From 5b8592de5a00e6ba369ec982de521e1a968cdfed Mon Sep 17 00:00:00 2001
From: William Chan <root@williamchan.me>
Date: Sat, 18 Apr 2026 07:24:30 +0800
Subject: [PATCH] feat(document): implement document.writeln

Document.writeln was not registered on the DOM prototype, so legacy
scripts that rely on it (e.g. script loaders that inject <script> tags
via `document.writeln`) crashed with `document.writeln is not a
function` and prevented the rest of the page from running.

Per HTML spec, writeln runs the document write steps with the joined
argument text followed by a U+000A LINE FEED character. Implement it
by sharing the existing write pipeline so all InvalidStateError and
parser behavior is identical to write().

Tests added to document/write.html covering:
- writeln appends a trailing newline text node after the written nodes
- multiple arguments are concatenated before the newline is appended

Spec: https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-document-writeln
---
 src/browser/tests/document/write.html | 34 +++++++++++++++++++++++++++
 src/browser/webapi/Document.zig       | 15 ++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/src/browser/tests/document/write.html b/src/browser/tests/document/write.html
index 25a9901c..c7e7f0ca 100644
--- a/src/browser/tests/document/write.html
+++ b/src/browser/tests/document/write.html
@@ -115,8 +115,42 @@
   testing.expectEqual('function', typeof document.open);
   testing.expectEqual('function', typeof document.close);
   testing.expectEqual('function', typeof document.write);
+  testing.expectEqual('function', typeof document.writeln);
 </script>
 
+<!-- Phase 4 Tests: document.writeln -->
+
+<script id=basic_writeln>
+  document.writeln('<p id="writeln_basic">Line</p>');
+  testing.expectEqual(true, true);
+</script>
+
+<script id=verify_writeln_basic>
+  const el = document.getElementById('writeln_basic');
+  testing.expectEqual('Line', el.textContent);
+  // writeln should append a newline after the written text; that newline
+  // becomes a text node sibling of the element.
+  const next = el.nextSibling;
+  testing.expectEqual(true, next !== null);
+  testing.expectEqual('#text', next.nodeName);
+  testing.expectEqual('\n', next.nodeValue);
+</script>
+
+<script id=writeln_multiple_args>
+  // writeln joins all arguments, then appends a single U+000A.
+  document.writeln('<span id="wln_a">A</span>', '<span id="wln_b">B</span>');
+  testing.expectEqual(true, true);
+</script>
+
+<script id=verify_writeln_multiple_args>
+  const a = document.getElementById('wln_a');
+  const b = document.getElementById('wln_b');
+  testing.expectEqual('A', a.textContent);
+  testing.expectEqual('B', b.textContent);
+  testing.expectEqual(b, a.nextElementSibling);
+</script>
+
+
 <!-- Phase 3 Tests: document.open/close (post-parsing with setTimeout) -->
 
 <div id="will_be_removed">This will be removed by document.open()</div>
diff --git a/src/browser/webapi/Document.zig b/src/browser/webapi/Document.zig
index 39c8a21c..3421d40b 100644
--- a/src/browser/webapi/Document.zig
+++ b/src/browser/webapi/Document.zig
@@ -621,6 +621,17 @@ fn looksLikeNewDocument(html: []const u8) bool {
 }
 
 pub fn write(self: *Document, text: []const []const u8, page: *Page) !void {
+    return self.writeInternal(text, false, page);
+}
+
+// https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-document-writeln
+// `writeln(...text)` runs the document write steps with `text` followed by a
+// U+000A LINE FEED character.
+pub fn writeln(self: *Document, text: []const []const u8, page: *Page) !void {
+    return self.writeInternal(text, true, page);
+}
+
+fn writeInternal(self: *Document, text: []const []const u8, append_newline: bool, page: *Page) !void {
     if (self._type == .xml) {
         return error.InvalidStateError;
     }
@@ -634,6 +645,9 @@ pub fn write(self: *Document, text: []const []const u8, page: *Page) !void {
         for (text) |str| {
             try joined.appendSlice(page.call_arena, str);
         }
+        if (append_newline) {
+            try joined.append(page.call_arena, '\n');
+        }
         break :blk joined.items;
     };
 
@@ -1052,6 +1066,7 @@ pub const JsApi = struct {
     pub const elementFromPoint = bridge.function(Document.elementFromPoint, .{});
     pub const elementsFromPoint = bridge.function(Document.elementsFromPoint, .{});
     pub const write = bridge.function(Document.write, .{ .dom_exception = true });
+    pub const writeln = bridge.function(Document.writeln, .{ .dom_exception = true });
     pub const open = bridge.function(Document.open, .{ .dom_exception = true });
     pub const close = bridge.function(Document.close, .{ .dom_exception = true });
     pub const doctype = bridge.accessor(Document.getDocType, null, .{});