4 files changed, 92 insertions, 0 deletions
diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index fe7450e723..a66c0eb03b 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -459,6 +459,7 @@ set(SOURCES
     Streams/ReadableStreamDefaultController.cpp
     Streams/ReadableStreamDefaultReader.cpp
     Streams/ReadableStreamGenericReader.cpp
+    Streams/UnderlyingSource.cpp
     SVG/AttributeNames.cpp
     SVG/AttributeParser.cpp
     SVG/SVGAnimatedLength.cpp
diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index f7e8a948c3..28f4d2ed68 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -422,6 +422,7 @@ class ReadableStreamDefaultController;
 class ReadableStreamDefaultReader;
 class ReadableStreamGenericReaderMixin;
 class ReadRequest;
+struct UnderlyingSource;
 }
 
 namespace Web::SVG {
diff --git a/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp
new file mode 100644
index 0000000000..7ea43d3e44
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp
@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/VM.h>
+#include <LibWeb/HTML/Scripting/Environments.h>
+#include <LibWeb/Streams/UnderlyingSource.h>
+#include <LibWeb/WebIDL/CallbackType.h>
+
+namespace Web::Streams {
+
+// Non-standard function to aid in converting a user-provided function into a WebIDL::Callback. This is essentially
+// what the Bindings generator would do at compile time, but at runtime instead.
+static JS::ThrowCompletionOr<JS::Handle<WebIDL::CallbackType>> property_to_callback(JS::VM& vm, JS::Value value, JS::PropertyKey const& property_key)
+{
+    auto property = TRY(value.get(vm, property_key));
+
+    if (property.is_undefined())
+        return JS::Handle<WebIDL::CallbackType> {};
+
+    if (!property.is_function())
+        return vm.throw_completion<JS::TypeError>(JS::ErrorType::NotAFunction, TRY_OR_THROW_OOM(vm, property.to_string_without_side_effects()));
+
+    return vm.heap().allocate_without_realm<WebIDL::CallbackType>(property.as_object(), HTML::incumbent_settings_object());
+}
+
+JS::ThrowCompletionOr<UnderlyingSource> UnderlyingSource::from_value(JS::VM& vm, JS::Value value)
+{
+    if (!value.is_object())
+        return UnderlyingSource {};
+
+    auto& object = value.as_object();
+
+    UnderlyingSource underlying_source {
+        .start = TRY(property_to_callback(vm, value, "start")),
+        .pull = TRY(property_to_callback(vm, value, "pull")),
+        .cancel = TRY(property_to_callback(vm, value, "cancel")),
+        .type = {},
+        .auto_allocate_chunk_size = {},
+    };
+
+    if (TRY(object.has_property("type"))) {
+        auto type_value = TRY(TRY(object.get("type")).to_string(object.vm()));
+        if (type_value == "bytes"sv) {
+            underlying_source.type = ReadableStreamType::Bytes;
+        } else {
+            return vm.throw_completion<JS::TypeError>(DeprecatedString::formatted("Unknown stream type '{}'", type_value));
+        }
+    }
+
+    if (TRY(object.has_property("autoAllocateChunkSize")))
+        underlying_source.auto_allocate_chunk_size = TRY(TRY(object.get("autoAllocateChunkSize")).to_bigint_int64(object.vm()));
+
+    return underlying_source;
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/Streams/UnderlyingSource.h b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.h
new file mode 100644
index 0000000000..08daab383a
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.h
@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <LibJS/Forward.h>
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/WebIDL/Promise.h>
+
+namespace Web::Streams {
+
+enum class ReadableStreamType {
+    Bytes
+};
+
+struct UnderlyingSource {
+    JS::Handle<WebIDL::CallbackType> start;
+    JS::Handle<WebIDL::CallbackType> pull;
+    JS::Handle<WebIDL::CallbackType> cancel;
+    Optional<ReadableStreamType> type;
+    Optional<i64> auto_allocate_chunk_size;
+
+    static JS::ThrowCompletionOr<UnderlyingSource> from_value(JS::VM&, JS::Value);
+};
+
+}