From 72fbd4c99a7b7dd84c2f86d8f1d80977dbde45d8 Mon Sep 17 00:00:00 2001
From: Ben Durrant <ben.durrant@marketdojo.com>
Date: Sun, 12 Jan 2025 23:55:58 +0000
Subject: [PATCH] fix(start): preserve multiple values for same key in
 `FormData` serialization (#3140)

Co-authored-by: SeanCassiere <33615041+SeanCassiere@users.noreply.github.com>
---
 .../serialize-formdata-fn-call.tsx            | 19 ++++---
 packages/react-router/src/index.tsx           |  1 +
 packages/react-router/src/transformer.ts      | 52 +++++++++++++------
 .../react-router/tests/transformer.test.tsx   | 20 +++++++
 4 files changed, 71 insertions(+), 21 deletions(-)

diff --git a/e2e/start/basic/app/routes/-server-fns/serialize-formdata-fn-call.tsx b/e2e/start/basic/app/routes/-server-fns/serialize-formdata-fn-call.tsx
index c33ff93686..d6bcc56d17 100644
--- a/e2e/start/basic/app/routes/-server-fns/serialize-formdata-fn-call.tsx
+++ b/e2e/start/basic/app/routes/-server-fns/serialize-formdata-fn-call.tsx
@@ -4,28 +4,32 @@ import { createServerFn } from '@tanstack/start'
 const testValues = {
   name: 'Sean',
   age: 25,
+  pet1: 'dog',
+  pet2: 'cat',
   __adder: 1,
 }
 
 export const greetUser = createServerFn()
-  .validator((data: unknown) => {
+  .validator((data: FormData) => {
     if (!(data instanceof FormData)) {
       throw new Error('Invalid! FormData is required')
     }
     const name = data.get('name')
     const age = data.get('age')
+    const pets = data.getAll('pet')
 
-    if (!name || !age) {
-      throw new Error('Name and age are required')
+    if (!name || !age || pets.length === 0) {
+      throw new Error('Name, age and pets are required')
     }
 
     return {
       name: name.toString(),
       age: parseInt(age.toString(), 10),
+      pets: pets.map((pet) => pet.toString()),
     }
   })
-  .handler(async ({ data: { name, age } }) => {
-    return `Hello, ${name}! You are ${age + testValues.__adder} years old.`
+  .handler(({ data: { name, age, pets } }) => {
+    return `Hello, ${name}! You are ${age + testValues.__adder} years old, and your favorite pets are ${pets.join(',')}.`
   })
 
 // Usage
@@ -40,7 +44,8 @@ export function SerializeFormDataFnCall() {
         <code>
           <pre data-testid="expected-serialize-formdata-server-fn-result">
             Hello, {testValues.name}! You are{' '}
-            {testValues.age + testValues.__adder} years old.
+            {testValues.age + testValues.__adder} years old, and your favorite{' '}
+            pets are {testValues.pet1},{testValues.pet2}.
           </pre>
         </code>
       </div>
@@ -55,6 +60,8 @@ export function SerializeFormDataFnCall() {
       >
         <input type="text" name="name" defaultValue={testValues.name} />
         <input type="number" name="age" defaultValue={testValues.age} />
+        <input type="text" name="pet" defaultValue={testValues.pet1} />
+        <input type="text" name="pet" defaultValue={testValues.pet2} />
         <button
           type="submit"
           data-testid="test-serialize-formdata-fn-calls-btn"
diff --git a/packages/react-router/src/index.tsx b/packages/react-router/src/index.tsx
index c4dd49bafc..7235f7b2bf 100644
--- a/packages/react-router/src/index.tsx
+++ b/packages/react-router/src/index.tsx
@@ -291,6 +291,7 @@ export type {
   RouterTransformer,
   TransformerParse,
   TransformerStringify,
+  DefaultSerializable,
   DefaultTransformerParse,
   DefaultTransformerStringify,
 } from './transformer'
diff --git a/packages/react-router/src/transformer.ts b/packages/react-router/src/transformer.ts
index 4320332bf7..fad1fb2f89 100644
--- a/packages/react-router/src/transformer.ts
+++ b/packages/react-router/src/transformer.ts
@@ -80,11 +80,11 @@ export const defaultTransformer: RouterTransformer = {
   },
 }
 
