[Flight] Support returning undefined

Author: eps1lon

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);

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>,
@@ -1107,12 +1112,15 @@ export function resolveModelToJSON(
 
   if (
     typeof value === 'boolean' ||
-    typeof value === 'number' ||
-    typeof value === 'undefined'
+    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));