4 files changed, 53 insertions, 0 deletions
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
index 28fdb48a01..a68ce1c19a 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 1185caf3a9..55ae5444d9 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 db68182a71..5cd97faf81 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<size_t> 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 587cc8616f..5ce1989480 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<size_t> 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);