diff --git a/dom/base/nsPlainTextSerializer.h b/dom/base/nsPlainTextSerializer.h
index 650a8e3e73..58aeb42078 100644
--- a/dom/base/nsPlainTextSerializer.h
+++ b/dom/base/nsPlainTextSerializer.h
@@ -16,6 +16,7 @@
 #include "mozilla/Attributes.h"
 #include "nsCOMPtr.h"
 #include "nsIAtom.h"
+#include "nsCycleCollectionParticipant.h"
 #include "nsIContentSerializer.h"
 #include "nsIDocumentEncoder.h"
 #include "nsILineBreaker.h"
diff --git a/dom/script/ModuleLoadRequest.cpp b/dom/script/ModuleLoadRequest.cpp
new file mode 100644
index 0000000000..e72edca2e8
--- /dev/null
+++ b/dom/script/ModuleLoadRequest.cpp
@@ -0,0 +1,102 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ModuleLoadRequest.h"
+#include "ModuleScript.h"
+#include "ScriptLoader.h"
+
+namespace mozilla {
+namespace dom {
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(ModuleLoadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ScriptLoadRequest)
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(ModuleLoadRequest, ScriptLoadRequest,
+                                   mBaseURL,
+                                   mLoader,
+                                   mParent,
+                                   mModuleScript,
+                                   mImports)
+
+NS_IMPL_ADDREF_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
+NS_IMPL_RELEASE_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
+
+ModuleLoadRequest::ModuleLoadRequest(nsIScriptElement* aElement,
+                                     uint32_t aVersion,
+                                     CORSMode aCORSMode,
+                                     const SRIMetadata &aIntegrity,
+                                     ScriptLoader* aLoader)
+  : ScriptLoadRequest(ScriptKind::Module,
+                      aElement,
+                      aVersion,
+                      aCORSMode,
+                      aIntegrity),
+    mIsTopLevel(true),
+    mLoader(aLoader)
+{}
+
+void ModuleLoadRequest::Cancel()
+{
+  ScriptLoadRequest::Cancel();
+  mModuleScript = nullptr;
+  mProgress = ScriptLoadRequest::Progress::Ready;
+  for (size_t i = 0; i < mImports.Length(); i++) {
+    mImports[i]->Cancel();
+  }
+  mReady.RejectIfExists(NS_ERROR_FAILURE, __func__);
+}
+
+void
+ModuleLoadRequest::SetReady()
+{
+#ifdef DEBUG
+  for (size_t i = 0; i < mImports.Length(); i++) {
+    MOZ_ASSERT(mImports[i]->IsReadyToRun());
+  }
+#endif
+
+  ScriptLoadRequest::SetReady();
+  mReady.ResolveIfExists(true, __func__);
+}
+
+void
+ModuleLoadRequest::ModuleLoaded()
+{
+  // A module that was found to be marked as fetching in the module map has now
+  // been loaded.
+
+  mModuleScript = mLoader->GetFetchedModule(mURI);
+  mLoader->StartFetchingModuleDependencies(this);
+}
+
+void
+ModuleLoadRequest::DependenciesLoaded()
+{
+  // The module and all of its dependencies have been successfully fetched and
+  // compiled.
+
+  if (!mLoader->InstantiateModuleTree(this)) {
+    LoadFailed();
+    return;
+  }
+
+  SetReady();
+  mLoader->ProcessLoadedModuleTree(this);
+  mLoader = nullptr;
+  mParent = nullptr;
+}
+
+void
+ModuleLoadRequest::LoadFailed()
+{
+  Cancel();
+  mLoader->ProcessLoadedModuleTree(this);
+  mLoader = nullptr;
+  mParent = nullptr;
+}
+
+} // dom namespace
+} // mozilla namespace
\ No newline at end of file
diff --git a/dom/script/ModuleLoadRequest.h b/dom/script/ModuleLoadRequest.h
new file mode 100644
index 0000000000..0119fad385
--- /dev/null
+++ b/dom/script/ModuleLoadRequest.h
@@ -0,0 +1,81 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ModuleLoadRequest_h
+#define mozilla_dom_ModuleLoadRequest_h
+
+#include "mozilla/dom/ScriptLoader.h"
+#include "mozilla/MozPromise.h"
+
+namespace mozilla {
+namespace dom {
+
+class ModuleScript;
+class ScriptLoader;
+
+// A load request for a module, created for every top level module script and
+// every module import. Load request can share a ModuleScript if there are
+// multiple imports of the same module.
+
+class ModuleLoadRequest final : public ScriptLoadRequest
+{
+  ~ModuleLoadRequest() {}
+
+  ModuleLoadRequest(const ModuleLoadRequest& aOther) = delete;
+  ModuleLoadRequest(ModuleLoadRequest&& aOther) = delete;
+
+public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
+
+  ModuleLoadRequest(nsIScriptElement* aElement,
+                    uint32_t aVersion,
+                    CORSMode aCORSMode,
+                    const SRIMetadata& aIntegrity,
+                    ScriptLoader* aLoader);
+
+  bool IsTopLevel() const {
+    return mIsTopLevel;
+  }
+
+  void SetReady() override;
+  void Cancel() override;
+
+  void ModuleLoaded();
+  void DependenciesLoaded();
+  void LoadFailed();
+
+  // Is this a request for a top level module script or an import?
+  bool mIsTopLevel;
+
+  // The base URL used for resolving relative module imports.
+  nsCOMPtr<nsIURI> mBaseURL;
+
+  // Pointer to the script loader, used to trigger actions when the module load
+  // finishes.
+  RefPtr<ScriptLoader> mLoader;
+
+  // The importing module, or nullptr for top level module scripts.  Used to
+  // implement the ancestor list checked when fetching module dependencies.
+  RefPtr<ModuleLoadRequest> mParent;
+
+  // Set to a module script object after a successful load or nullptr on
+  // failure.
+  RefPtr<ModuleScript> mModuleScript;
+
+  // A promise that is completed on successful load of this module and all of
+  // its dependencies, indicating that the module is ready for instantiation and
+  // evaluation.
+  MozPromiseHolder<GenericPromise> mReady;
+
+  // Array of imported modules.
+  nsTArray<RefPtr<ModuleLoadRequest>> mImports;
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_ModuleLoadRequest_h
\ No newline at end of file
diff --git a/dom/script/ModuleScript.cpp b/dom/script/ModuleScript.cpp
new file mode 100644
index 0000000000..34ef4dec46
--- /dev/null
+++ b/dom/script/ModuleScript.cpp
@@ -0,0 +1,100 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A class that handles loading and evaluation of <script> elements.
+ */
+
+#include "ModuleScript.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "ScriptLoader.h"
+
+namespace mozilla {
+namespace dom {
+
+// A single module script.  May be used to satisfy multiple load requests.
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ModuleScript)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(ModuleScript)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ModuleScript)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mLoader)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mBaseURL)
+  tmp->UnlinkModuleRecord();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(ModuleScript)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLoader)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(ModuleScript)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mModuleRecord)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mException)
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ModuleScript)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ModuleScript)
+
+ModuleScript::ModuleScript(ScriptLoader *aLoader, nsIURI* aBaseURL,
+                           JS::Handle<JSObject*> aModuleRecord)
+ : mLoader(aLoader),
+   mBaseURL(aBaseURL),
+   mModuleRecord(aModuleRecord),
+   mInstantiationState(Uninstantiated)
+{
+  MOZ_ASSERT(mLoader);
+  MOZ_ASSERT(mBaseURL);
+  MOZ_ASSERT(mModuleRecord);
+  MOZ_ASSERT(mException.isUndefined());
+
+  // Make module's host defined field point to this module script object.
+  // This is cleared in the UnlinkModuleRecord().
+  JS::SetModuleHostDefinedField(mModuleRecord, JS::PrivateValue(this));
+  HoldJSObjects(this);
+}
+
+void
+ModuleScript::UnlinkModuleRecord()
+{
+  // Remove module's back reference to this object request if present.
+  if (mModuleRecord) {
+    MOZ_ASSERT(JS::GetModuleHostDefinedField(mModuleRecord).toPrivate() ==
+               this);
+    JS::SetModuleHostDefinedField(mModuleRecord, JS::UndefinedValue());
+  }
+  mModuleRecord = nullptr;
+  mException.setUndefined();
+}
+
+ModuleScript::~ModuleScript()
+{
+  if (mModuleRecord) {
+    // The object may be destroyed without being unlinked first.
+    UnlinkModuleRecord();
+  }
+  DropJSObjects(this);
+}
+
+void
+ModuleScript::SetInstantiationResult(JS::Handle<JS::Value> aMaybeException)
+{
+  MOZ_ASSERT(mInstantiationState == Uninstantiated);
+  MOZ_ASSERT(mModuleRecord);
+  MOZ_ASSERT(mException.isUndefined());
+
+  if (aMaybeException.isUndefined()) {
+    mInstantiationState = Instantiated;
+  } else {
+    mModuleRecord = nullptr;
+    mException = aMaybeException;
+    mInstantiationState = Errored;
+  }
+}
+
+} // dom namespace
+} // mozilla namespace
diff --git a/dom/script/ModuleScript.h b/dom/script/ModuleScript.h
new file mode 100644
index 0000000000..dd0d07e847
--- /dev/null
+++ b/dom/script/ModuleScript.h
@@ -0,0 +1,67 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ModuleScript_h
+#define mozilla_dom_ModuleScript_h
+
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "jsapi.h"
+
+class nsIURI;
+
+namespace mozilla {
+namespace dom {
+
+class ScriptLoader;
+
+class ModuleScript final : public nsISupports
+{
+  enum InstantiationState {
+    Uninstantiated,
+    Instantiated,
+    Errored
+  };
+
+  RefPtr<ScriptLoader> mLoader;
+  nsCOMPtr<nsIURI> mBaseURL;
+  JS::Heap<JSObject*> mModuleRecord;
+  JS::Heap<JS::Value> mException;
+  InstantiationState mInstantiationState;
+
+  ~ModuleScript();
+
+public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ModuleScript)
+
+  ModuleScript(ScriptLoader* aLoader,
+               nsIURI* aBaseURL,
+               JS::Handle<JSObject*> aModuleRecord);
+
+  ScriptLoader* Loader() const { return mLoader; }
+  JSObject* ModuleRecord() const { return mModuleRecord; }
+  JS::Value Exception() const { return mException; }
+  nsIURI* BaseURL() const { return mBaseURL; }
+
+  void SetInstantiationResult(JS::Handle<JS::Value> aMaybeException);
+  bool IsUninstantiated() const {
+    return mInstantiationState == Uninstantiated;
+  }
+  bool IsInstantiated() const {
+    return mInstantiationState == Instantiated;
+  }
+  bool InstantiationFailed() const {
+    return mInstantiationState == Errored;
+  }
+
+  void UnlinkModuleRecord();
+};
+
+} // dom namespace
+} // mozilla namespace
+
+#endif // mozilla_dom_ModuleScript_h
\ No newline at end of file
diff --git a/dom/script/ScriptLoadHandler.cpp b/dom/script/ScriptLoadHandler.cpp
new file mode 100644
index 0000000000..80fb70f6ae
--- /dev/null
+++ b/dom/script/ScriptLoadHandler.cpp
@@ -0,0 +1,216 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A class that handles loading and evaluation of <script> elements.
