From 43b2dd78a2cb96c810299d40ac86fae911545018 Mon Sep 17 00:00:00 2001
From: Nigro Simone <nigro.simone@gmail.com>
Date: Sat, 27 Sep 2025 13:39:04 +0200
Subject: [PATCH 1/4] fix: isWellFormed

---
 lib/serializer.js   |  6 +++++-
 test/string.test.js | 15 +++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/lib/serializer.js b/lib/serializer.js
index 0f4dde76..1f2ab201 100644
--- a/lib/serializer.js
+++ b/lib/serializer.js
@@ -1,5 +1,8 @@
 'use strict'
 
+// eslint-disable-next-line
+const ASCII_ESCAPE = /[\u0000-\u001f\u0022\u005c]/
+
 module.exports = class Serializer {
   constructor (options) {
     switch (options && options.rounding) {
@@ -89,6 +92,7 @@ module.exports = class Serializer {
 
   asString (str) {
     const len = str.length
+
     if (len === 0) {
       return '""'
     } else if (len < 42) {
@@ -116,7 +120,7 @@ module.exports = class Serializer {
         }
       }
       return (last === -1 && ('"' + str + '"')) || ('"' + result + str.slice(last) + '"')
-    } else if (len < 5000 && str.isWellFormed()) {
+    } else if (len < 5000 && str.isWellFormed() && ASCII_ESCAPE.test(str) === false) {
       // Only use the regular expression for shorter input. The overhead is otherwise too much.
       return '"' + str + '"'
     } else {
diff --git a/test/string.test.js b/test/string.test.js
index 518513da..0b3a76c8 100644
--- a/test/string.test.js
+++ b/test/string.test.js
@@ -34,6 +34,21 @@ test('serialize short string', (t) => {
   t.assert.equal(JSON.parse(output), input)
 })
 
+test('serialize medium string', (t) => {
+  t.plan(2)
+
+  const schema = {
+    type: 'string'
+  }
+
+  const input = new Array(150).fill('\x00').join('')
+  const stringify = build(schema)
+  const output = stringify(input)
+
+  t.assert.equal(output, `"${new Array(150).fill('\\u0000').join('')}"`)
+  t.assert.equal(JSON.parse(output), input)
+})
+
 test('serialize long string', (t) => {
   t.plan(2)
 

From b287ce4c874f5f491b549dab5982eb32703fa534 Mon Sep 17 00:00:00 2001
From: Nigro Simone <nigro.simone@gmail.com>
Date: Sat, 27 Sep 2025 14:53:00 +0200
Subject: [PATCH 2/4] fix: enhance string serialization with well-formed check

---
 lib/serializer.js | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/lib/serializer.js b/lib/serializer.js
index 1f2ab201..8e017a73 100644
--- a/lib/serializer.js
+++ b/lib/serializer.js
@@ -95,7 +95,13 @@ module.exports = class Serializer {
 
     if (len === 0) {
       return '""'
-    } else if (len < 42) {
+    }
+
+    if (!str.isWellFormed()) {
+      return JSON.stringify(str)
+    }
+
+    if (len < 42) {
       // magically escape strings for json
       // relying on their charCodeAt
       // everything below 32 needs JSON.stringify()
@@ -114,13 +120,13 @@ module.exports = class Serializer {
           last === -1 && (last = 0)
           result += str.slice(last, i) + '\\'
           last = i
-        } else if (point < 32 || (point >= 0xD800 && point <= 0xDFFF)) {
+        } else if (point < 32) {
           // The current character is non-printable characters or a surrogate.
           return JSON.stringify(str)
         }
       }
       return (last === -1 && ('"' + str + '"')) || ('"' + result + str.slice(last) + '"')
-    } else if (len < 5000 && str.isWellFormed() && ASCII_ESCAPE.test(str) === false) {
+    } else if (len < 5000 && ASCII_ESCAPE.test(str) === false) {
       // Only use the regular expression for shorter input. The overhead is otherwise too much.
       return '"' + str + '"'
     } else {

From bfc2beab255c701d75a74af6ffee7e0b7d486cdd Mon Sep 17 00:00:00 2001
From: Nigro Simone <nigro.simone@gmail.com>
Date: Sun, 28 Sep 2025 08:43:10 +0200
Subject: [PATCH 3/4] fix: revert to Uzlopak proposal, is more stable and
 faster on more cases on node 24

---
 lib/serializer.js | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/lib/serializer.js b/lib/serializer.js
index 8e017a73..eadaa074 100644
--- a/lib/serializer.js
+++ b/lib/serializer.js
@@ -95,13 +95,7 @@ module.exports = class Serializer {
 
     if (len === 0) {
       return '""'
-    }
-
-    if (!str.isWellFormed()) {
-      return JSON.stringify(str)
-    }
-
-    if (len < 42) {
+    } else if (len < 42) {
       // magically escape strings for json
       // relying on their charCodeAt
       // everything below 32 needs JSON.stringify()
@@ -120,13 +114,13 @@ module.exports = class Serializer {
           last === -1 && (last = 0)
           result += str.slice(last, i) + '\\'
           last = i
-        } else if (point < 32) {
+        } else if (point < 32 || (point >= 0xd800 && point <= 0xdfff)) {
           // The current character is non-printable characters or a surrogate.
           return JSON.stringify(str)
         }
       }
       return (last === -1 && ('"' + str + '"')) || ('"' + result + str.slice(last) + '"')
-    } else if (len < 5000 && ASCII_ESCAPE.test(str) === false) {
+    } else if (len < 5000 && str.isWellFormed() && ASCII_ESCAPE.test(str) === false) {
       // Only use the regular expression for shorter input. The overhead is otherwise too much.
       return '"' + str + '"'
     } else {

From 2403bdd0ef2be055bb1d6cefae8373cc996c037e Mon Sep 17 00:00:00 2001
From: Nigro Simone <nigro.simone@gmail.com>
Date: Sun, 28 Sep 2025 09:25:37 +0200
Subject: [PATCH 4/4] feat: add empty string into benchmark

---
 benchmark/bench.js | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/benchmark/bench.js b/benchmark/bench.js
index acb87b94..cd4bda87 100644
--- a/benchmark/bench.js
+++ b/benchmark/bench.js
@@ -40,6 +40,13 @@ for (let i = 0; i < SHORT_ARRAY_SIZE; i++) {
 }
 
 const benchmarks = [
+  {
+    name: 'empty string',
+    schema: {
+      type: 'string'
+    },
+    input: ''
+  },
   {
     name: 'short string',
     schema: {