File fields are not instances of Blob

[jaydenseric]

A File field should be an instance of Blob when read via FormData.get().

For example, in Chrome:

I'm trying to assert in a project that a Blob instance (globally polyfilled via fetch-blob) gets correctly added to a FormData instance, and one of the things to check is that the type of the field that was added is a Blob.

[octet-stream]

Please read the docs:

Returns the first value associated with the given name. If the field has Blob, Buffer or any Readable and ReadableStream (and when options.size is set for this stream) value, the File-like object will be returned.

I don't polyfill anything, instead I use objects which have same methods and properties as those from browsers API. It allows you to use your own Blob and File implementations if they're compatible. If you want to check whether returning field is a Blob, File or ReadableStream, you must check it other way than just call instanceof. See: util/isWHATWGReadable.mjs and util/isBlob

Just to mention, this is how node-fetch handles the same checks: utils/is.js

[jaydenseric]

That may be how it works now, but it could be better.

• If Blob global is present, use it, otherwise use a local Blob class.
• If File global is present, use it, otherwise extend Blob (either from a global or a local class, see above) to create and use a local File class.

[octet-stream]

Now that's sounds good to me. I will take a look at that. Thanks again!

[octet-stream]

hey, looks like fetch-blob has Symbol.hasInstance, so the myFile instanceof Blob should work.

See: https://github.com/node-fetch/fetch-blob/pull/46

[jaydenseric]

Hmm, this used to not work so maybe something changed 🤔

instanceof Blob is working now in my tests, so the original reason for this issue is resolved. I'll leave it up to you to close it or not here, because I'm not sure how anything other than the originally suggested fix could resolve other potential issues.

[octet-stream]

Hi @jaydenseric 👋 I think since Node.js has the first class Blob support, I can use fetch-blob in this package and then eventually add native Blob support.

[octet-stream]

So, this change available in https://github.com/octet-stream/form-data/releases/tag/v3.0.0. I believe this issue is now resolved.

[octet-stream]

Also, note that public API has been changed, so be careful and read release note before upgrading :)