+ */
+
+#include "ScriptLoadHandler.h"
+#include "ScriptLoader.h"
+#include "nsContentUtils.h"
+
+#include "mozilla/dom/EncodingUtils.h"
+
+namespace mozilla {
+namespace dom {
+
+ScriptLoadHandler::ScriptLoadHandler(ScriptLoader *aScriptLoader,
+                                     ScriptLoadRequest *aRequest,
+                                     mozilla::dom::SRICheckDataVerifier *aSRIDataVerifier)
+  : mScriptLoader(aScriptLoader),
+    mRequest(aRequest),
+    mSRIDataVerifier(aSRIDataVerifier),
+    mSRIStatus(NS_OK),
+    mDecoder(),
+    mBuffer()
+{}
+
+ScriptLoadHandler::~ScriptLoadHandler()
+{}
+
+NS_IMPL_ISUPPORTS(ScriptLoadHandler, nsIIncrementalStreamLoaderObserver)
+
+NS_IMETHODIMP
+ScriptLoadHandler::OnIncrementalData(nsIIncrementalStreamLoader* aLoader,
+                                     nsISupports* aContext,
+                                     uint32_t aDataLength,
+                                     const uint8_t* aData,
+                                     uint32_t *aConsumedLength)
+{
+  if (mRequest->IsCanceled()) {
+    // If request cancelled, ignore any incoming data.
+    *aConsumedLength = aDataLength;
+    return NS_OK;
+  }
+
+  if (!EnsureDecoder(aLoader, aData, aDataLength,
+                     /* aEndOfStream = */ false)) {
+    return NS_OK;
+  }
+
+  // Below we will/shall consume entire data chunk.
+  *aConsumedLength = aDataLength;
+
+  // Decoder has already been initialized. -- trying to decode all loaded bytes.
+  nsresult rv = TryDecodeRawData(aData, aDataLength,
+                                 /* aEndOfStream = */ false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // If SRI is required for this load, appending new bytes to the hash.
+  if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
+    mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
+  }
+
+  return rv;
+}
+
+nsresult
+ScriptLoadHandler::TryDecodeRawData(const uint8_t* aData,
+                                    uint32_t aDataLength,
+                                    bool aEndOfStream)
+{
+  int32_t srcLen = aDataLength;
+  const char* src = reinterpret_cast<const char *>(aData);
+  int32_t dstLen;
+  nsresult rv =
+    mDecoder->GetMaxLength(src, srcLen, &dstLen);
+
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  uint32_t haveRead = mBuffer.length();
+
+  CheckedInt<uint32_t> capacity = haveRead;
+  capacity += dstLen;
+
+  if (!capacity.isValid() || !mBuffer.reserve(capacity.value())) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  rv = mDecoder->Convert(src,
+                         &srcLen,
+                         mBuffer.begin() + haveRead,
+                         &dstLen);
+
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  haveRead += dstLen;
+  MOZ_ASSERT(haveRead <= capacity.value(), "mDecoder produced more data than expected");
+  MOZ_ALWAYS_TRUE(mBuffer.resizeUninitialized(haveRead));
+
+  return NS_OK;
+}
+
+bool
+ScriptLoadHandler::EnsureDecoder(nsIIncrementalStreamLoader *aLoader,
+                                 const uint8_t* aData,
+                                 uint32_t aDataLength,
+                                 bool aEndOfStream)
+{
+  // Check if decoder has already been created.
+  if (mDecoder) {
+    return true;
+  }
+
+  nsAutoCString charset;
+
+  // JavaScript modules are always UTF-8.
+  if (mRequest->IsModuleRequest()) {
+    charset = "UTF-8";
+    mDecoder = EncodingUtils::DecoderForEncoding(charset);
+    return true;
+  }
+
+  // Determine if BOM check should be done.  This occurs either
+  // if end-of-stream has been reached, or at least 3 bytes have
+  // been read from input.
+  if (!aEndOfStream && (aDataLength < 3)) {
+    return false;
+  }
+
+  // Do BOM detection.
+  if (nsContentUtils::CheckForBOM(aData, aDataLength, charset)) {
+    mDecoder = EncodingUtils::DecoderForEncoding(charset);
+    return true;
+  }
+
+  // BOM detection failed, check content stream for charset.
