From f3ab1dcf78124345db1e6affbfa9053931ec9439 Mon Sep 17 00:00:00 2001
From: Karl Seguin <k@openmymind.io>
Date: Mon, 27 Apr 2026 17:17:50 +0800
Subject: [PATCH] Add constructor arity

Depends on https://github.com/lightpanda-io/zig-v8-fork/pull/174

Builds ontop of https://github.com/lightpanda-io/browser/pull/2272 largely
because it's driven by the same uievents WPT tests.

https://github.com/lightpanda-io/browser/pull/1498 added the length to
functions, and this does it for constructors.
---
 src/browser/js/Snapshot.zig                   |  7 ++-
 src/browser/js/bridge.zig                     | 56 +++++++++++++------
 .../tests/element/html/script/script.html     |  6 ++
 3 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/src/browser/js/Snapshot.zig b/src/browser/js/Snapshot.zig
index 533d479c2..93bf9f6bf 100644
--- a/src/browser/js/Snapshot.zig
+++ b/src/browser/js/Snapshot.zig
@@ -569,7 +569,12 @@ pub fn generateConstructor(comptime JsApi: type, isolate: *v8.Isolate) *const v8
         break :blk illegalConstructorCallback;
     };
 
-    const template = v8.v8__FunctionTemplate__New__DEFAULT2(isolate, callback).?;
+    const arity: c_int = if (@hasDecl(JsApi, "constructor")) JsApi.constructor.arity else 0;
+    const template = v8.v8__FunctionTemplate__New__Config(isolate, &.{
+        .length = arity,
+        .callback = callback,
+        .behavior = v8.kConstructorBehavior_Allow,
+    }).?;
     {
         const internal_field_count = comptime countInternalFields(JsApi);
         if (internal_field_count > 0) {
diff --git a/src/browser/js/bridge.zig b/src/browser/js/bridge.zig
index 73ddccab3..ebe96a1f2 100644
--- a/src/browser/js/bridge.zig
+++ b/src/browser/js/bridge.zig
@@ -107,6 +107,7 @@ pub fn Builder(comptime T: type) type {
 }
 
 pub const Constructor = struct {
+    arity: c_int,
     func: *const fn (?*const v8.FunctionCallbackInfo) callconv(.c) void,
 
     const Opts = struct {
@@ -118,21 +119,24 @@ pub const Constructor = struct {
     };
 
     fn init(comptime T: type, comptime func: anytype, comptime opts: Opts) Constructor {
-        return .{ .func = struct {
-            fn wrap(handle: ?*const v8.FunctionCallbackInfo) callconv(.c) void {
-                const v8_isolate = v8.v8__FunctionCallbackInfo__GetIsolate(handle).?;
-                var caller: Caller = undefined;
-                if (!caller.init(v8_isolate)) {
-                    return;
-                }
-                defer caller.deinit();
+        return .{
+            .arity = comptime Function.getArity(@TypeOf(func), if (opts.new_target) 1 else 0),
+            .func = struct {
+                fn wrap(handle: ?*const v8.FunctionCallbackInfo) callconv(.c) void {
+                    const v8_isolate = v8.v8__FunctionCallbackInfo__GetIsolate(handle).?;
+                    var caller: Caller = undefined;
+                    if (!caller.init(v8_isolate)) {
+                        return;
+                    }
+                    defer caller.deinit();
 
-                caller.constructor(T, func, handle.?, .{
-                    .dom_exception = opts.dom_exception,
-                    .new_target = opts.new_target,
-                });
-            }
-        }.wrap };
+                    caller.constructor(T, func, handle.?, .{
+                        .dom_exception = opts.dom_exception,
+                        .new_target = opts.new_target,
+                    });
+                }
+            }.wrap,
+        };
     }
 };
 
@@ -149,7 +153,7 @@ pub const Function = struct {
             .cache = opts.cache,
             .static = opts.static,
             .wpt_only = opts.wpt_only,
-            .arity = getArity(@TypeOf(func)),
+            .arity = getArity(@TypeOf(func), 1),
             .func = if (opts.noop) noopFunction else struct {
                 fn wrap(handle: ?*const v8.FunctionCallbackInfo) callconv(.c) void {
                     Caller.Function.call(T, handle.?, func, opts);
@@ -160,14 +164,32 @@ pub const Function = struct {
 
     pub fn noopFunction(_: ?*const v8.FunctionCallbackInfo) callconv(.c) void {}
 
-    fn getArity(comptime T: type) usize {
+    fn getArity(comptime T: type, comptime start: usize) usize {
+        const Execution = js.Execution;
+
+        const Page = @import("../Page.zig");
+        const Session = @import("../Session.zig");
+
         var count: usize = 0;
         var params = @typeInfo(T).@"fn".params;
-        for (params[1..]) |p| { // start at 1, skip self
+        for (params[start..]) |p| { // start at 1, skip self
             const PT = p.type.?;
             if (PT == *Frame or PT == *const Frame) {
                 break;
             }
+
+            if (PT == *Page or PT == *const Page) {
+                break;
+            }
+
+            if (PT == *Execution or PT == *const Execution) {
+                break;
+            }
+
+            if (PT == *Session or PT == *const Session) {
+                break;
+            }
+
             if (@typeInfo(PT) == .optional) {
                 break;
             }
diff --git a/src/browser/tests/element/html/script/script.html b/src/browser/tests/element/html/script/script.html
index d49d025ac..0b7fc0f8d 100644
--- a/src/browser/tests/element/html/script/script.html
+++ b/src/browser/tests/element/html/script/script.html
@@ -35,5 +35,11 @@
 
    const append = Object.getOwnPropertyDescriptor(Element.prototype, 'append');
    testing.expectEqual('append', append.value.name);
+
+   // Constructor.length should equal the number of required arguments (1 for
+   // events: the type). Sentinel for the New__Config wiring in Snapshot.zig.
+   testing.expectEqual(1, MouseEvent.length);
+   testing.expectEqual(1, KeyboardEvent.length);
+   testing.expectEqual(1, Event.length);
 }
 </script>