[Flight] Support returning undefined

Author: eps1lon

packages/react-client/src/ReactFlightClient.js

@@ -556,6 +556,10 @@ 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);

packages/react-server/src/ReactFlightServer.js

@@ -117,6 +117,7 @@ export type ReactClientValue =
   | number
   | symbol
   | null
+  | void
   | Iterable<ReactClientValue>
   | Array<ReactClientValue>
   | ReactClientObject
@@ -546,6 +547,10 @@ function serializeProviderReference(name: string): string {
   return '$P' + name;
 }
 
+function serializeUndefined(): string {
+  return '$U';
+}
+
 function serializeClientReference(
   request: Request,
   parent:
@@ -1134,14 +1139,14 @@ 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));