Add exists vs. not interfaces for type guards

Author: jamesdaniels

src/firestore/interfaces.ts

@@ -1,10 +1,19 @@
 import { Subscriber } from 'rxjs';
-import { DocumentChangeType, DocumentSnapshot as _DocumentSnapshot, SnapshotOptions, QueryDocumentSnapshot as _QueryDocumentSnapshot, DocumentChange as _DocumentChange, CollectionReference, Query } from '@firebase/firestore-types';
+import { DocumentChangeType, FieldPath, DocumentSnapshot as _DocumentSnapshot, SnapshotOptions, QueryDocumentSnapshot as _QueryDocumentSnapshot, DocumentChange as _DocumentChange, CollectionReference, Query } from '@firebase/firestore-types';
 
-export interface DocumentSnapshot<T> extends _DocumentSnapshot {
-  data(options?: SnapshotOptions): T | undefined;
+export interface DocumentSnapshotExists<T> extends _DocumentSnapshot {
+  readonly exists: true;
+  data(options?: SnapshotOptions): T;
+}
+
+export interface DocumentSnapshotDoesNotExist extends _DocumentSnapshot {
+  readonly exists: false;
+  data(options?: SnapshotOptions): undefined;
+  get(fieldPath: string | FieldPath, options?: SnapshotOptions): undefined;
 }
 
+export type DocumentSnapshot<T> = DocumentSnapshotExists<T> | DocumentSnapshotDoesNotExist;
+
 export interface QueryDocumentSnapshot<T> extends _QueryDocumentSnapshot {
   data(options?: SnapshotOptions): T;
 }