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

CHANGELOG.md

@@ -46,6 +46,7 @@ should change the heading of the (upcoming) version to include a major version b
 - Updated `computedDefaults` (used by `getDefaultFormState`) to skip saving the computed default if it's an empty object unless `includeUndefinedValues` is truthy, fixing [#2150](https://github.com/rjsf-team/react-jsonschema-form/issues/2150) and [#2708](https://github.com/rjsf-team/react-jsonschema-form/issues/2708)
 - Expanded the `getDefaultFormState` util's `includeUndefinedValues` prop to accept a boolean or `"excludeObjectChildren"` if it does not want to include undefined values in nested objects
 - Updated `mergeObjects` to add new `preventDuplicates` mode when concatenating arrays so that only unique values from the source object array are copied to the destination object array
+- Fix `isObject` to correctly identify 'Date' as not an object, similar to 'File', thus preventing them from being merged with Object default values.
 
 ## Dev / docs / playground
 - Removed extraneous leading space on the examples in the validation documentation, fixing [#3282](https://github.com/rjsf-team/react-jsonschema-form/issues/3282)

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

@@ -0,0 +1,31 @@
+import { isObject } from "../src";
+
+const NON_OBJECTS = ["string", 10, NaN, true, null, undefined];
+
+const OBJECTS = [{ plain: "object" }, new Object(), new Map()];
+
+describe("isObject()", () => {
+  it("returns false when a non-object is provided", () => {
+    NON_OBJECTS.forEach(
+      (nonObject: string | number | boolean | null | undefined) => {
+        expect(isObject(nonObject)).toBe(false);
+      }
+    );
+  });
+  it("returns false when a File is provided", () => {
+    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);
+  });
+  it("returns true when an object is provided", () => {
+    OBJECTS.forEach((object: any) => {
+      expect(isObject(object)).toBe(true);
+    });
+  });
+});