parseMultipartRequest.ts

import {Multipart, MultipartFile, MultipartValue} from '@fastify/multipart';
import {FastifyRequest} from 'fastify';

function isMultipartFile(m: Multipart): m is MultipartFile {
  return !!m['toBuffer'];
}

async function getValueOrBuffer(m: Multipart): Promise<string | Buffer> {
  if (isMultipartFile(m)) {
    return m.toBuffer();
  } else {
    return (m as MultipartValue<string>).value;
  }
}

export async function parseMultipartRequest(
  req: FastifyRequest
): Promise<Record<string, string | Buffer | (string | Buffer)[]>> {
  let data: MultipartFile;
  for await (const part of req.files()) {
    await part.toBuffer();
    data = part;
  }

  const result: Record<string, string | Buffer | (string | Buffer)[]> = {};
  if (!data) {
    return result;
  }

  for await (const [key, fieldObject] of Object.entries(data.fields)) {
    let value: string | Buffer | (string | Buffer)[];
    if (Array.isArray(fieldObject)) {
      value = await Promise.all(fieldObject.map((field) => getValueOrBuffer(field)));
    } else {
      value = await getValueOrBuffer(fieldObject);
    }

    result[key] = value;
  }

  return result;
}