diff --git a/js/loader/ModuleLoaderBase.cpp b/js/loader/ModuleLoaderBase.cpp
index 53717944d23b..cc5284702979 100644
--- a/js/loader/ModuleLoaderBase.cpp
+++ b/js/loader/ModuleLoaderBase.cpp
@@ -1385,6 +1385,42 @@ void ModuleLoaderBase::RegisterImportMap(UniquePtr<ImportMap> aImportMap) {
   mImportMap = std::move(aImportMap);
 }
 
+void ModuleLoaderBase::CopyModulesTo(ModuleLoaderBase* aDest) {
+  MOZ_ASSERT(aDest->mFetchingModules.IsEmpty());
+  MOZ_ASSERT(aDest->mFetchedModules.IsEmpty());
+  MOZ_ASSERT(mFetchingModules.IsEmpty());
+
+  for (const auto& entry : mFetchedModules) {
+    RefPtr<ModuleScript> moduleScript = entry.GetData();
+    if (!moduleScript) {
+      continue;
+    }
+    aDest->mFetchedModules.InsertOrUpdate(entry.GetKey(), moduleScript);
+  }
+}
+
+void ModuleLoaderBase::MoveModulesTo(ModuleLoaderBase* aDest) {
+  MOZ_ASSERT(mFetchingModules.IsEmpty());
+  MOZ_ASSERT(aDest->mFetchingModules.IsEmpty());
+
+  for (const auto& entry : mFetchedModules) {
+    RefPtr<ModuleScript> moduleScript = entry.GetData();
+    if (!moduleScript) {
+      continue;
+    }
+
+#ifdef DEBUG
+    if (auto existingEntry = aDest->mFetchedModules.Lookup(entry.GetKey())) {
+      MOZ_ASSERT(moduleScript == existingEntry.Data());
+    }
+#endif
+
+    aDest->mFetchedModules.InsertOrUpdate(entry.GetKey(), moduleScript);
+  }
+
+  mFetchedModules.Clear();
+}
+
 #undef LOG
 #undef LOG_ENABLED
 
diff --git a/js/loader/ModuleLoaderBase.h b/js/loader/ModuleLoaderBase.h
index 1b66fed3a348..86879db84bb9 100644
--- a/js/loader/ModuleLoaderBase.h
+++ b/js/loader/ModuleLoaderBase.h
@@ -351,6 +351,21 @@ class ModuleLoaderBase : public nsISupports {
 
   void ResetOverride();
 
+  // Copy fetched modules to `aDest`.
+  // `this` shouldn't have any fetching.
+  // `aDest` shouldn't have any fetching or fetched modules.
+  //
+  // This is used when starting sync module load, to replicate the module cache
+  // in the sync module loader pointed by `aDest`.
+  void CopyModulesTo(ModuleLoaderBase* aDest);
+
+  // Move all fetched modules to `aDest`.
+  // Both `this` and `aDest` shouldn't have any fetching.
+  //
+  // This is used when finishing sync module load, to reflect the loaded modules
+  // to the async module loader pointed by `aDest`.
+  void MoveModulesTo(ModuleLoaderBase* aDest);
+
   // Internal methods.
 
  private: