isObject should return false for Date as well as File, null, and Array

Without this, assigning dates to the form's value instead merges them with a default value for objects of `{}` (in mergeDefaultsWithFormData), which stops them being dates.

CHANGELOG.md

@@ -34,6 +34,7 @@ should change the heading of the (upcoming) version to include a major version b
 ## @rjsf/core
 - Added `ref` definition to `ThemeProps` fixing [#2135](https://github.com/rjsf-team/react-jsonschema-form/issues/2135)
 - Updated the `onChange` handler in `Form` to use the new `preventDuplicates` mode of `mergeObjects()` when merging `extraErrors` when live validation is off, fixing [#3169](https://github.com/rjsf-team/react-jsonschema-form/issues/3169)
+- Fix isObject to correctly identify 'Date' as not mergable with Object default value.
 
 ## @rjsf/material-ui
 - Fix RangeWidget missing htmlFor and schema.title [#3281](https://github.com/rjsf-team/react-jsonschema-form/pull/3281)

packages/utils/src/isObject.ts

@@ -8,5 +8,8 @@ export default function isObject(thing: any) {
   if (typeof File !== "undefined" && thing instanceof File) {
     return false;
   }
+  if (typeof Date !== "undefined" && thing instanceof Date) {
+    return false;
+  }
   return typeof thing === "object" && thing !== null && !Array.isArray(thing);
 }

packages/utils/test/isObject.test.ts

@@ -25,6 +25,10 @@ describe('isObject()', () => {
     const file = new File(["test"], "test.txt");
     expect(isObject(file)).toBe(false);
   });
+  it('returns false when a Date is provided', () => {
+    const date = new Date();
+    expect(isObject(date)).toBe(false);
+  });
   it('returns false when an array is provided', () => {
     expect(isObject(['foo'])).toBe(false);
   });