form-data-encoder

What is form-data-encoder?

The form-data-encoder package is designed to encode FormData objects into a variety of formats suitable for HTTP transmission. It is particularly useful for preparing multipart/form-data payloads without relying on the browser's native FormData implementation, which can be beneficial in non-browser environments like Node.js.

What are form-data-encoder's main functionalities?

Encoding FormData for multipart/form-data

This feature allows you to encode a FormData object into a multipart/form-data format, which is suitable for HTTP requests. The code sample demonstrates how to create a FormData object, append fields and files, and then use the FormDataEncoder to encode the data and retrieve the necessary headers for an HTTP request.

const { FormDataEncoder } = require('form-data-encoder');
const { FormData, File } = require('formdata-node');

const formData = new FormData();
formData.append('field', 'value');
formData.append('file', new File(['content'], 'file.txt'));

const encoder = new FormDataEncoder(formData);

// You can now use encoder to get headers and read the encoded form data
const headers = encoder.headers;
const body = Readable.from(encoder);

Streaming encoded data

This feature is useful for streaming the encoded form data, which can be helpful when dealing with large files or when you want to send the data in chunks over a network. The code sample shows how to iterate over the encoder to handle each chunk of data.

const { FormDataEncoder } = require('form-data-encoder');
const { FormData } = require('formdata-node');

const formData = new FormData();
formData.append('field', 'value');

const encoder = new FormDataEncoder(formData);

// Stream the encoded form data
for await (const chunk of encoder) {
  // Handle each chunk of data
}

Other packages similar to form-data-encoder

form-data

The form-data package is a Node.js module that allows you to create readable 'multipart/form-data' streams. It can be used to submit forms and file uploads to other web applications. It is similar to form-data-encoder but also provides the ability to append streams, buffers, and simple values to form data.

busboy

Busboy is a Node.js module for parsing incoming HTML form data. It handles multipart/form-data, which is primarily used for uploading files. While form-data-encoder is focused on encoding and preparing form data for transmission, busboy is designed to parse and handle form data received by a server.

multiparty

Multiparty is a Node.js module for parsing multipart/form-data requests, which are typically used for file uploads. Unlike form-data-encoder, which is for encoding form data, multiparty is used on the server side to process the uploaded data.

README

form-data-encoder

Encode FormData content into the multipart/form-data format

Usage

1. To start the encoding process, you need to create a new Encoder instance with the FormData you want to encode:

import {Readable} from "stream"

import {FormData, File} from "formdata-node"
import {Encoder} from "form-data-encoder"

import fetch from "node-fetch"

const fd = new FormData()

fd.set("greeting", "Hello, World!")
fd.set("file", new File(["On Soviet Moon landscape see binoculars through YOU"], "file.txt"))

const encoder = new Encoder(fd)

const options = {
  method: "post",

  // Set request headers provided by the Encoder.
  // The `headers` property has `Content-Type` and `Content-Length` headers.
  headers: encoder.headers,

  // Create a Readable stream from the Encoder.
  // You can omit usage of `Readable.from` for HTTP clients whose support async iterables.
  // The Encoder will yield FormData content portions encoded into the multipart/form-data format as node-fetch consumes the stream.
  body: Readable.from(encoder.encode()) // or Readable.from(encoder)
}

const response = await fetch("https://httpbin.org/post", options)

console.log(await response.json())

1. Encoder support different spec-compatible FormData implementations. Let's try it with formdata-polyfill:

import {Readable} from "stream"

import {Encoder} from "form-data-encoder"
import {FormData, File} from "formdata-polyfill/esm-min.js"

const fd = new FormData()

fd.set("field", "Some value")
fd.set("file", new File(["File content goes here"], "file.txt"))

const encoder = new Encoder(fd)

const options = {
  method: "post",
  headers: encoder.headers,
  body: Readable.from(encoder)
}

await fetch("https://httpbin.org/post", options)

1. Because the Encoder is async iterable, you can use it with different targets. Let's say you want to put FormData content into Blob, for that you can write a function like this:

import {FormData} from "formdata-polyfill/esm-min.js"
import {blobFrom} from "fetch-blob/from.js"
import {Encoder} from "form-data-encoder"

import Blob from "fetch-blob"
import fetch from "node-fetch"

async function toBlob(form) {
  const encoder = new Encoder(form)
  const chunks = []

  for await (const chunk of encoder) {
    chunks.push(chunk)
  }

  return new Blob(chunks, {type: encoder.contentType})
}

const fd = new FormData()

fd.set("name", "John Doe")
fd.set("avatar", await blobFrom("path/to/an/avatar.png"), "avatar.png")

const options = {
  method: "post",
  body: await toBlob(fd)
}

await fetch("https://httpbin.org/post", options)

