improve code

Author: cherylEnkidu

packages/firestore/src/lite-api/reference.ts

@@ -627,12 +627,6 @@ export function doc<AppModelType, DbModelType extends DocumentData>(
   path?: string,
   ...pathSegments: string[]
 ): DocumentReference<AppModelType, DbModelType> {
-  if (parent === undefined) {
-    throw new FirestoreError(
-      Code.INVALID_ARGUMENT,
-      'Function doc() cannot be called with an undefined first argument.'
-    );
-  }
   parent = getModularInstance(parent);
 
   // We allow omission of 'pathString' but explicitly prohibit passing in both

packages/firestore/test/lite/integration.test.ts

@@ -286,13 +286,7 @@ describe('doc', () => {
   it('validates path', () => {
     return withTestDb(db => {
       expect(() =>
-        // @ts-ignore
-        doc(undefined, 'coll/doc')
-      ).to.throw(
-        'Function doc() cannot be called with an undefined first argument.'
-      );
-      expect(() =>
-        // @ts-ignore
+        // @ts-expect-error
         doc({}, 'coll/doc')
       ).to.throw(
         'Expected first argument to doc() to be a CollectionReference, a DocumentReference or FirebaseFirestore'