fix(start): serialize FormData for server function input

e2e/start/basic/app/routes/-server-fns/serialize-formdata-fn-call.tsx

@@ -0,0 +1,73 @@
+import * as React from 'react'
+import { createServerFn } from '@tanstack/start'
+
+const testValues = {
+  name: 'Sean',
+  age: 25,
+  __adder: 1,
+}
+
+export const greetUser = createServerFn()
+  .validator((data: unknown) => {
+    if (!(data instanceof FormData)) {
+      throw new Error('Invalid! FormData is required')
+    }
+    const name = data.get('name')
+    const age = data.get('age')
+
+    if (!name || !age) {
+      throw new Error('Name and age are required')
+    }
+
+    return {
+      name: name.toString(),
+      age: parseInt(age.toString(), 10),
+    }
+  })
+  .handler(async ({ data: { name, age } }) => {
+    return `Hello, ${name}! You are ${age + testValues.__adder} years old.`
+  })
+
+// Usage
+export function SerializeFormDataFnCall() {
+  const [formDataResult, setFormDataResult] = React.useState({})
+
+  return (
+    <div className="p-2 border m-2 grid gap-2">
+      <h3>Serialize FormData Fn POST Call</h3>
+      <div className="overflow-y-auto">
+        It should return{' '}
+        <code>
+          <pre data-testid="expected-serialize-formdata-server-fn-result">
+            Hello, {testValues.name}! You are{' '}
+            {testValues.age + testValues.__adder} years old.
+          </pre>
+        </code>
+      </div>
+      <form
+        className="flex flex-col gap-2"
+        data-testid="serialize-formdata-form"
+        onSubmit={(evt) => {
+          evt.preventDefault()
+          const data = new FormData(evt.currentTarget)
+          greetUser({ data }).then(setFormDataResult)
+        }}
+      >
+        <input type="text" name="name" defaultValue={testValues.name} />
+        <input type="number" name="age" defaultValue={testValues.age} />
+        <button
+          type="submit"
+          data-testid="test-serialize-formdata-fn-calls-btn"
+          className="rounded-md bg-white px-2.5 py-1.5 text-sm font-semibold text-gray-900 shadow-sm ring-1 ring-inset ring-gray-300 hover:bg-gray-50"
+        >
+          Submit
+        </button>
+      </form>
+      <div className="overflow-y-auto">
+        <pre data-testid="serialize-formdata-form-response">
+          {JSON.stringify(formDataResult)}
+        </pre>
+      </div>
+    </div>
+  )
+}

e2e/start/basic/app/routes/server-fns.tsx

@@ -4,6 +4,7 @@ import { createFileRoute } from '@tanstack/react-router'
 import { ConsistentServerFnCalls } from './-server-fns/consistent-fn-calls'
 import { MultipartServerFnCall } from './-server-fns/multipart-formdata-fn-call'
 import { AllowServerFnReturnNull } from './-server-fns/allow-fn-return-null'
+import { SerializeFormDataFnCall } from './-server-fns/serialize-formdata-fn-call'
 
 export const Route = createFileRoute('/server-fns')({
   component: RouteComponent,
@@ -15,6 +16,7 @@ function RouteComponent() {
       <ConsistentServerFnCalls />
       <MultipartServerFnCall />
       <AllowServerFnReturnNull />
+      <SerializeFormDataFnCall />
     </>
   )
 }

e2e/start/basic/tests/base.spec.ts

@@ -223,3 +223,23 @@ test('Server function can return null for GET and POST calls', async ({
     page.getByTestId('allow_return_null_postFn-response'),
   ).toContainText(JSON.stringify(null))
 })
+
+test('Server function can correctly send and receive FormData', async ({
+  page,
+}) => {
+  await page.goto('/server-fns')
+
+  await page.waitForLoadState('networkidle')
+  const expected =
+    (await page
+      .getByTestId('expected-serialize-formdata-server-fn-result')
+      .textContent()) || ''
+  expect(expected).not.toBe('')
+
+  await page.getByTestId('test-serialize-formdata-fn-calls-btn').click()
+  await page.waitForLoadState('networkidle')
+
+  await expect(
+    page.getByTestId('serialize-formdata-form-response'),
+  ).toContainText(expected)
+})

packages/react-router/src/transformer.ts

@@ -33,7 +33,7 @@ export const defaultTransformer: RouterTransformer = {
       return val
     }),
   encode: (value: any) => {
-    // When encodign, dive first
+    // When encoding, dive first
     if (Array.isArray(value)) {
       return value.map((v) => defaultTransformer.encode(v))
     }
@@ -125,6 +125,28 @@ const transformers = [
     // From
     (v) => Object.assign(new Error(v.message), v),
   ),
+  createTransformer(
+    // Key
+    'formData',
+    // Check
+    (v) => v instanceof FormData,
+    // To
+    (v: FormData) => {
+      const entries: Record<string, any> = {}
+      v.forEach((value, key) => {
+        entries[key] = value
+      })
+      return entries
+    },
+    // From
+    (v) => {
+      const formData = new FormData()
+      Object.entries(v).forEach(([key, value]) => {
+        formData.append(key, value as string | Blob)
+      })
+      return formData
+    },
+  ),
 ] as const
 
 export type TransformerStringify<T, TSerializable> = T extends TSerializable