fix(assignIn): Correct undefined assignment (#959)

* fix(assignIn): Ensure properties with undefined values are assigned correctly

* test(assignIn): Add test case for correct assignment of undefined values

src/compat/object/assignIn.spec.ts

@@ -105,4 +105,10 @@ describe('assignIn', () => {
   it(`\`assignIn\` should coerce string sources to objects`, () => {
     expect(func({}, 'a')).toEqual({ 0: 'a' });
   });
+
+  it(`\`assignIn\` should assign properties with undefined values correctly`, () => {
+    const values = [{ workId: undefined }, { exerciseId: '1' }];
+    const result = assignIn({}, ...values);
+    expect(result).toEqual({ workId: undefined, exerciseId: '1' });
+  });
 });

src/compat/object/assignIn.ts

@@ -142,8 +142,7 @@ function assignInImpl(object: any, source: any): any {
 
   for (let i = 0; i < keys.length; i++) {
     const key = keys[i];
-
-    if (!eq(object[key], source[key])) {
+    if (!(key in object) || !eq(object[key], source[key])) {
       object[key] = source[key];
     }
   }