LibWeb: Add select and option collection set length

Author: https://github.com/bplaat
Commit: 5c277144d8
Pull-request: https://github.com/SerenityOS/serenity/pull/23895
Reviewed-by: https://github.com/shannonbooth

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.cpp

@@ -5,10 +5,12 @@
  */
 
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/ElementFactory.h>
 #include <LibWeb/HTML/HTMLOptGroupElement.h>
 #include <LibWeb/HTML/HTMLOptionElement.h>
 #include <LibWeb/HTML/HTMLOptionsCollection.h>
 #include <LibWeb/HTML/HTMLSelectElement.h>
+#include <LibWeb/Namespace.h>
 #include <LibWeb/WebIDL/DOMException.h>
 
 namespace Web::HTML {
@@ -33,6 +35,41 @@ void HTMLOptionsCollection::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(HTMLOptionsCollection);
 }
 
+// https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#dom-htmloptionscollection-length
+WebIDL::ExceptionOr<void> HTMLOptionsCollection::set_length(WebIDL::UnsignedLong value)
+{
+    // 1. Let current be the number of nodes represented by the collection.
+    auto current = static_cast<WebIDL::UnsignedLong>(length());
+
+    // 2. If the given value is greater than current, then:
+    if (value > current) {
+        // 2.1. If the given value is greater than 100,000, then return.
+        if (value > 100'000)
+            return {};
+
+        // 2.2. Let n be value − current.
+        auto n = value - current;
+
+        // 2.3. Append n new option elements with no attributes and no child nodes to the select element on which this is rooted.
+        // Mutation events must be fired as if a DocumentFragment containing the new option elements had been inserted.
+        auto root_element = root();
+        for (WebIDL::UnsignedLong i = 0; i < n; i++)
+            TRY(root_element->append_child(TRY(DOM::create_element(root_element->document(), HTML::TagNames::option, Namespace::HTML))));
+    }
+
+    // 3. If the given value is less than current, then:
+    if (value < current) {
+        // 3.1. Let n be current − value.
+        auto n = current - value;
+
+        // 3.2. Remove the last n nodes in the collection from their parent nodes.
+        for (WebIDL::UnsignedLong i = current - 1; i >= current - n; i--)
+            this->item(i)->remove();
+    }
+
+    return {};
+}
+
 // https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#dom-htmloptionscollection-add
 WebIDL::ExceptionOr<void> HTMLOptionsCollection::add(HTMLOptionOrOptGroupElement element, Optional<HTMLElementOrElementIndex> before)
 {