From 19af2bebc78cf093aa2de6a35bd167296490b0a0 Mon Sep 17 00:00:00 2001
From: Shannon Booth <shannon@serenityos.org>
Date: Fri, 13 Mar 2026 16:51:52 +0100
Subject: [PATCH] LibWeb/Bindings: Implement LegacyLenientThis extended
 attribute

---
 .../BindingsGenerator/IDLGenerators.cpp       | 29 +++++++++++++++----
 .../ecmascript-binding/setter-argument.txt    |  5 ++--
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
index aaab2ac3713..395e83e4963 100644
--- a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
@@ -4130,13 +4130,20 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
     // 4. Let idlObject be null.
     // 5. If attribute is a regular attribute:
 
-    // FIXME: 1. Let jsValue be the this value, if it is not null or undefined, or realm’s global object otherwise. (This will subsequently cause a TypeError in a few steps, if the global object does not implement target and [LegacyLenientThis] is not specified.)
-    auto* impl = TRY(impl_from(vm));
-
+    // 1. Let jsValue be the this value, if it is not null or undefined, or realm’s global object otherwise.
+    //   (This will subsequently cause a TypeError in a few steps, if the global object does not implement target and [LegacyLenientThis] is not specified.)
     // FIXME: 2. If jsValue is a platform object, then perform a security check, passing jsValue, id, and "setter".
-    // FIXME: 3. Let validThis be true if jsValue implements target, or false otherwise.
-    // FIXME: 4. If validThis is false and attribute was not specified with the [LegacyLenientThis] extended attribute, then throw a TypeError.
+    // 3. Let validThis be true if jsValue implements target, or false otherwise.
+    auto maybe_impl = impl_from(vm);
+
+    // 4. If validThis is false and attribute was not specified with the [LegacyLenientThis] extended attribute, then throw a TypeError.
 )~~~");
+    if (!attribute.extended_attributes.contains("LegacyLenientThis")) {
+        attribute_generator.append(R"~~~(
+    auto impl = TRY(maybe_impl);
+)~~~");
+    }
+
     // For [CEReactions]: https://html.spec.whatwg.org/multipage/custom-elements.html#cereactions
 
     if (attribute.extended_attributes.contains("CEReactions")) {
@@ -4160,7 +4167,17 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
         return;
     }
 
-    // FIXME: 6. If validThis is false, then return undefined.
+    // 6. If validThis is false, then return undefined.
+    // NB: This is only possible if LegacyLenientThis is defined.
+    if (attribute.extended_attributes.contains("LegacyLenientThis")) {
+        attribute_generator.append(R"~~~(
+    if (maybe_impl.is_error())
+        return JS::js_undefined();
+
+    auto impl = maybe_impl.release_value();
+)~~~");
+    }
+
     // 7. If attribute is declared with a [LegacyLenientSetter] extended attribute, then return undefined.
     if (auto legacy_lenient_setter_identifier = attribute.extended_attributes.get("LegacyLenientSetter"sv); legacy_lenient_setter_identifier.has_value()) {
         attribute_generator.append(R"~~~(
diff --git a/Tests/LibWeb/Text/expected/wpt-import/webidl/ecmascript-binding/setter-argument.txt b/Tests/LibWeb/Text/expected/wpt-import/webidl/ecmascript-binding/setter-argument.txt
index bfe3747a9de..c07e7c649f1 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/webidl/ecmascript-binding/setter-argument.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/webidl/ecmascript-binding/setter-argument.txt
@@ -2,8 +2,7 @@ Harness status: OK
 
 Found 17 tests
 
-16 Pass
-1 Fail
+17 Pass
 Pass	String attribute setter should throw if this value is invalid
 Pass	String attribute setter should treat no arguments as undefined
 Pass	String attribute setter called with undefined should behave in the same way as no arguments
@@ -14,7 +13,7 @@ Pass	Object attribute setter called with undefined should behave in the same way
 Pass	[Replaceable] setter should treat no arguments as undefined
 Pass	[Replaceable] setter called with undefined should behave in the same way as no arguments
 Pass	[Replaceable] setter called with other value should just work
-Fail	[LegacyLenientThis] setter should not throw even if this value is invalid, regardless of the arguments count
+Pass	[LegacyLenientThis] setter should not throw even if this value is invalid, regardless of the arguments count
 Pass	[LegacyLenientThis] setter should treat no arguments as undefined
 Pass	[LegacyLenientThis] setter called with undefined should behave in the same way as no arguments
 Pass	[LegacyLenientSetter] setter should treat no arguments as undefined