fix: Short-circuit File and Date constructor access in isObject

CHANGELOG.md

@@ -22,6 +22,10 @@ should change the heading of the (upcoming) version to include a major version b
 
 - Fix default value population when switching between options in `MultiSchemaField` [#4375](https://github.com/rjsf-team/react-jsonschema-form/pull/4375). Fixes [#4367](https://github.com/rjsf-team/react-jsonschema-form/issues/4367)
 
+## @rjsf/utils
+
+- Short-circuit `File` and `Date` constructor access in isObject to optimize performance in scenarios where `globalThis` is a `Proxy` that incurs overhead for each class constructor access ([#4413](https://github.com/rjsf-team/react-jsonschema-form/pull/4413)). Fixes [#4409](https://github.com/rjsf-team/react-jsonschema-form/issues/4409)
+
 ## @rjsf/validator-ajv8
 
 - Fixed issue where `ui:title` in anyOf/oneOf is not shown in error messages. Fixes [#4368](https://github.com/rjsf-team/react-jsonschema-form/issues/4368)

packages/utils/src/isObject.ts

@@ -1,15 +1,22 @@
-/** Determines whether a `thing` is an object for the purposes of RSJF. In this case, `thing` is an object if it has
+/** Determines whether a `thing` is an object for the purposes of RJSF. In this case, `thing` is an object if it has
  * the type `object` but is NOT null, an array or a File.
  *
  * @param thing - The thing to check to see whether it is an object
  * @returns - True if it is a non-null, non-array, non-File object
  */
 export default function isObject(thing: any) {
-  if (typeof File !== 'undefined' && thing instanceof File) {
+  if (typeof thing !== 'object' || thing === null) {
     return false;
   }
-  if (typeof Date !== 'undefined' && thing instanceof Date) {
+  // lastModified is guaranteed to be a number on a File instance
+  // as per https://w3c.github.io/FileAPI/#dfn-lastModified
+  if (typeof thing.lastModified === 'number' && typeof File !== 'undefined' && thing instanceof File) {
     return false;
   }
-  return typeof thing === 'object' && thing !== null && !Array.isArray(thing);
+  // getMonth is guaranteed to be a method on a Date instance
+  // as per https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-date.prototype.getmonth
+  if (typeof thing.getMonth === 'function' && typeof Date !== 'undefined' && thing instanceof Date) {
+    return false;
+  }
+  return !Array.isArray(thing);
 }

packages/utils/test/isObject.test.ts

@@ -26,4 +26,38 @@ describe('isObject()', () => {
       expect(isObject(object)).toBe(true);
     });
   });
+  describe('without accessing File and Date classes', () => {
+    const NativeFile = File;
+    const NativeDate = Date;
+
+    beforeEach(() => {
+      Object.defineProperty(global, 'File', {
+        get() {
+          throw new Error('File should not have been accessed');
+        },
+      });
+      Object.defineProperty(global, 'Date', {
+        get() {
+          throw new Error('Date should not have been accessed');
+        },
+      });
+    });
+
+    afterEach(() => {
+      Object.defineProperty(global, 'File', NativeFile);
+      Object.defineProperty(global, 'Date', NativeDate);
+    });
+
+    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 true when an object is provided', () => {
+      OBJECTS.forEach((object: any) => {
+        expect(isObject(object)).toBe(true);
+      });
+    });
+  });
 });