From 22e9ae84da807d4144b3d25102bf8e5239380b58 Mon Sep 17 00:00:00 2001
From: Matteo Collina <hello@matteocollina.com>
Date: Sat, 7 Nov 2026 20:06:10 +0100
Subject: [PATCH] stream: drop rejection step for sync transforms

transformStreamDefaultControllerPerformTransform chained a rejection
step onto the result of every transform() call. A non-thenable result
cannot reject, so return the shared resolved promise directly and save
a promise allocation and a microtask per chunk.

The write promise of a transform whose transform() returns a
non-thenable now settles one microtask earlier than in the reference
implementation. The relative order of all stream events is unchanged.

Signed-off-by: Matteo Collina <hello@matteocollina.com>
---
 lib/internal/webstreams/transformstream.js | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/lib/internal/webstreams/transformstream.js b/lib/internal/webstreams/transformstream.js
index bb1d1aa8396d..2943ce3fe099 100644
--- a/lib/internal/webstreams/transformstream.js
+++ b/lib/internal/webstreams/transformstream.js
@@ -592,11 +592,11 @@ function transformStreamDefaultControllerError(controller, error) {
 
 // Mirrors the reference implementation's
 // `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
-// the returned promise settles one microtask after the (coerced) result
-// does, and a rejection errors the transform stream before propagating.
-// The raw transform callback plus the shared resolved promise for
-// non-thenable results replace the previous async wrapper's two implicit
-// promises per chunk.
+// a rejection errors the transform stream before propagating. A
+// non-thenable result cannot reject, so it maps to the shared resolved
+// promise without the rejection step; the write then completes one
+// microtask earlier than in the reference implementation, with the
+// relative order of all stream events unchanged.
 function transformStreamDefaultControllerPerformTransform(controller, chunk) {
   const controllerState = controller[kState];
   const transformAlgorithm = controllerState.transformAlgorithm;
@@ -612,10 +612,9 @@ function transformStreamDefaultControllerPerformTransform(controller, chunk) {
   }
   if (result === null ||
       (typeof result !== 'object' && typeof result !== 'function')) {
-    result = kResolvedPromise;
-  } else {
-    result = PromiseResolve(result);
+    return kResolvedPromise;
   }
+  result = PromiseResolve(result);
   controllerState.performTransformRejected ??= (error) => {
     transformStreamError(controller[kState].stream, error);
     throw error;