+  nsCOMPtr<nsIRequest> req;
+  nsresult rv = aLoader->GetRequest(getter_AddRefs(req));
+  NS_ASSERTION(req, "StreamLoader's request went away prematurely");
+  NS_ENSURE_SUCCESS(rv, false);
+
+  nsCOMPtr<nsIChannel> channel = do_QueryInterface(req);
+
+  if (channel &&
+      NS_SUCCEEDED(channel->GetContentCharset(charset)) &&
+      EncodingUtils::FindEncodingForLabel(charset, charset)) {
+    mDecoder = EncodingUtils::DecoderForEncoding(charset);
+    return true;
+  }
+
+  // Check the hint charset from the script element or preload
+  // request.
+  nsAutoString hintCharset;
+  if (!mRequest->IsPreload()) {
+    mRequest->mElement->GetScriptCharset(hintCharset);
+  } else {
+    nsTArray<ScriptLoader::PreloadInfo>::index_type i =
+      mScriptLoader->mPreloads.IndexOf(mRequest, 0,
+            ScriptLoader::PreloadRequestComparator());
+
+    NS_ASSERTION(i != mScriptLoader->mPreloads.NoIndex,
+                 "Incorrect preload bookkeeping");
+    hintCharset = mScriptLoader->mPreloads[i].mCharset;
+  }
+
+  if (EncodingUtils::FindEncodingForLabel(hintCharset, charset)) {
+    mDecoder = EncodingUtils::DecoderForEncoding(charset);
+    return true;
+  }
+
+  // Get the charset from the charset of the document.
+  if (mScriptLoader->mDocument) {
+    charset = mScriptLoader->mDocument->GetDocumentCharacterSet();
+    mDecoder = EncodingUtils::DecoderForEncoding(charset);
+    return true;
+  }
+
+  // Curiously, there are various callers that don't pass aDocument. The
+  // fallback in the old code was ISO-8859-1, which behaved like
+  // windows-1252. Saying windows-1252 for clarity and for compliance
+  // with the Encoding Standard.
+  charset = "windows-1252";
+  mDecoder = EncodingUtils::DecoderForEncoding(charset);
+  return true;
+}
+
+NS_IMETHODIMP
+ScriptLoadHandler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
+                                    nsISupports* aContext,
+                                    nsresult aStatus,
+                                    uint32_t aDataLength,
+                                    const uint8_t* aData)
+{
+  if (!mRequest->IsCanceled()) {
+    DebugOnly<bool> encoderSet =
+      EnsureDecoder(aLoader, aData, aDataLength, /* aEndOfStream = */ true);
+    MOZ_ASSERT(encoderSet);
+    DebugOnly<nsresult> rv = TryDecodeRawData(aData, aDataLength,
+                                              /* aEndOfStream = */ true);
+
+    // If SRI is required for this load, appending new bytes to the hash.
+    if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
+      mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
+    }
+  }
+
+  // we have to mediate and use mRequest.
+  return mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
+                                         mBuffer, mSRIDataVerifier);
+}
+
+} // dom namespace
+} // mozilla namespace
\ No newline at end of file
diff --git a/dom/script/ScriptLoadHandler.h b/dom/script/ScriptLoadHandler.h
new file mode 100644
index 0000000000..b70f873974
--- /dev/null
+++ b/dom/script/ScriptLoadHandler.h
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A class that handles loading and evaluation of <script> elements.
+ */
+
+#ifndef mozilla_dom_ScriptLoadHandler_h
+#define mozilla_dom_ScriptLoadHandler_h
+
+#include "nsIIncrementalStreamLoader.h"
+#include "nsIUnicodeDecoder.h"
+#include "nsAutoPtr.h"
+#include "mozilla/Vector.h"
+
+namespace mozilla {
+namespace dom {
+
+class ScriptLoadRequest;
+class ScriptLoader;
+class SRICheckDataVerifier;
+
+class ScriptLoadHandler final : public nsIIncrementalStreamLoaderObserver
+{
+public:
+  explicit ScriptLoadHandler(ScriptLoader* aScriptLoader,
+                             ScriptLoadRequest* aRequest,
+                             SRICheckDataVerifier* aSRIDataVerifier);
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIINCREMENTALSTREAMLOADEROBSERVER
+
+private:
+  virtual ~ScriptLoadHandler();
+
+  /*
+   * Try to decode some raw data.
+   */
+  nsresult TryDecodeRawData(const uint8_t* aData, uint32_t aDataLength,
+                            bool aEndOfStream);
+
+  /*
+   * Discover the charset by looking at the stream data, the script
+   * tag, and other indicators.  Returns true if charset has been
+   * discovered.
+   */
+  bool EnsureDecoder(nsIIncrementalStreamLoader *aLoader,
+                     const uint8_t* aData, uint32_t aDataLength,
+                     bool aEndOfStream);
+
+  // ScriptLoader which will handle the parsed script.
+  RefPtr<ScriptLoader>        mScriptLoader;
+
+  // The ScriptLoadRequest for this load.
+  RefPtr<ScriptLoadRequest>   mRequest;
+
+  // SRI data verifier.
+  nsAutoPtr<SRICheckDataVerifier> mSRIDataVerifier;
+
+  // Status of SRI data operations.
+  nsresult                      mSRIStatus;
+
+  // Unicode decoder for charset.
+  nsCOMPtr<nsIUnicodeDecoder>   mDecoder;
+
+  // Accumulated decoded char buffer.
