Add a missing method when checking if an object is a spec-compliant FormData object

[xxczaki]

MDN: https://developer.mozilla.org/en-US/docs/Web/API/FormData/has]
Spec: https://xhr.spec.whatwg.org/#dom-formdata-has

[jimmywarting]

OCD kicking in? :P
shouldn't we put them in alphabetic order while we are at it? :P

[tinovyatkin]

Not exactly related to this PR, but I think we should start this discussion again 😄
I'm pretty skeptical about this way of object testing that we are using in node-fetch. It's completely different things to check an object provided by an environment (DOM for example) just by Symbol.toStringTag vs concluding out of this symbol that some object will act as we believe in an environment where it doesn't belong natively (read FormData in Node.js).
Should we test methods arguments too (set.length)?
I would prefer to have some small reference implementations of Blob, FormData, and AbortController inside the node-fetch organization and just do instanceof - as it can be extended by any developer if needed.
For example, the following class will pass your test perfectly, but will not work as expected:

class FormData extends URLSearchParams {
  get [Symbol.toStringTag]() {
    return "FormData";
  }
}

[jimmywarting]

I would prefer to have some small reference implementations of Blob, FormData, and AbortController inside the node-fetch organization and just do instanceof

That would increase the size of node-fetch. And you wouldn't be able to use similar polyfills
(hint hint -> #751)

I still prefer to keep none fetch stuff outside of node-fetch. I mean formdata-node has a custom File class that is not inherit by fetch-blob and can't be a instance of the same

[tinovyatkin]

@jimmywarting we have Blob already, FormData (without serialization) is almost complete in my example above and we have serialization built-in into node-fetch. AbortController is small too.

instanceof can be extended by base class via Symbol.hasInstance as I did today for fetch-blob or can be spoofed via Proxy, as in the example in #848, nothing particularly difficult.

[jimmywarting]

@tinovyatkin Would like @bitinn or someone else opinion on that first before we decide on it - create a issue about it first - the hasInstance is grate doe :)

[jimmywarting]

I don't think this is worth merging if #1314 gets merged.
it basically brings in formdata-polyfill and fixes some stuff like the missing body.formData() while also using symbol.hasInstance to check if something is a spec compatible formData

this util function got wiped out completely by
https://github.com/node-fetch/node-fetch/pull/1314/files#diff-598e8a8f4fefeab6b7f348b81147005e33df4922ec007a4d9bb0c2422055bfd8L49

[LinusU]

Ah, right, #1314 seems to be very close to getting merged so lets close this one ✔️