fix: If request does not have a static backend defined, return undefined for the Request.prototype.backend getter (#722)

Author: JakeChampion

runtime/js-compute-runtime/builtins/request-response.cpp

@@ -1263,11 +1263,7 @@ bool Request::bodyAll(JSContext *cx, unsigned argc, JS::Value *vp) {
 bool Request::backend_get(JSContext *cx, unsigned argc, JS::Value *vp) {
   METHOD_HEADER(0)
   JS::RootedValue backend(cx, JS::GetReservedSlot(self, static_cast<uint32_t>(Slots::Backend)));
-  if (backend.isNullOrUndefined()) {
-    args.rval().setString(JS_GetEmptyString(cx));
-  } else {
-    args.rval().set(backend);
-  }
+  args.rval().set(backend);
 
   return true;
 }

types/globals.d.ts

@@ -1084,7 +1084,7 @@ interface Request extends Body {
   clone(): Request;
 
   // Fastly extensions
-  backend: string;
+  backend?: string;
   setCacheOverride(override: import('fastly:cache-override').CacheOverride): void;
   setCacheKey(key: string): void;
   setManualFramingHeaders(manual: boolean): void;