+  mozilla::Vector<char16_t>     mBuffer;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif //mozilla_dom_ScriptLoader_h
diff --git a/dom/script/ScriptLoader.cpp b/dom/script/ScriptLoader.cpp
index b5c8053e88..adc046b7c4 100644
--- a/dom/script/ScriptLoader.cpp
+++ b/dom/script/ScriptLoader.cpp
@@ -4,11 +4,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-/*
- * A class that handles loading and evaluation of <script> elements.
- */
-
 #include "ScriptLoader.h"
+#include "ScriptLoadHandler.h"
+#include "ModuleLoadRequest.h"
+#include "ModuleScript.h"
 
 #include "prsystem.h"
 #include "jsapi.h"
@@ -134,96 +133,6 @@ ScriptLoadRequest::MaybeCancelOffThreadScript()
   mOffThreadToken = nullptr;
 }
 
-//////////////////////////////////////////////////////////////
-// ModuleLoadRequest
-//////////////////////////////////////////////////////////////
-
-// A load request for a module, created for every top level module script and
-// every module import. Load request can share a ModuleScript if there are
-// multiple imports of the same module.
-
-class ModuleLoadRequest final : public ScriptLoadRequest
-{
-  ~ModuleLoadRequest() {}
-
-  ModuleLoadRequest(const ModuleLoadRequest& aOther) = delete;
-  ModuleLoadRequest(ModuleLoadRequest&& aOther) = delete;
-
-public:
-  NS_DECL_ISUPPORTS_INHERITED
-  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
-
-  ModuleLoadRequest(nsIScriptElement* aElement,
-                    uint32_t aVersion,
-                    CORSMode aCORSMode,
-                    const SRIMetadata& aIntegrity,
-                    ScriptLoader* aLoader);
-
-  bool IsTopLevel() const {
-    return mIsTopLevel;
-  }
-
-  void SetReady() override;
-  void Cancel() override;
-
-  void ModuleLoaded();
-  void DependenciesLoaded();
-  void LoadFailed();
-
-  // Is this a request for a top level module script or an import?
-  bool mIsTopLevel;
-
-  // The base URL used for resolving relative module imports.
-  nsCOMPtr<nsIURI> mBaseURL;
-
-  // Pointer to the script loader, used to trigger actions when the module load
-  // finishes.
-  RefPtr<ScriptLoader> mLoader;
-
-  // The importing module, or nullptr for top level module scripts.  Used to
-  // implement the ancestor list checked when fetching module dependencies.
-  RefPtr<ModuleLoadRequest> mParent;
-
-  // Set to a module script object after a successful load or nullptr on
-  // failure.
-  RefPtr<ModuleScript> mModuleScript;
-
-  // A promise that is completed on successful load of this module and all of
-  // its dependencies, indicating that the module is ready for instantiation and
-  // evaluation.
-  MozPromiseHolder<GenericPromise> mReady;
-
-  // Array of imported modules.
-  nsTArray<RefPtr<ModuleLoadRequest>> mImports;
-};
-
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(ModuleLoadRequest)
-NS_INTERFACE_MAP_END_INHERITING(ScriptLoadRequest)
-
-NS_IMPL_CYCLE_COLLECTION_INHERITED(ModuleLoadRequest, ScriptLoadRequest,
-                                   mBaseURL,
-                                   mLoader,
-                                   mParent,
-                                   mModuleScript,
-                                   mImports)
-
-NS_IMPL_ADDREF_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
-NS_IMPL_RELEASE_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
-
-ModuleLoadRequest::ModuleLoadRequest(nsIScriptElement* aElement,
-                                     uint32_t aVersion,
-                                     CORSMode aCORSMode,
-                                     const SRIMetadata &aIntegrity,
-                                     ScriptLoader* aLoader)
-  : ScriptLoadRequest(ScriptKind::Module,
-                      aElement,
-                      aVersion,
-                      aCORSMode,
-                      aIntegrity),
-    mIsTopLevel(true),
-    mLoader(aLoader)
-{}
-
 inline ModuleLoadRequest*
 ScriptLoadRequest::AsModuleRequest()
 {
@@ -231,194 +140,6 @@ ScriptLoadRequest::AsModuleRequest()
   return static_cast<ModuleLoadRequest*>(this);
 }
 
-void ModuleLoadRequest::Cancel()
-{
-  ScriptLoadRequest::Cancel();
-  mModuleScript = nullptr;
-  mProgress = ScriptLoadRequest::Progress::Ready;
-  for (size_t i = 0; i < mImports.Length(); i++) {
-    mImports[i]->Cancel();
-  }
-  mReady.RejectIfExists(NS_ERROR_FAILURE, __func__);
-}
-
-void
-ModuleLoadRequest::SetReady()
-{
-#ifdef DEBUG
-  for (size_t i = 0; i < mImports.Length(); i++) {
-    MOZ_ASSERT(mImports[i]->IsReadyToRun());
-  }
-#endif
-
-  ScriptLoadRequest::SetReady();
-  mReady.ResolveIfExists(true, __func__);
-}
-
-void
-ModuleLoadRequest::ModuleLoaded()
-{
-  // A module that was found to be marked as fetching in the module map has now
-  // been loaded.
-
-  mModuleScript = mLoader->GetFetchedModule(mURI);
-  mLoader->StartFetchingModuleDependencies(this);
-}
-
-void
-ModuleLoadRequest::DependenciesLoaded()
-{
-  // The module and all of its dependencies have been successfully fetched and
-  // compiled.
