From 3a3119022835ce58a1ff03c529434b86b10bb360 Mon Sep 17 00:00:00 2001
From: marko1olo <marko1olo@users.noreply.github.com>
Date: Sat, 22 Aug 2026 21:44:59 +0400
Subject: [PATCH] fix: serialize BigInt in integer anyOf/oneOf and multi-type
 schemas

AJV does not recognise BigInt as type 'integer', so values like 12n were
always falling through to the TypeError branch in anyOf/oneOf union code,
and in multi-type (type array) serializers the Number.isInteger() guard
also rejects BigInt.

Two targeted changes:
- buildMultiTypeSerializer: add 	ypeof input === 'bigint' alongside
  Number.isInteger() in the integer branch condition.
- buildOneOf: when an option schema declares type 'integer', prefix the
  AJV validator.validate() call with a BigInt type-guard so BigInt values
  are correctly routed to asInteger() (which already handles BigInt).

asInteger() in lib/serializer.js already converts BigInt to string via
.toString(), so no change is needed there.

Fixes #501
---
 index.js                | 13 +++++++++++--
 test/typesArray.test.js | 19 +++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/index.js b/index.js
index e692d5e9..aa087805 100644
--- a/index.js
+++ b/index.js
@@ -946,7 +946,7 @@ function buildMultiTypeSerializer (context, location, input) {
       }
       case 'integer': {
         code += `
-          ${statement}(Number.isInteger(${input}) || ${input} === null) {
+          ${statement}(Number.isInteger(${input}) || typeof ${input} === 'bigint' || ${input} === null) {
             ${nestedResult}
           }
         `
@@ -1214,8 +1214,17 @@ function buildOneOf (context, location, input) {
     const schemaRef = getValidatorSchemaRef(context, optionLocation)
     context.validatorSchemaRefs.add(schemaRef)
 
+    // AJV does not recognise BigInt as type "integer".
+    // Pre-route BigInt to the first integer-typed schema so it reaches
+    // asInteger() (which correctly handles bigint) instead of falling
+    // through to the TypeError branch.
+    const isIntegerSchema = optionSchema.type === 'integer'
+    const condition = isIntegerSchema
+      ? `typeof ${input} === 'bigint' || validator.validate("${schemaRef}", ${input})`
+      : `validator.validate("${schemaRef}", ${input})`
+
     code += `
-      ${index === 0 ? 'if' : 'else if'}(validator.validate("${schemaRef}", ${input})) {
+      ${index === 0 ? 'if' : 'else if'}(${condition}) {
         ${nestedResult}
       }
     `
diff --git a/test/typesArray.test.js b/test/typesArray.test.js
index dc09d9ba..a9e0b86a 100644
--- a/test/typesArray.test.js
+++ b/test/typesArray.test.js
@@ -66,6 +66,25 @@ test('possibly nullable integer primitive alternative with null value', (t) => {
   t.assert.equal(value, '{"data":0}')
 })
 
+
+test('bigint serialized as integer in type array [integer, null]', (t) => {
+  t.plan(3)
+  const schema = { type: 'object', properties: { data: { type: ['integer', 'null'] } } }
+  const stringify = build(schema)
+  t.assert.equal(stringify({ data: 12n }), '{"data":12}')
+  t.assert.equal(stringify({ data: -5n }), '{"data":-5}')
+  t.assert.equal(stringify({ data: null }), '{"data":null}')
+})
+
+test('bigint serialized as integer in anyOf union', (t) => {
+  t.plan(3)
+  const schema = { type: 'object', properties: { data: { anyOf: [{ type: 'integer' }, { type: 'null' }] } } }
+  const stringify = build(schema)
+  t.assert.equal(stringify({ data: 12n }), '{"data":12}')
+  t.assert.equal(stringify({ data: -5n }), '{"data":-5}')
+  t.assert.equal(stringify({ data: null }), '{"data":null}')
+})
+
 test('possibly nullable number primitive alternative with null value', (t) => {
   t.plan(1)