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

Type-safe integration of FormData with tRPC using Standard Schema.

tRPC can't type a FormData input on its own: files have to travel as multipart parts, every other field arrives as an untyped string, and input shows up undefined. This package restores end-to-end type safety by carrying the non-file fields as a serialized JSON payload alongside the file parts, and validating the whole thing against a schema you control.

Installation

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

Requires @trpc/client and @trpc/server v11 as peer dependencies. zod v4 is an optional peer dependency, only needed if you use the /zod entry point.

Usage

Three pieces work together — all of them are required:

1. Client: the typedFormDataLink serializes the non-file fields into the request.
2. Server: the plugin middleware deserializes them back before validation.
3. Schema: typedFormData() wraps your input schema and validates files + fields together.

Server

Build the plugin from your tRPC instance, add its middleware to the procedures that accept uploads, and wrap the input schema in typedFormData():

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

const t = initTRPC.create()

const typedFormDataPlugin = createTypedFormDataPlugin(t) // 👈 create the plugin

// 👇 add the middleware to a procedure
const uploadProcedure = t.procedure.concat(typedFormDataPlugin.middleware)

export const router = t.router({
  upload: uploadProcedure
    .input(
      // 👇 wrap any Standard Schema with `typedFormData`
      typedFormData(
        z.object({
          userId: z.string(),
          file: file({ maxSize: 10 * 1024 * 1024 }),
        }),
      ),
    )
    .mutation(async ({ input }) => {
      const { userId, file } = input // input is fully typed

      await file.arrayBuffer() // works as expected
    }),
})

export type AppRouter = typeof router

Client

Add typedFormDataLink to your links (before the terminating link), then build the payload with createTypedFormData and pass it straight to .mutate():

import { createTypedFormData, typedFormDataLink } from '@falcondev-oss/trpc-typed-form-data/client'
import { createTRPCClient, httpLink } from '@trpc/client'
import type { AppRouter } from './server'

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

await trpc.upload.mutate(
  createTypedFormData({
    // fields are type-checked against the schema
    userId: '123',
    file: new File(['file contents'], 'example.txt'),
  }),
)

Arrays of files are supported — pass a File[] under a key and every file is uploaded under it.

Note: FormData requests can't be batched. Terminate the upload path with httpLink, not httpBatchLink.

File validation

Instead of z.instanceof(File) or z.file(), use the built-in file() validator to enforce size and MIME-type constraints. It also gives you a stable FileValue type across the server → client boundary — unlike z.file(), whose inferred type degrades to any in environments without a DOM File (notably React Native), silently dropping type-checking.

For a zod schema, import the zod-native file so it nests inside z.object():

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

typedFormData(
  z.object({
    userId: z.string(),
    avatar: file({
      maxSize: 10_000_000,
      mimeTypes: ['image/png', 'image/jpeg'],
    }),
  }),
)

For any other Standard Schema stack, the same validator is available as a Standard Schema from @falcondev-oss/trpc-typed-form-data/server (or the root export). All options — maxSize, minSize, mimeTypes — are optional; byte limits are inclusive.

The isFile(value) type guard narrows an unknown value to FileValue for ad-hoc checks outside a schema.

React Native / Expo

React Native's FormData only sends parts that carry a uri — it never reads a Blob's bytes. Use ReactNativeFile (a real File subclass that also carries a uri) so uploads work and pass validation. Build it synchronously from a picker result:

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

const { assets } = await ImagePicker.launchImageLibraryAsync()
const asset = assets[0]

const file = new ReactNativeFile({
  uri: asset.uri, // local file URI; used for the upload
  name: asset.fileName ?? 'upload.jpg', // sent as the multipart filename
  type: asset.mimeType, // MIME type, e.g. 'image/jpeg'
  size: asset.fileSize, // bytes — enables size checks like file({ maxSize })
})

await trpc.upload.mutate(createTypedFormData({ userId: '123', file }))

Only uri and name are required. Pass size if you use size constraints — the empty blob backing a ReactNativeFile otherwise reports 0.

You can also build one from a remote URL. fromUrl sends a HEAD request to read the content type and length; the body is not downloaded, the upload streams from the URL:

// url from e.g. expo-file-system, expo-image-picker or similar libraries
const file = await ReactNativeFile.fromUrl('https://example.com/photo.jpg')

await trpc.upload.mutate(createTypedFormData({ userId: '123', file }))

Configuration

Transformers

If your tRPC router uses a data transformer (e.g. superjson), pass it to the link so the serialized payload matches on both ends:

typedFormDataLink({ transformer: superjson })

The payload is serialized with the link's transformer and deserialized with the router's; a mismatch throws a BAD_REQUEST on the server.

Transfer field

The non-file payload travels in a ~data field by default. To change it, set the same transferDataKey on both sides:

// client
typedFormDataLink({ transferDataKey: '__data' })
// server
createTypedFormDataPlugin(t, { transferDataKey: '__data' })

API

Exported from @falcondev-oss/trpc-typed-form-data/...:

Entry point	Exports
/client	createTypedFormData, typedFormDataLink, ReactNativeFile, file, isFile
/server	typedFormData, createTypedFormDataPlugin, file, isFile
/zod	file (zod-native), isFile
. (root)	file (Standard Schema), isFile

Types (FileValue, FileValidationOptions, TypedFormData) are re-exported from every entry point.

License

MIT