-
-  if (!mLoader->InstantiateModuleTree(this)) {
-    LoadFailed();
-    return;
-  }
-
-  SetReady();
-  mLoader->ProcessLoadedModuleTree(this);
-  mLoader = nullptr;
-  mParent = nullptr;
-}
-
-void
-ModuleLoadRequest::LoadFailed()
-{
-  Cancel();
-  mLoader->ProcessLoadedModuleTree(this);
-  mLoader = nullptr;
-  mParent = nullptr;
-}
-
-//////////////////////////////////////////////////////////////
-// ModuleScript
-//////////////////////////////////////////////////////////////
-
-// A single module script.  May be used to satisfy multiple load requests.
-
-class ModuleScript final : public nsISupports
-{
-  enum InstantiationState {
-    Uninstantiated,
-    Instantiated,
-    Errored
-  };
-
-  RefPtr<ScriptLoader> mLoader;
-  nsCOMPtr<nsIURI> mBaseURL;
-  JS::Heap<JSObject*> mModuleRecord;
-  JS::Heap<JS::Value> mException;
-  InstantiationState mInstantiationState;
-
-  ~ModuleScript();
-
-public:
-  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ModuleScript)
-
-  ModuleScript(ScriptLoader* aLoader,
-               nsIURI* aBaseURL,
-               JS::Handle<JSObject*> aModuleRecord);
-
-  ScriptLoader* Loader() const { return mLoader; }
-  JSObject* ModuleRecord() const { return mModuleRecord; }
-  JS::Value Exception() const { return mException; }
-  nsIURI* BaseURL() const { return mBaseURL; }
-
-  void SetInstantiationResult(JS::Handle<JS::Value> aMaybeException);
-  bool IsUninstantiated() const {
-    return mInstantiationState == Uninstantiated;
-  }
-  bool IsInstantiated() const {
-    return mInstantiationState == Instantiated;
-  }
-  bool InstantiationFailed() const {
-    return mInstantiationState == Errored;
-  }
-
-  void UnlinkModuleRecord();
-};
-
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ModuleScript)
-NS_INTERFACE_MAP_END
-
-NS_IMPL_CYCLE_COLLECTION_CLASS(ModuleScript)
-
-NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ModuleScript)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mLoader)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mBaseURL)
-  tmp->UnlinkModuleRecord();
-NS_IMPL_CYCLE_COLLECTION_UNLINK_END
-
-NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(ModuleScript)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLoader)
-NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
-
-NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(ModuleScript)
-  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mModuleRecord)
-  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mException)
-NS_IMPL_CYCLE_COLLECTION_TRACE_END
-
-NS_IMPL_CYCLE_COLLECTING_ADDREF(ModuleScript)
-NS_IMPL_CYCLE_COLLECTING_RELEASE(ModuleScript)
-
-ModuleScript::ModuleScript(ScriptLoader *aLoader, nsIURI* aBaseURL,
-                           JS::Handle<JSObject*> aModuleRecord)
- : mLoader(aLoader),
-   mBaseURL(aBaseURL),
-   mModuleRecord(aModuleRecord),
-   mInstantiationState(Uninstantiated)
-{
-  MOZ_ASSERT(mLoader);
-  MOZ_ASSERT(mBaseURL);
-  MOZ_ASSERT(mModuleRecord);
-  MOZ_ASSERT(mException.isUndefined());
-
-  // Make module's host defined field point to this module script object.
-  // This is cleared in the UnlinkModuleRecord().
-  JS::SetModuleHostDefinedField(mModuleRecord, JS::PrivateValue(this));
-  HoldJSObjects(this);
-}
-
-void
-ModuleScript::UnlinkModuleRecord()
-{
-  // Remove module's back reference to this object request if present.
-  if (mModuleRecord) {
-    MOZ_ASSERT(JS::GetModuleHostDefinedField(mModuleRecord).toPrivate() ==
-               this);
-    JS::SetModuleHostDefinedField(mModuleRecord, JS::UndefinedValue());
-  }
-  mModuleRecord = nullptr;
-  mException.setUndefined();
-}
-
-ModuleScript::~ModuleScript()
-{
-  if (mModuleRecord) {
-    // The object may be destroyed without being unlinked first.
-    UnlinkModuleRecord();
-  }
-  DropJSObjects(this);
-}
-
-void
-ModuleScript::SetInstantiationResult(JS::Handle<JS::Value> aMaybeException)
-{
-  MOZ_ASSERT(mInstantiationState == Uninstantiated);
-  MOZ_ASSERT(mModuleRecord);
-  MOZ_ASSERT(mException.isUndefined());
-
-  if (aMaybeException.isUndefined()) {
-    mInstantiationState = Instantiated;
-  } else {
-    mModuleRecord = nullptr;
-    mException = aMaybeException;
-    mInstantiationState = Errored;
-  }
-}
-
 //////////////////////////////////////////////////////////////
 
 // ScriptLoadRequestList
@@ -2860,204 +2581,5 @@ ScriptLoader::MaybeRemovedDeferRequests()
   return false;
 }
 
-//////////////////////////////////////////////////////////////
-// ScriptLoadHandler
-//////////////////////////////////////////////////////////////
-
-ScriptLoadHandler::ScriptLoadHandler(ScriptLoader *aScriptLoader,
-                                     ScriptLoadRequest *aRequest,
-                                     mozilla::dom::SRICheckDataVerifier *aSRIDataVerifier)
-  : mScriptLoader(aScriptLoader),
-    mRequest(aRequest),
-    mSRIDataVerifier(aSRIDataVerifier),
-    mSRIStatus(NS_OK),
-    mDecoder(),
-    mBuffer()
-{}
-
-ScriptLoadHandler::~ScriptLoadHandler()
-{}
-
-NS_IMPL_ISUPPORTS(ScriptLoadHandler, nsIIncrementalStreamLoaderObserver)
-
-NS_IMETHODIMP
-ScriptLoadHandler::OnIncrementalData(nsIIncrementalStreamLoader* aLoader,
-                                     nsISupports* aContext,
-                                     uint32_t aDataLength,
-                                     const uint8_t* aData,
-                                     uint32_t *aConsumedLength)
-{
-  if (mRequest->IsCanceled()) {
-    // If request cancelled, ignore any incoming data.
