isFormDataLike fails with bundled code

[penalosa]

Bug Description

If you bundle undici with esbuild (or other bundlers that mangle names), the check in https://github.com/nodejs/undici/blob/main/lib/core/util.js#L378 fails, since the name of the FormData class is not necessarily equal to "FormData". Previously, the name of the FormData class was hardcoded with

class FormData {
  static name = 'FormData'
  ...
}

but this was removed in #1742 (https://github.com/nodejs/undici/pull/1742/files#diff-bf6d8875713a8e4d7917687069b9d3c16323b34155573b442c156d77765aa999).

For instance, esbuild bundles:

class FormData {
  ...
}

to

var FormData4 = class {
  ...
}

(or some other number).

Reproducible By

// index.js
import { FormData, Request } from 'undici';
import { writeFileSync } from 'fs';

const formData = new FormData();
formData.append('hello', 'world');
new Request('http://example.com', {
  method: 'POST',
  body: formData,
})
  .arrayBuffer()
  .then((r) => writeFileSync('./body.bin', new Uint8Array(r)));

Run npx esbuild --bundle index.js --outfile=out.js --platform=node --format=cjs && node out.js

The file body.bin contains the content:

[object FormData]

Expected Behavior

It should contain:

------formdata-undici-082107041324
Content-Disposition: form-data; name="hello"

world
------formdata-undici-082107041324--

Environment

macOS Ventura arm64, node 18.7

Additional context

This is fixed by changing isFormDataLike to check chunk[Symbol.toStringTag] rather than chunk.constructor.name, or re-adding the hardcoded class name, but I'm not sure if that might break other stuff.

[KhafraDev]

Adding in the hardcoded class name is incorrect, however using Symbol.toStringTag over the constructor name is a valid fix. Would you like to send in a PR?

[ronag]

@KhafraDev I believe our isFormData

[penalosa]

Amazing, thank you @ronag! Do you know when this might be in the next release?