fix: clonedeep source key if its an object

packages/mui-utils/src/deepmerge.test.ts

@@ -45,4 +45,19 @@ describe('deepmerge', () => {
       bar: 'test',
     });
   });
+
+  it('should deep clone source key object if target key does not exist', () => {
+    const foo = { foo: { baz: 'test' } };
+    const bar = {};
+
+    const result = deepmerge(bar, foo);
+
+    expect(result).to.deep.equal({ foo: { baz: 'test' } });
+
+    // @ts-ignore
+    result.foo.baz = 'new test';
+
+    expect(result).to.deep.equal({ foo: { baz: 'new test' } });
+    expect(foo).to.deep.equal({ foo: { baz: 'test' } });
+  });
 });

packages/mui-utils/src/deepmerge.ts

@@ -1,3 +1,5 @@
+import { cloneDeep } from 'lodash';
+
 export function isPlainObject(item: unknown): item is Record<keyof any, unknown> {
   return item !== null && typeof item === 'object' && item.constructor === Object;
 }
@@ -24,7 +26,9 @@ export default function deepmerge<T>(
         // Since `output` is a clone of `target` and we have narrowed `target` in this block we can cast to the same type.
         (output as Record<keyof any, unknown>)[key] = deepmerge(target[key], source[key], options);
       } else {
-        (output as Record<keyof any, unknown>)[key] = source[key];
+        (output as Record<keyof any, unknown>)[key] = isPlainObject(source[key])
+          ? cloneDeep(source[key])
+          : source[key];
       }
     });
   }