-    *aConsumedLength = aDataLength;
-    return NS_OK;
-  }
-
-  if (!EnsureDecoder(aLoader, aData, aDataLength,
-                     /* aEndOfStream = */ false)) {
-    return NS_OK;
-  }
-
-  // Below we will/shall consume entire data chunk.
-  *aConsumedLength = aDataLength;
-
-  // Decoder has already been initialized. -- trying to decode all loaded bytes.
-  nsresult rv = TryDecodeRawData(aData, aDataLength,
-                                 /* aEndOfStream = */ false);
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  // If SRI is required for this load, appending new bytes to the hash.
-  if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
-    mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
-  }
-
-  return rv;
-}
-
-nsresult
-ScriptLoadHandler::TryDecodeRawData(const uint8_t* aData,
-                                    uint32_t aDataLength,
-                                    bool aEndOfStream)
-{
-  int32_t srcLen = aDataLength;
-  const char* src = reinterpret_cast<const char *>(aData);
-  int32_t dstLen;
-  nsresult rv =
-    mDecoder->GetMaxLength(src, srcLen, &dstLen);
-
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  uint32_t haveRead = mBuffer.length();
-
-  CheckedInt<uint32_t> capacity = haveRead;
-  capacity += dstLen;
-
-  if (!capacity.isValid() || !mBuffer.reserve(capacity.value())) {
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
-
-  rv = mDecoder->Convert(src,
-                         &srcLen,
-                         mBuffer.begin() + haveRead,
-                         &dstLen);
-
-  NS_ENSURE_SUCCESS(rv, rv);
-
-  haveRead += dstLen;
-  MOZ_ASSERT(haveRead <= capacity.value(), "mDecoder produced more data than expected");
-  MOZ_ALWAYS_TRUE(mBuffer.resizeUninitialized(haveRead));
-
-  return NS_OK;
-}
-
-bool
-ScriptLoadHandler::EnsureDecoder(nsIIncrementalStreamLoader *aLoader,
-                                 const uint8_t* aData,
-                                 uint32_t aDataLength,
-                                 bool aEndOfStream)
-{
-  // Check if decoder has already been created.
-  if (mDecoder) {
-    return true;
-  }
-
-  nsAutoCString charset;
-
-  // JavaScript modules are always UTF-8.
-  if (mRequest->IsModuleRequest()) {
-    charset = "UTF-8";
-    mDecoder = EncodingUtils::DecoderForEncoding(charset);
-    return true;
-  }
-
-  // Determine if BOM check should be done.  This occurs either
-  // if end-of-stream has been reached, or at least 3 bytes have
-  // been read from input.
-  if (!aEndOfStream && (aDataLength < 3)) {
-    return false;
-  }
-
-  // Do BOM detection.
-  if (nsContentUtils::CheckForBOM(aData, aDataLength, charset)) {
-    mDecoder = EncodingUtils::DecoderForEncoding(charset);
-    return true;
-  }
-
-  // BOM detection failed, check content stream for charset.
-  nsCOMPtr<nsIRequest> req;
-  nsresult rv = aLoader->GetRequest(getter_AddRefs(req));
-  NS_ASSERTION(req, "StreamLoader's request went away prematurely");
-  NS_ENSURE_SUCCESS(rv, false);
-
-  nsCOMPtr<nsIChannel> channel = do_QueryInterface(req);
-
-  if (channel &&
-      NS_SUCCEEDED(channel->GetContentCharset(charset)) &&
-      EncodingUtils::FindEncodingForLabel(charset, charset)) {
-    mDecoder = EncodingUtils::DecoderForEncoding(charset);
-    return true;
-  }
-
-  // Check the hint charset from the script element or preload
-  // request.
-  nsAutoString hintCharset;
-  if (!mRequest->IsPreload()) {
-    mRequest->mElement->GetScriptCharset(hintCharset);
-  } else {
-    nsTArray<ScriptLoader::PreloadInfo>::index_type i =
-      mScriptLoader->mPreloads.IndexOf(mRequest, 0,
-            ScriptLoader::PreloadRequestComparator());
-
-    NS_ASSERTION(i != mScriptLoader->mPreloads.NoIndex,
-                 "Incorrect preload bookkeeping");
-    hintCharset = mScriptLoader->mPreloads[i].mCharset;
-  }
-
-  if (EncodingUtils::FindEncodingForLabel(hintCharset, charset)) {
-    mDecoder = EncodingUtils::DecoderForEncoding(charset);
-    return true;
-  }
-
-  // Get the charset from the charset of the document.
