From e446b7c588dbe07d186695374e69c93710666628 Mon Sep 17 00:00:00 2001
From: Alexander Kireev <ak.chatman.media@gmail.com>
Date: Sun, 21 Jun 2026 01:11:06 +0700
Subject: [PATCH] fix(runtime): serialize BigInt attribute values instead of
 throwing

`pug_attr` serialized any non-string attribute value with
`JSON.stringify`, which throws `TypeError: Do not know how to serialize a
BigInt` for `bigint` values. As a result an attribute whose value is a
BigInt (e.g. `input(value=count)` with `count` a BigInt local) crashed
rendering instead of producing `value="10"` as it does for a Number.

Coerce `bigint` values to their decimal string before the
`JSON.stringify` branch, mirroring how Numbers are handled and preserving
precision beyond `Number.MAX_SAFE_INTEGER`.

Fixes #3437
---
 packages/pug-runtime/index.js           |  3 +++
 packages/pug-runtime/test/index.test.js | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/packages/pug-runtime/index.js b/packages/pug-runtime/index.js
index 648af65db..d29cd4b10 100644
--- a/packages/pug-runtime/index.js
+++ b/packages/pug-runtime/index.js
@@ -147,6 +147,9 @@ function pug_attr(key, val, escaped, terse) {
   ) {
     val = val.toJSON();
   }
+  if (typeof val === 'bigint') {
+    val = val.toString();
+  }
   if (typeof val !== 'string') {
     val = JSON.stringify(val);
     if (!escaped && val.indexOf('"') !== -1) {
diff --git a/packages/pug-runtime/test/index.test.js b/packages/pug-runtime/test/index.test.js
index 6d1d0c737..4cecb8497 100644
--- a/packages/pug-runtime/test/index.test.js
+++ b/packages/pug-runtime/test/index.test.js
@@ -96,6 +96,16 @@ addTest('attr', function(attr) {
   expect(attr('key', 500, true, false)).toBe(' key="500"');
   expect(attr('key', 500, false, false)).toBe(' key="500"');
 
+  // BigInt attributes
+  expect(attr('key', BigInt(500), true, true)).toBe(' key="500"');
+  expect(attr('key', BigInt(500), false, true)).toBe(' key="500"');
+  expect(attr('key', BigInt(500), true, false)).toBe(' key="500"');
+  expect(attr('key', BigInt(500), false, false)).toBe(' key="500"');
+  // BigInt preserves precision beyond Number.MAX_SAFE_INTEGER
+  expect(attr('key', BigInt('9007199254740993'), true, true)).toBe(
+    ' key="9007199254740993"'
+  );
+
   // String attributes
   expect(attr('key', 'foo', true, true)).toBe(' key="foo"');
   expect(attr('key', 'foo', false, true)).toBe(' key="foo"');