[Flight] Support FormData from Server to Client (#28754)

We currently support FormData for Replies mainly for Form Actions. This
supports it in the other direction too which lets you return it from an
action as the response. Mainly for parity.

We don't really recommend that you just pass the original form data back
because the action is supposed to be able to clear fields and such but
you could potentially at least use this as the format and could clear
some fields.

We could potentially optimize this with a temporary reference if the
same object was passed to a reply in case you use it as a round trip to
avoid serializing it back again. That way the action has the ability to
override it to clear fields but if it doesn't you get back the same as
you sent.

#28755 adds support for Blobs when the `enableBinaryFlight` is enabled
which allows them to be used inside FormData too.

packages/react-client/src/ReactFlightClient.js

@@ -746,6 +746,16 @@ function parseModelString(
         }
         return undefined;
       }
+      case 'K': {
+        // FormData
+        const id = parseInt(value.slice(2), 16);
+        const data = getOutlinedModel(response, id);
+        const formData = new FormData();
+        for (let i = 0; i < data.length; i++) {
+          formData.append(data[i][0], data[i][1]);
+        }
+        return formData;
+      }
       case 'I': {
         // $Infinity
         return Infinity;

packages/react-client/src/ReactFlightReplyClient.js

@@ -75,14 +75,14 @@ export type ReactServerValue =
   | string
   | boolean
   | number
-  | symbol
   | null
   | void
   | bigint
   | Iterable<ReactServerValue>
   | Array<ReactServerValue>
   | Map<ReactServerValue, ReactServerValue>
   | Set<ReactServerValue>
+  | FormData
   | Date
   | ReactServerObject
   | Promise<ReactServerValue>; // Thenable<ReactServerValue>

packages/react-client/src/__tests__/ReactFlight-test.js

@@ -468,6 +468,40 @@ describe('ReactFlight', () => {
       `);
   });
 
+  if (typeof FormData !== 'undefined') {
+    it('can transport FormData (no blobs)', async () => {
+      function ComponentClient({prop}) {
+        return `
+          formData: ${prop instanceof FormData}
+          hi: ${prop.get('hi')}
+          multiple: ${prop.getAll('multiple')}
+          content: ${JSON.stringify(Array.from(prop))}
+        `;
+      }
+      const Component = clientReference(ComponentClient);
+
+      const formData = new FormData();
+      formData.append('hi', 'world');
+      formData.append('multiple', 1);
+      formData.append('multiple', 2);
+
+      const model = <Component prop={formData} />;
+
+      const transport = ReactNoopFlightServer.render(model);
+
+      await act(async () => {
+        ReactNoop.render(await ReactNoopFlightClient.read(transport));
+      });
+
+      expect(ReactNoop).toMatchRenderedOutput(`
+          formData: true
+          hi: world
+          multiple: 1,2
+          content: [["hi","world"],["multiple","1"],["multiple","2"]]
+        `);
+    });
+  }
+
   it('can transport cyclic objects', async () => {
     function ComponentClient({prop}) {
       expect(prop.obj.obj.obj).toBe(prop.obj.obj);

packages/react-server/src/ReactFlightServer.js

@@ -239,6 +239,7 @@ export type ReactClientValue =
   | Array<ReactClientValue>
   | Map<ReactClientValue, ReactClientValue>
   | Set<ReactClientValue>
+  | FormData
   | $ArrayBufferView
   | ArrayBuffer
   | Date
@@ -1186,6 +1187,12 @@ function serializeMap(
   return '$Q' + id.toString(16);
 }
 
+function serializeFormData(request: Request, formData: FormData): string {
+  const entries = Array.from(formData.entries());
+  const id = outlineModel(request, (entries: any));
+  return '$K' + id.toString(16);
+}
+
 function serializeSet(request: Request, set: Set<ReactClientValue>): string {
   const entries = Array.from(set);
   for (let i = 0; i < entries.length; i++) {
@@ -1595,6 +1602,10 @@ function renderModelDestructive(
     if (value instanceof Set) {
       return serializeSet(request, value);
     }
+    // TODO: FormData is not available in old Node. Remove the typeof later.
+    if (typeof FormData === 'function' && value instanceof FormData) {
+      return serializeFormData(request, value);
+    }
 
     if (enableBinaryFlight) {
       if (value instanceof ArrayBuffer) {
@@ -2139,6 +2150,10 @@ function renderConsoleValue(
     if (value instanceof Set) {
       return serializeSet(request, value);
     }
+    // TODO: FormData is not available in old Node. Remove the typeof later.
+    if (typeof FormData === 'function' && value instanceof FormData) {
+      return serializeFormData(request, value);
+    }
 
     if (enableBinaryFlight) {
       if (value instanceof ArrayBuffer) {