form-data-parser

form-data-parser is a wrapper around request.formData() that provides streaming support for handling file uploads. This is useful in server contexts where large files should be streamed to disk or some cloud storage service like AWS S3 or Cloudflare R2 instead of being buffered in memory.

Features

• Drop-in replacement for request.formData() with support for streaming file uploads
• Built on the standard web Streams API and File API
• Does not buffer any content, minimal memory usage
• Automatically falls back to native request.formData() implementation for non-multipart/form-data requests

The Problem

The web fetch API's built-in request.formData() method is not a great fit for server environments because it doesn't provide a way to stream file uploads. This means that when you call request.formData() in a server environment on a request that was submitted by a <form enctype="multipart/form-data">, any file uploads contained in the request are buffered in memory. For small files this may not be an issue, but it's a total non-starter for large files that exceed the server's memory capacity.

form-data-parser fixes this issue by providing an API to handle streaming file data.

Installation

Install from npm:

npm install @mjackson/form-data-parser

Usage

import { LocalFileStorage } from '@mjackson/file-storage/local';
import { type FileUpload, parseFormData } from '@mjackson/form-data-parser';

const fileStorage = new LocalFileStorage('/uploads/user-avatars');

async function uploadHandler(fileUpload: FileUpload) {
  // Is this file upload from the <input type="file" name="user-avatar"> field?
  if (fileUpload.fieldName === 'user-avatar') {
    let storageKey = `user-${user.id}-avatar`;

    // FileUpload objects are not meant to stick around for very long (they are
    // streaming data from the request.body!) so we should store them as soon as
    // possible.
    await fileStorage.set(storageKey, fileUpload);

    // Return a File for the FormData object. This is a LazyFile that knows how
    // to access the file's content if needed (using e.g. file.stream()) but
    // waits until it is requested to actually read anything.
    return fileStorage.get(storageKey);
  }

  // Ignore any files we don't recognize the name of...
}

async function requestHandler(request: Request) {
  let formData = await parseFormData(request, uploadHandler);

  let file = formData.get('user-avatar'); // File
  file.name; // "my-avatar.jpg" (name of the file on the user's computer)
  file.size; // number
  file.type; // "image/jpeg"
}

Related Packages

• file-storage - A simple key/value interface for storing FileUpload objects you get from the parser
• multipart-parser - The parser used internally for parsing multipart/form-data HTTP messages

License

See LICENSE