fix(utils): avoid enumerating string defaults in mergeDefaultsWithFormData (#5062)

* fix(utils): avoid Object.entries on string defaults in mergeDefaultsWithFormData

* Apply suggestion from @heath-freenome

* Update CHANGELOG.md

* Update CHANGELOG.md

---------

Co-authored-by: Heath C <51679588+heath-freenome@users.noreply.github.com>

Author: abhinavgautam01

CHANGELOG.md

@@ -29,6 +29,7 @@ should change the heading of the (upcoming) version to include a major version b
 - Added `deprecatedHandling` to `GlobalUISchemaOptions` and updated `StrictRJSFSchema` to be recursive, ensuring the `deprecated` keyword (and future extensions) are supported in all nested schema structures without requiring type casts, fixing [#5024](https://github.com/rjsf-team/react-jsonschema-form/issues/5024)
   - Also added `DeprecatedLabel` to `TranslatableString` to support internationalization of the deprecated field suffix
 - Added `SchemaFieldPath` (`string | FieldPathList`) for `getFromSchema` / `findFieldInSchema`; fixed schema descent when a segment is numeric `0` (previously skipped due to falsy check); use string keys for `required` / oneOf fallback matching
+- Fixed `mergeDefaultsWithFormData` calling `Object.entries` on non-object defaults (e.g. long `data:` URL strings for single file fields), which iterated every character and caused severe UI freezes, fixing [#5055](https://github.com/rjsf-team/react-jsonschema-form/issues/5055)
 
 # 6.5.2
 

packages/utils/src/mergeDefaultsWithFormData.ts

@@ -68,7 +68,8 @@ export default function mergeDefaultsWithFormData<T = any>(
       const keyExistsInDefaults = isObject(defaults) && key in (defaults as GenericObjectType);
       const keyExistsInFormData = key in (formData as GenericObjectType);
       const keyDefault = get(defaults, key) ?? {};
-      const defaultValueIsNestedObject = keyExistsInDefaults && Object.entries(keyDefault).some(([, v]) => isObject(v));
+      const defaultValueIsNestedObject =
+        keyExistsInDefaults && isObject(keyDefault) && Object.values(keyDefault).some((v) => isObject(v));
 
       const keyDefaultIsObject = keyExistsInDefaults && isObject(get(defaults, key));
       const keyHasFormDataObject = keyExistsInFormData && isObject(keyValue);

packages/utils/test/mergeDefaultsWithFormData.test.ts

@@ -148,6 +148,14 @@ describe('mergeDefaultsWithFormData()', () => {
     expect(mergeDefaultsWithFormData(obj1, obj2)?.a).toBeInstanceOf(File);
   });
 
+  it('should merge quickly when the default for a key is a long string (e.g. data-url file value)', () => {
+    const largeDefault = `data:application/octet-stream;base64,${'A'.repeat(200_000)}`;
+    const formValue = 'data:application/octet-stream;base64,B';
+    expect(mergeDefaultsWithFormData<{ file: string }>({ file: largeDefault }, { file: formValue })).toEqual({
+      file: formValue,
+    });
+  });
+
   describe('test with overrideFormDataWithDefaults set to true', () => {
     it('should return data in formData when no defaults', () => {
       expect(mergeDefaultsWithFormData(undefined, [2], undefined, undefined, true)).toEqual([2]);