From b25c8132422be406b94cd3a30e69a966af1e7adb Mon Sep 17 00:00:00 2001
From: eps1lon <silbermann.sebastian@gmail.com>
Date: Wed, 8 Mar 2023 21:36:42 +0100
Subject: [PATCH] [Flight] Support returning undefined

---
 packages/react-client/src/ReactFlightClient.js |  4 ++++
 packages/react-server/src/ReactFlightServer.js | 15 ++++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/packages/react-client/src/ReactFlightClient.js b/packages/react-client/src/ReactFlightClient.js
index aa864b6895b46..24da4242e949f 100644
--- a/packages/react-client/src/ReactFlightClient.js
+++ b/packages/react-client/src/ReactFlightClient.js
@@ -553,6 +553,10 @@ export function parseModelString(
             throw chunk.reason;
         }
       }
+      case 'U': {
+        // Special encoding for `undefined` which can't be serialized as JSON otherwise.
+        return undefined;
+      }
       default: {
         // We assume that anything else is a reference ID.
         const id = parseInt(value.substring(1), 16);
diff --git a/packages/react-server/src/ReactFlightServer.js b/packages/react-server/src/ReactFlightServer.js
index 17e1c7aca83c5..e3a7b12ebe2bb 100644
--- a/packages/react-server/src/ReactFlightServer.js
+++ b/packages/react-server/src/ReactFlightServer.js
@@ -105,6 +105,7 @@ export type ReactModel =
   | number
   | symbol
   | null
+  | void
   | Iterable<ReactModel>
   | ReactModelObject
   | Promise<ReactModel>;
@@ -533,6 +534,10 @@ function serializeProviderReference(name: string): string {
   return '$P' + name;
 }
 
+function serializeUndefined(): string {
+  return '$U';
+}
+
 function serializeClientReference(
   request: Request,
   parent: {+[key: string | number]: ReactModel} | $ReadOnlyArray<ReactModel>,
@@ -1105,14 +1110,14 @@ export function resolveModelToJSON(
     return escapeStringValue(value);
   }
 
-  if (
-    typeof value === 'boolean' ||
-    typeof value === 'number' ||
-    typeof value === 'undefined'
-  ) {
+  if (typeof value === 'boolean' || typeof value === 'number') {
     return value;
   }
 
+  if (typeof value === 'undefined') {
+    return serializeUndefined();
+  }
+
   if (typeof value === 'function') {
     if (isClientReference(value)) {
       return serializeClientReference(request, parent, key, (value: any));