Trpc Typed Form Data

@falcondev-oss/trpc-typed-form-data

tRPC can't type a FormData input: files must ride as multipart parts, everything else is untyped strings, and input arrives undefined. This package restores end-to-end type safety by carrying the non-file fields as a serialized sidecar — a JSON blob in one extra FormData field (~data) — while files stay as real multipart parts. Client, server, and schema each handle one half of that trick.

Four entry points:

file, isFile, and the types (FileValue, FileValidationOptions, TypedFormData) are re-exported from /client and /server too, so import them from wherever you already are.

Three touch points — all required

The sidecar only works if every piece is wired. Miss one and validation throws a message naming what's missing.

1. Client link — serializes the sidecar into the FormData. Place it before the terminating link:

import { typedFormDataLink } from '@falcondev-oss/trpc-typed-form-data/client'
import { createTRPCClient, httpLink } from '@trpc/client'

const trpc = createTRPCClient<AppRouter>({
  links: [
    typedFormDataLink(), // 👈 before httpLink
    httpLink({ url: '/api/trpc' }),
  ],
})

2. Server middleware — deserializes the sidecar back onto the FormData. Build the plugin from your tRPC instance and concat its middleware onto the procedures that accept uploads:

import { createTypedFormDataPlugin } from '@falcondev-oss/trpc-typed-form-data/server'

const t = initTRPC.create()
const typedFormDataPlugin = createTypedFormDataPlugin(t)

export const uploadProcedure = t.procedure.concat(typedFormDataPlugin.middleware)

3. Input schema wrapper — wrap the input schema in typedFormData(). It reads the sidecar + the file parts and validates them together:

import { typedFormData } from '@falcondev-oss/trpc-typed-form-data/server'
import { z } from 'zod'

export const router = t.router({
  upload: uploadProcedure
    .input(
      typedFormData(
        z.object({
          userId: z.string(),
          file: z.instanceof(File),
        }),
      ),
    )
    .mutation(({ input }) => {
      // input is fully typed: { userId: string; file: File }
    }),
})

Calling it — build the payload with createTypedFormData and pass it straight to .mutate(); the fields are type-checked against the schema:

import { createTypedFormData } from '@falcondev-oss/trpc-typed-form-data/client'

await trpc.upload.mutate(
  createTypedFormData({
    userId: '123',
    file: new File(['contents'], 'example.txt'),
  }),
)

Arrays of files work: pass File[] under a key — each file is appended under that key and read back with getAll.

File validation — prefer file() over z.file()

Reach for this package's file() when the schema is shared with a React Native / Expo client. z.file()'s inferred type degrades to any in environments without a DOM File (React Native), silently dropping type-checking across the tRPC server→client boundary. file() infers a stable FileValue — a structural subset of the Web File/Blob API (name, size, type, arrayBuffer, stream, slice, text) — everywhere.

Two flavours, same options ({ maxSize?, minSize?, mimeTypes? }, all optional, byte limits inclusive):

// zod stack — nests inside z.object like z.file()
import { file } from '@falcondev-oss/trpc-typed-form-data/zod'
typedFormData(z.object({
  avatar: file({ maxSize: 10_000_000, mimeTypes: ['image/png', 'image/jpeg'] }),
}))

// non-zod / any Standard Schema stack
import { file } from '@falcondev-oss/trpc-typed-form-data/server'

isFile(value) is a type guard narrowing unknown → FileValue for ad-hoc checks outside a schema. Annotate values that came out of these schemas (a procedure input, an upload helper) with FileValue, not File.

Gotchas

• All three touch points, or nothing. Missing the link, the middleware, or the wrapper each produces a distinct validation error — read the message, it names the missing piece.
• Transformer must match. If your router uses a transformer (e.g. superjson), pass it to typedFormDataLink({ transformer }) — the sidecar is serialized with the link's transformer and deserialized with the router's. A mismatch throws BAD_REQUEST on the server.
• Custom transfer field. The sidecar rides in a ~data field by default. Override with transferDataKey — set the same value on both typedFormDataLink({ transferDataKey }) and createTypedFormDataPlugin(t, { transferDataKey }).
• FormData isn't batchable. Terminate with httpLink, not httpBatchLink, on the upload path.
• Middleware only fires for mutations with no plain input (the FormData path). Queries and subscriptions pass through untouched.

React Native / Expo

Uploading from an image/document picker needs ReactNativeFile — RN's FormData only sends parts with a uri and never reads a Blob's bytes. Read reference/react-native.md before wiring an Expo upload.