From 13fb1477b52329c322dd93ea7f40030ef3be1eaa Mon Sep 17 00:00:00 2001
From: Slava Rudkovskiy <xraskolx@gmail.com>
Date: Thu, 9 Apr 2026 17:09:04 +0300
Subject: [PATCH] fix: preserve null values in cast

---
 src/utils/cast.ts             | 16 ++++++++++------
 test/specs/utils/cast.spec.ts | 12 ++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/utils/cast.ts b/src/utils/cast.ts
index cc9da0c..88d4e76 100644
--- a/src/utils/cast.ts
+++ b/src/utils/cast.ts
@@ -2,8 +2,8 @@ const isBooleanString = (value: string): value is 'true' | 'false' => {
   return value === 'true' || value === 'false';
 };
 
-const isNullString = (value: string): value is 'null' => {
-  return value === 'null';
+const isNull = (value: string | null): value is 'null' | null => {
+  return value === 'null' || value === null;
 };
 
 const isJsonStructure = (value: string): boolean => {
@@ -43,8 +43,8 @@ const parseBoolean = (value: string): boolean | string => {
   return value;
 };
 
-const parseNull = (value: string): null | string => {
-  return value === 'null' ? null : value;
+const parseNull = (value: string | null): null | string => {
+  return isNull(value) ? null : value;
 };
 
 const parseJson = (value: string): unknown => {
@@ -66,7 +66,7 @@ const parseAuto = (value: string): unknown => {
     return value === 'true';
   }
 
-  if (isNullString(value)) {
+  if (isNull(value)) {
     return null;
   }
 
@@ -85,7 +85,11 @@ const parseAuto = (value: string): unknown => {
   return value;
 };
 
-export const cast = (value: string, type: CastType = 'auto'): unknown => {
+export const cast = (value: string | null, type: CastType = 'auto'): unknown => {
+  if (value === null) {
+    return null;
+  }
+
   switch (type) {
     case 'string':
       return value;
diff --git a/test/specs/utils/cast.spec.ts b/test/specs/utils/cast.spec.ts
index 4938e32..148fccc 100644
--- a/test/specs/utils/cast.spec.ts
+++ b/test/specs/utils/cast.spec.ts
@@ -12,6 +12,10 @@ describe('cast', () => {
       expect(cast('null')).toBeNull();
     });
 
+    it('casts null value', () => {
+      expect(cast(null as unknown as string)).toBeNull();
+    });
+
     it('casts integer strings', () => {
       expect(cast('0')).toBe(0);
       expect(cast('1')).toBe(1);
@@ -62,6 +66,14 @@ describe('cast', () => {
       expect(cast('01', 'string')).toBe('01');
     });
 
+    it('keeps null value as null for any rule', () => {
+      expect(cast(null, 'string')).toBeNull();
+      expect(cast(null, 'number')).toBeNull();
+      expect(cast(null, 'boolean')).toBeNull();
+      expect(cast(null, 'null')).toBeNull();
+      expect(cast(null, 'json')).toBeNull();
+    });
+
     it('casts value to number for number rule when possible', () => {
       expect(cast('11', 'number')).toBe(11);
       expect(cast('1.5', 'number')).toBe(1.5);