1. Another way to convert FormData parts to blob using form-data-encoder is making a Blob-ish class:

import {Readable} from "stream"

import {FormData} from "formdata-polyfill/esm-min.js"
import {blobFrom} from "fetch-blob/from.js"
import {Encoder} from "form-data-encoder"

import Blob from "fetch-blob"
import fetch from "node-fetch"

class BlobDataItem {
  constructor(encoder) {
    this.#encoder = encoder
    this.#size = encoder.headers["Content-Length"]
    this.#type = encoder.headers["Content-Type"]
  }

  get type() {
    return this.#type
  }

  get size() {
    return this.#size
  }

  stream() {
    return Readable.from(this.#encoder)
  }

  get [Symbol.toStringTag]() {
    return "Blob"
  }
}

const fd = new FormData()

fd.set("name", "John Doe")
fd.set("avatar", await blobFrom("path/to/an/avatar.png"), "avatar.png")

const encoder = new Encoder(fd)

// Note that node-fetch@2 performs more strictness tests for Blob objects, so you may need to do extra steps before you set up request body (like, maybe you'll need to instaniate a Blob with BlobDataItem as one of its blobPart)
const blob = new BlobDataItem(enocoder) // or new Blob([new BlobDataItem(enocoder)], {type: encoder.contentType})

const options = {
  method: "post",
  body: blob
}

await fetch("https://httpbin.org/post", options)

1. In this example we will pull FormData content into the ReadableStream:

 // This module is only necessary when you targeting Node.js or need web streams that implement Symbol.asyncIterator
import {ReadableStream} from "web-streams-polyfill/ponyfill/es2018"

import {Encoder} from "form-data-encoder"
import {FormData} from "formdata-node"

import fetch from "node-fetch"

const toReadableStream = iterator => new ReadableStream({
  async pull(controller) {
    const {value, done} = await iterator.next()

    if (done) {
      return controller.close()
    }

    controller.enqueue(value)
  }
})

const fd = new FormData()

fd.set("field", "My hovercraft is full of eels")

const encoder = new Encoder(fd)

const options = {
  method: "post",
  headers: encoder.headers,
  body: toReadableStream(encoder.encode())
}

// Note that this example requires `fetch` to support Symbol.asyncIterator, which node-fetch lacks of (but will support eventually)
await fetch("https://httpbin.org/post", options)

1. Speaking of async iterables - if HTTP client supports them, you can use encoder like this:

import {Encoder} from "form-data-encoder"
import {FormData} from "formdata-node"

import fetch from "node-fetch"

const fd = new FormData()

fd.set("field", "My hovercraft is full of eels")

const encoder = new Encoder(fd)

const options = {
  method: "post",
  headers: encoder.headers,
  body: encoder
}

await fetch("https://httpbin.org/post", options)

1. ...And for those client whose supporting form-data-encoder out of the box, the usage will be much, much more simpler:

import {FormData} from "formdata-node" // Or any other spec-compatible implementation

import fetch from "node-fetch"

const fd = new FormData()

fd.set("field", "My hovercraft is full of eels")

const options = {
  method: "post",
  body: fd
}

// Note that node-fetch does NOT support form-data-encoder
await fetch("https://httpbin.org/post", options)

Installation

You can install this package using npm:

npm install form-data-encoder

Or yarn:

yarn add form-data-encoder

Or pnpm:

pnpm add form-data-encoder

API

class Encoder

constructor(form[, boundary]) -> {Encoder}

• {FormDataLike} form - FormData object to encode. This object must be a spec-compatible FormData implementation.
• {string} boundary - An optional boundary string that will be used by the encoder. If there's no boundary string is present, Encoder will generate it automatically.

Creates a multipart/form-data encoder.

Instance properties

boundary -> {string}

Returns boundary string

contentType -> {string}

Returns Content-Type header for multipart/form-data

headers -> {object}

Returns headers object with Content-Type and Content-Length header

Instance methods

encode() -> {AsyncGenerator<Uint8Array, void, undefined>}

Creates an async iterator allowing to perform the encoding by portions.

[Symbol.asyncIterator]() -> {AsyncGenerator<Uint8Array, void, undefined>}

An alias for Encoder#encode() method.

isFileLike(value) -> {boolean}

Check if given value is a File-ish object.

• {unknown} value - a value to test

isFormDataLike(value) -> {boolean}

Check if is a FormData-ish object.

• {unknown} value - a value to test