fix: mergeDefaultsWithFormData should treat null as a defined default value

CHANGELOG.md

@@ -37,6 +37,7 @@ should change the heading of the (upcoming) version to include a major version b
 
 - BREAKING CHANGE: Removed `formContext` from the following interfaces because it is available on `registry`:
   - `ErrorListProps`, `FieldProps`, `FieldTemplateProps`, `ArrayFieldTemplateProps` and `WidgetProps`
+- Update `mergeDefaultsWithFormData` to properly handle overriding `undefined` formData with a `null` default value, fixing [#4734](https://github.com/rjsf-team/react-jsonschema-form/issues/4734)
 
 ## Dev / docs / playground
 

packages/utils/src/mergeDefaultsWithFormData.ts

@@ -102,7 +102,7 @@ export default function mergeDefaultsWithFormData<T = any>(
    */
   if (
     (defaultSupercedesUndefined &&
-      ((!isNil(defaults) && isNil(formData)) || (typeof formData === 'number' && isNaN(formData)))) ||
+      ((!(defaults === undefined) && isNil(formData)) || (typeof formData === 'number' && isNaN(formData)))) ||
     (overrideFormDataWithDefaults && !isNil(formData))
   ) {
     return defaults;

packages/utils/test/mergeDefaultsWithFormData.test.ts

@@ -29,6 +29,13 @@ describe('mergeDefaultsWithFormData()', () => {
     expect(mergeDefaultsWithFormData({}, null, undefined, true)).toEqual({});
   });
 
+  it('should return null if default is null and formData is undefined and defaultSupercedesUndefined is true', () => {
+    const defaultValue = null;
+    const formData = undefined;
+    const defaultSupercedesUndefined = true;
+    expect(mergeDefaultsWithFormData(defaultValue, formData, undefined, defaultSupercedesUndefined)).toBeNull();
+  });
+
   it('should return undefined when formData is undefined', () => {
     expect(mergeDefaultsWithFormData(undefined, undefined)).toBeUndefined();
   });

packages/utils/test/schema/getDefaultFormStateTest.ts

@@ -4990,5 +4990,19 @@ export default function getDefaultFormStateTest(testValidator: TestValidatorType
         }),
       ).toEqual({ stringArray: [undefined], numberArray: [] });
     });
+    it('handles a `null` default value', () => {
+      const schema: RJSFSchema = {
+        type: 'object',
+        properties: {
+          empty: {
+            type: 'null',
+            default: null,
+          },
+        },
+      };
+      expect(getDefaultFormState(testValidator, schema, {}, schema)).toEqual({
+        empty: null,
+      });
+    });
   });
 }