (const value of values?.split(';') ?? []) {
          headers.append(key, value.trim())
        }
      }
    }
    const contentDisposition = headerValuesAsObject(
      headers.get('Content-Disposition') ?? '',
    )
    const parsed = {
      contentDisposition,
      headers,
      body: partBody.trim(),
    }
    return parsed
  })

  return parts
}

export {parse}