-const createTransformer = <T extends string>(
-  key: T,
-  check: (value: any) => boolean,
-  toValue: (value: any) => any = (v) => v,
-  fromValue: (value: any) => any = (v) => v,
+const createTransformer = <TKey extends string, TInput, TSerialized>(
+  key: TKey,
+  check: (value: any) => value is TInput,
+  toValue: (value: TInput) => TSerialized,
+  fromValue: (value: TSerialized) => TInput,
 ) => ({
   key,
   stringifyCondition: check,
@@ -94,13 +94,14 @@ const createTransformer = <T extends string>(
 })
 
 // Keep these ordered by predicted frequency
+// Make sure to keep DefaultSerializeable in sync with these transformers
 // Also, make sure that they are unit tested in transformer.test.tsx
 const transformers = [
   createTransformer(
     // Key
     'undefined',
     // Check
-    (v) => v === undefined,
+    (v): v is undefined => v === undefined,
     // To
     () => 0,
     // From
@@ -110,7 +111,7 @@ const transformers = [
     // Key
     'date',
     // Check
-    (v) => v instanceof Date,
+    (v): v is Date => v instanceof Date,
     // To
     (v) => v.toISOString(),
     // From
@@ -120,7 +121,7 @@ const transformers = [
     // Key
     'error',
     // Check
-    (v) => v instanceof Error,
+    (v): v is Error => v instanceof Error,
     // To
     (v) => ({ ...v, message: v.message, stack: v.stack, cause: v.cause }),
     // From
@@ -130,12 +131,24 @@ const transformers = [
     // Key
     'formData',
     // Check
-    (v) => v instanceof FormData,
+    (v): v is FormData => v instanceof FormData,
     // To
-    (v: FormData) => {
-      const entries: Record<string, any> = {}
+    (v) => {
+      const entries: Record<
+        string,
+        Array<FormDataEntryValue> | FormDataEntryValue
+      > = {}
       v.forEach((value, key) => {
-        entries[key] = value
+        const entry = entries[key]
+        if (entry !== undefined) {
+          if (Array.isArray(entry)) {
+            entry.push(value)
+          } else {
+            entries[key] = [entry, value]
+          }
+        } else {
+          entries[key] = value
+        }
       })
       return entries
     },
@@ -143,7 +156,11 @@ const transformers = [
     (v) => {
       const formData = new FormData()
       Object.entries(v).forEach(([key, value]) => {
-        formData.append(key, value as string | Blob)
+        if (Array.isArray(value)) {
+          value.forEach((val) => formData.append(key, val))
+        } else {
+          formData.append(key, value)
+        }
       })
       return formData
     },
@@ -162,9 +179,14 @@ export type TransformerParse<T, TSerializable> = T extends TSerializable
     ? ReadableStream
     : { [K in keyof T]: TransformerParse<T[K], TSerializable> }
 
+export type DefaultSerializable = Date | undefined | Error | FormData
+
 export type DefaultTransformerStringify<T> = TransformerStringify<
   T,
-  Date | undefined
+  DefaultSerializable
 >
 
-export type DefaultTransformerParse<T> = TransformerParse<T, Date | undefined>
+export type DefaultTransformerParse<T> = TransformerParse<
+  T,
+  DefaultSerializable
+>
diff --git a/packages/react-router/tests/transformer.test.tsx b/packages/react-router/tests/transformer.test.tsx
index fbe9f0f20d..e4cf06110f 100644
--- a/packages/react-router/tests/transformer.test.tsx
+++ b/packages/react-router/tests/transformer.test.tsx
@@ -46,6 +46,14 @@ describe('transformer.stringify', () => {
       `"{"$formData":{"foo":"bar","name":"Sean"}}"`,
     )
   })
+  it('should stringify FormData with multiple values for the same key', () => {
+    const formData = new FormData()
+    formData.append('foo', 'bar')
+    formData.append('foo', 'baz')
+    expect(tf.stringify(formData)).toMatchInlineSnapshot(
+      `"{"$formData":{"foo":["bar","baz"]}}"`,
+    )
+  })
 })
 
 describe('transformer.parse', () => {
@@ -99,4 +107,16 @@ describe('transformer.parse', () => {
       ['name', 'Sean'],
     ])
   })
+  it('should parse FormData with multiple values for the same key', () => {
+    const formData = new FormData()
+    formData.append('foo', 'bar')
+    formData.append('foo', 'baz')
+    const str = tf.stringify(formData)
+    const parsed = tf.parse(str) as FormData
+    expect(parsed).toBeInstanceOf(FormData)
+    expect([...parsed.entries()]).toEqual([
+      ['foo', 'bar'],
+      ['foo', 'baz'],
+    ])
+  })
 })