feat: allow form data through default body serializer

[thatsprettyfaroutman]

I was trying to use openapi-fetch for file uploading, but by default that wasn't possible (using multipart/form-data). Looking at the source code I noticed that part of the solution for fixing it seemed to be missing as defaultBodySerializer never allows FormData to be passed through it:

if (requestInit.body) {
  // Here, bodySerializer always returns a JSON string by default
  requestInit.body = bodySerializer(requestInit.body);
  // remove `Content-Type` if serialized body is FormData; browser will correctly set Content-Type & boundary expression
  if (requestInit.body instanceof FormData) {
    requestInit.headers.delete("Content-Type");
  }
}

I'm trying to fix that in this PR by allowing FormData to be passed through the defaultBodySerializer enabling file uploads with multipart/form-data by default.

Changes

Allow FormData to be passed through defaultBodySerializer as body to enable file uploading by default.

How to Review

Upload a file as part of multipart/form-data

Checklist

• Unit tests updated
• docs/ updated (if necessary)
• pnpm run update:examples run (only applicable for openapi-typescript)

[changeset-bot]

🦋 Changeset detected

Latest commit: 0abd93c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
openapi-fetch	Patch

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[drwpow]

Thank you for this fix! I’d accept this change if you also added a test that was failing before your fix, and passes with.

Also be sure to add a Changeset for this that adds a patch to openapi-fetch so it can be released on merge!

[thatsprettyfaroutman]

How to get this to accept FormData?

[drwpow]

Oh hm do we need to adjust the types here? Is there an improvement we could make in packages/openapi-fetch/index.d.ts for body?

[drwpow]

Oh this will be tricky. This library’s whole purpose is to assert types for body based on your OpenAPI schema. https://github.com/openapi-ts/openapi-typescript/blob/main/packages/openapi-fetch/src/index.d.ts#L90

We don’t want to always allow FormData as that would mess up some people’s code. So we want to conditionally allow FormData in some instances where it’s possible (e.g. not for GET). So we’ll need to surgically add FormData as a union into request bodies, but only when it may be possible/desired.

Given the complexity, I’m OK with merging this PR without that just to unblock the runtime, and the types can be tackled separately (sidenote: that’s why the runtime and types are separated—the type complexity isn’t possible to express with normal runtime code)

[drwpow]

What are your thoughts? Do you want to try and solve the type errors? Or merge by requiring manual overrides?

[thatsprettyfaroutman]

I'm ok with merging now. I think letting FormData through the body serializer is a nice little addition in itself. Oh, I was wondering why the types are separated, that's very cool to know 👍

[drwpow]

Sounds good! Will merge.

[thatsprettyfaroutman]

Thanks @drwpow! I added the test and a changeset entry

[drwpow]

Looks good! But I see we have a type error in the tests we should probably fix in openapi-fetch/index.d.ts.