-  if (mScriptLoader->mDocument) {
-    charset = mScriptLoader->mDocument->GetDocumentCharacterSet();
-    mDecoder = EncodingUtils::DecoderForEncoding(charset);
-    return true;
-  }
-
-  // Curiously, there are various callers that don't pass aDocument. The
-  // fallback in the old code was ISO-8859-1, which behaved like
-  // windows-1252. Saying windows-1252 for clarity and for compliance
-  // with the Encoding Standard.
-  charset = "windows-1252";
-  mDecoder = EncodingUtils::DecoderForEncoding(charset);
-  return true;
-}
-
-NS_IMETHODIMP
-ScriptLoadHandler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
-                                    nsISupports* aContext,
-                                    nsresult aStatus,
-                                    uint32_t aDataLength,
-                                    const uint8_t* aData)
-{
-  if (!mRequest->IsCanceled()) {
-    DebugOnly<bool> encoderSet =
-      EnsureDecoder(aLoader, aData, aDataLength, /* aEndOfStream = */ true);
-    MOZ_ASSERT(encoderSet);
-    DebugOnly<nsresult> rv = TryDecodeRawData(aData, aDataLength,
-                                              /* aEndOfStream = */ true);
-
-    // If SRI is required for this load, appending new bytes to the hash.
-    if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
-      mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
-    }
-  }
-
-  // we have to mediate and use mRequest.
-  return mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
-                                         mBuffer, mSRIDataVerifier);
-}
-
 } // dom namespace
-} // mozilla namespace
\ No newline at end of file
+} // mozilla namespace
diff --git a/dom/script/ScriptLoader.h b/dom/script/ScriptLoader.h
index 6fe76eca88..3c428deea6 100644
--- a/dom/script/ScriptLoader.h
+++ b/dom/script/ScriptLoader.h
@@ -4,10 +4,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-/*
- * A class that handles loading and evaluation of <script> elements.
- */
-
 #ifndef mozilla_dom_ScriptLoader_h
 #define mozilla_dom_ScriptLoader_h
 
@@ -25,7 +21,6 @@
 #include "mozilla/CORSMode.h"
 #include "mozilla/dom/SRIMetadata.h"
 #include "mozilla/dom/SRICheck.h"
-#include "mozilla/LinkedList.h"
 #include "mozilla/MozPromise.h"
 #include "mozilla/net/ReferrerPolicy.h"
 #include "mozilla/Vector.h"
@@ -221,6 +216,7 @@ public:
     NS_RELEASE(aElem);
   }
 };
+class ScriptLoadHandler;
 
 //////////////////////////////////////////////////////////////
 // Script loader implementation
@@ -643,53 +639,6 @@ private:
   nsCOMPtr<nsIConsoleReportCollector> mReporter;
 };
 
-class ScriptLoadHandler final : public nsIIncrementalStreamLoaderObserver
-{
-public:
-  explicit ScriptLoadHandler(ScriptLoader* aScriptLoader,
-                             ScriptLoadRequest *aRequest,
-                             mozilla::dom::SRICheckDataVerifier *aSRIDataVerifier);
-
-  NS_DECL_ISUPPORTS
-  NS_DECL_NSIINCREMENTALSTREAMLOADEROBSERVER
-
-private:
-  virtual ~ScriptLoadHandler();
-
-  /*
-   * Try to decode some raw data.
-   */
-  nsresult TryDecodeRawData(const uint8_t* aData, uint32_t aDataLength,
-                            bool aEndOfStream);
-
-  /*
-   * Discover the charset by looking at the stream data, the script
-   * tag, and other indicators.  Returns true if charset has been
-   * discovered.
-   */
-  bool EnsureDecoder(nsIIncrementalStreamLoader *aLoader,
-                     const uint8_t* aData, uint32_t aDataLength,
-                     bool aEndOfStream);
-
-  // ScriptLoader which will handle the parsed script.
-  RefPtr<ScriptLoader>        mScriptLoader;
-
-  // The ScriptLoadRequest for this load.
-  RefPtr<ScriptLoadRequest>   mRequest;
-
-  // SRI data verifier.
-  nsAutoPtr<mozilla::dom::SRICheckDataVerifier> mSRIDataVerifier;
-
-  // Status of SRI data operations.
-  nsresult                      mSRIStatus;
-
-  // Unicode decoder for charset.
-  nsCOMPtr<nsIUnicodeDecoder>   mDecoder;
-
-  // Accumulated decoded char buffer.
-  mozilla::Vector<char16_t>     mBuffer;
-};
-
 class nsAutoScriptLoaderDisabler
 {
 public:
diff --git a/dom/script/moz.build b/dom/script/moz.build
index 063d154265..280850ed4a 100644
--- a/dom/script/moz.build
+++ b/dom/script/moz.build
@@ -18,8 +18,11 @@ EXPORTS.mozilla.dom += [
 ]
 
 SOURCES += [
+    'ModuleLoadRequest.cpp',
+    'ModuleScript.cpp',
     'ScriptElement.cpp',
     'ScriptLoader.cpp',
+    'ScriptLoadHandler.cpp',
     'ScriptSettings.cpp',
 ]