diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
index 28fdb48a01d..a68ce1c19a1 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
@@ -742,6 +742,28 @@ BrowsingContext* BrowsingContext::choose_a_browsing_context(StringView name, boo
return chosen;
}
+// https://html.spec.whatwg.org/multipage/browsers.html#document-tree-child-browsing-context
+size_t BrowsingContext::document_tree_child_browsing_context_count() const
+{
+ size_t count = 0;
+
+ // A browsing context child is a document-tree child browsing context of parent if child is a child browsing context and child's container is in a document tree.
+ for_each_child([this, &count](BrowsingContext const& child) {
+ if (child.is_child_of(*this) && child.container()->in_a_document_tree())
+ ++count;
+ });
+
+ return count;
+}
+
+// https://html.spec.whatwg.org/multipage/browsers.html#child-browsing-context
+bool BrowsingContext::is_child_of(BrowsingContext const& parent) const
+{
+ // A browsing context child is said to be a child browsing context of another browsing context parent,
+ // if child's container document is non-null and child's container document's browsing context is parent.
+ return container_document() && container_document()->browsing_context() == &parent;
+}
+
// https://html.spec.whatwg.org/multipage/dom.html#still-on-its-initial-about:blank-document
bool BrowsingContext::still_on_its_initial_about_blank_document() const
{
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
index 1185caf3a95..55ae5444d92 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
@@ -80,6 +80,10 @@ public:
BrowsingContext* choose_a_browsing_context(StringView name, bool noopener);
+ size_t document_tree_child_browsing_context_count() const;
+
+ bool is_child_of(BrowsingContext const&) const;
+
HTML::BrowsingContextContainer* container() { return m_container; }
HTML::BrowsingContextContainer const* container() const { return m_container; }
diff --git a/Userland/Libraries/LibWeb/HTML/Window.cpp b/Userland/Libraries/LibWeb/HTML/Window.cpp
index db68182a714..5cd97faf811 100644
--- a/Userland/Libraries/LibWeb/HTML/Window.cpp
+++ b/Userland/Libraries/LibWeb/HTML/Window.cpp
@@ -794,6 +794,7 @@ void Window::initialize(JS::Realm& realm)
define_native_accessor(realm, "pageXOffset", scroll_x_getter, {}, attr);
define_native_accessor(realm, "scrollY", scroll_y_getter, {}, attr);
define_native_accessor(realm, "pageYOffset", scroll_y_getter, {}, attr);
+ define_native_accessor(realm, "length", length_getter, {}, attr);
define_native_function(realm, "scroll", scroll, 2, attr);
define_native_function(realm, "scrollTo", scroll, 2, attr);
@@ -1074,6 +1075,29 @@ JS_DEFINE_NATIVE_FUNCTION(Window::btoa)
return JS::js_string(vm, move(encoded));
}
+// https://html.spec.whatwg.org/multipage/window-object.html#number-of-document-tree-child-browsing-contexts
+JS::ThrowCompletionOr Window::document_tree_child_browsing_context_count() const
+{
+ auto* impl = TRY(impl_from(vm()));
+
+ // 1. If W's browsing context is null, then return 0.
+ auto* this_browsing_context = impl->associated_document().browsing_context();
+ if (!this_browsing_context)
+ return 0;
+
+ // 2. Return the number of document-tree child browsing contexts of W's browsing context.
+ return this_browsing_context->document_tree_child_browsing_context_count();
+}
+
+// https://html.spec.whatwg.org/multipage/window-object.html#dom-length
+JS_DEFINE_NATIVE_FUNCTION(Window::length_getter)
+{
+ auto* impl = TRY(impl_from(vm));
+
+ // The length getter steps are to return the number of document-tree child browsing contexts of this.
+ return TRY(impl->document_tree_child_browsing_context_count());
+}
+
// https://html.spec.whatwg.org/multipage/browsers.html#dom-top
JS_DEFINE_NATIVE_FUNCTION(Window::top_getter)
{
diff --git a/Userland/Libraries/LibWeb/HTML/Window.h b/Userland/Libraries/LibWeb/HTML/Window.h
index 587cc8616f6..5ce1989480d 100644
--- a/Userland/Libraries/LibWeb/HTML/Window.h
+++ b/Userland/Libraries/LibWeb/HTML/Window.h
@@ -52,6 +52,8 @@ public:
HTML::BrowsingContext const* browsing_context() const;
HTML::BrowsingContext* browsing_context();
+ JS::ThrowCompletionOr document_tree_child_browsing_context_count() const;
+
void alert_impl(String const&);
bool confirm_impl(String const&);
String prompt_impl(String const&, String const&);
@@ -201,6 +203,7 @@ public:
Bindings::CrossOriginPropertyDescriptorMap& cross_origin_property_descriptor_map() { return m_cross_origin_property_descriptor_map; }
private:
+ JS_DECLARE_NATIVE_FUNCTION(length_getter);
JS_DECLARE_NATIVE_FUNCTION(top_getter);
JS_DECLARE_NATIVE_FUNCTION(document_getter);