Support globalThis.FormData

[mcollina]

Since #3502, we only support our own classes, and that is the right move. However, we need to relax this condition to allow for globalThis.FormData, as that is a different copy of undici.

In essence, this change make the following fork

// import { fetch, FormData } from 'undici'
import { fetch } from 'undici'
import fastify from 'fastify'
import fastifyMultipart from '@fastify/multipart'

const app = fastify()

app.register(fastifyMultipart, {
  attachFieldsToBody: true
})

app.post('/upload', async (req, reply) => {
  console.log('received', req.body.file.filename)
  return 'ok'
})

await app.listen({ port: 3000 })

const body = new FormData()
body.append('file', new Blob(['hello world'], { type: 'text/plain' }), 'hello.txt')

const res = await fetch('http://localhost:3000/upload', {
  method: 'POST',
  body
})

console.log(res.status)
console.log(await res.text())

[tsctx]

Suggested change

	} else if (webidl.is.FormData(object) || (globalThis.FormData && object instanceof FormData)) {
	} else if (webidl.is.FormData(object)) {

[tsctx]

How about something like this?

// lib/web/fetch/webidl.js
webidl.util.MakeTypeAssertionMultiple = function (List) {
  return (O) => List.some((I) => FunctionPrototypeSymbolHasInstance(I, O))
}

Usage:

webidl.is.FormData = webidl.util.MakeTypeAssertionMultiple([FormData, gloablThis.FormData].filter(Boolean))

[KhafraDev]

I don't understand the need for this change, it's a known limitation that different versions of undici create different versions of classes. It's also a pain to deal with.

Won't work:

new Request(new globalThis.Request('http://a'))

await fetch(new globalThis.Request(...))

Works because of webidl:

• Headers (due to the iterators)
• Response (since the members of a ResponseInit are getters, it's treated like a plain object)

[mcollina]

Ok, let's close this. We would need to document this a bit better, as I suspect a flurry of issues.