diff --git a/dom/streams/StreamUtils.h b/dom/streams/StreamUtils.h
index ddfff89c1c97..08a6fad13ba3 100644
--- a/dom/streams/StreamUtils.h
+++ b/dom/streams/StreamUtils.h
@@ -6,6 +6,9 @@
 #define mozilla_dom_StreamUtils_h
 
 #include "mozilla/ErrorResult.h"
+#include "mozilla/dom/Promise.h"
+
+class nsIGlobalObject;
 
 namespace mozilla::dom {
 
@@ -14,6 +17,37 @@ struct QueuingStrategy;
 double ExtractHighWaterMark(const QueuingStrategy& aStrategy,
                             double aDefaultHWM, ErrorResult& aRv);
 
+// Promisification algorithm, shared among:
+// Step 2 and 3 of https://streams.spec.whatwg.org/#readablestream-set-up
+// Step 2 and 3 of
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Step 2 and 3 of https://streams.spec.whatwg.org/#writablestream-set-up
+// Step 5 and 6 of https://streams.spec.whatwg.org/#transformstream-set-up
+template <typename T>
+MOZ_CAN_RUN_SCRIPT static already_AddRefed<Promise> PromisifyAlgorithm(
+    nsIGlobalObject* aGlobal, T aFunc, mozilla::ErrorResult& aRv) {
+  // Step 1. Let result be the result of running (algorithm). If this throws an
+  // exception e, return a promise rejected with e.
+  RefPtr<Promise> result;
+  if constexpr (!std::is_same<decltype(aFunc(aRv)), void>::value) {
+    result = aFunc(aRv);
+  } else {
+    aFunc(aRv);
+  }
+
+  if (aRv.Failed()) {
+    return Promise::CreateRejectedWithErrorResult(aGlobal, aRv);
+  }
+
+  // Step 2. If result is a Promise, then return result.
+  if (result) {
+    return result.forget();
+  }
+
+  // Step 3. Return a promise resolved with undefined.
+  return Promise::CreateResolvedWithUndefined(aGlobal, aRv);
+}
+
 }  // namespace mozilla::dom
 
 #endif  // mozilla_dom_StreamUtils_h
diff --git a/dom/streams/TransformerCallbackHelpers.cpp b/dom/streams/TransformerCallbackHelpers.cpp
index 445d9704730e..606c24adda7c 100644
--- a/dom/streams/TransformerCallbackHelpers.cpp
+++ b/dom/streams/TransformerCallbackHelpers.cpp
@@ -6,6 +6,7 @@
 
 #include "TransformerCallbackHelpers.h"
 
+#include "StreamUtils.h"
 #include "mozilla/dom/Promise.h"
 #include "mozilla/dom/TransformStreamDefaultController.h"
 
@@ -84,30 +85,11 @@ already_AddRefed<Promise> TransformerAlgorithms::FlushCallback(
              CallbackObject::eRethrowExceptions);
 }
 
-// https://streams.spec.whatwg.org/#transformstream-set-up
-// Step 5 and 6.
-template <typename T>
-MOZ_CAN_RUN_SCRIPT static already_AddRefed<Promise> Promisify(
-    nsIGlobalObject* aGlobal, T aFunc, mozilla::ErrorResult& aRv) {
-  // Step 1. Let result be the result of running (algorithm). If this throws an
-  // exception e, return a promise rejected with e.
-  aFunc(aRv);
-  if (aRv.Failed()) {
-    return Promise::CreateRejectedWithErrorResult(aGlobal, aRv);
-  }
-
-  // Step 2. If result is a Promise, then return result.
-  // (This supports no return value since currently no subclass needs one)
-
-  // Step 3. Return a promise resolved with undefined.
-  return Promise::CreateResolvedWithUndefined(aGlobal, aRv);
-}
-
 already_AddRefed<Promise> TransformerAlgorithmsWrapper::TransformCallback(
     JSContext*, JS::Handle<JS::Value> aChunk,
     TransformStreamDefaultController& aController, ErrorResult& aRv) {
   nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
-  return Promisify(
+  return PromisifyAlgorithm(
       global,
       [this, &aChunk, &aController](ErrorResult& aRv)
           MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION {
@@ -120,7 +102,7 @@ already_AddRefed<Promise> TransformerAlgorithmsWrapper::FlushCallback(
     JSContext*, TransformStreamDefaultController& aController,
     ErrorResult& aRv) {
   nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
-  return Promisify(
+  return PromisifyAlgorithm(
       global,
       [this, &aController](ErrorResult& aRv) MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION {
         return FlushCallbackImpl(aController, aRv);