feat(util-rxjs): add finalizeWithLatest operator

Author: mkazlauskas

packages/util-rxjs/src/finalizeWithLatest.ts

@@ -0,0 +1,11 @@
+import { Observable, finalize, tap } from 'rxjs';
+
+export const finalizeWithLatest =
+  <T>(callback: (latest: T | null) => void) =>
+  (value$: Observable<T>) => {
+    let latest: T | null = null;
+    return value$.pipe(
+      tap((value) => (latest = value)),
+      finalize(() => callback(latest))
+    );
+  };

packages/util-rxjs/src/index.ts

@@ -2,4 +2,5 @@ export * from './TrackerSubject';
 export * from './blockingWithLatestFrom';
 export * from './toEmpty';
 export * from './passthrough';
+export * from './finalizeWithLatest';
 export * from './types';

packages/util-rxjs/test/finalizeWithLatest.test.ts

@@ -0,0 +1,20 @@
+import { EMPTY, from, lastValueFrom } from 'rxjs';
+import { finalizeWithLatest } from '../src';
+
+describe('finalizeWithLatest', () => {
+  describe('source completes without emitting', () => {
+    it('calls the callback function with "null"', async () => {
+      const callback = jest.fn();
+      EMPTY.pipe(finalizeWithLatest(callback)).subscribe();
+      expect(callback).toBeCalledWith(null);
+    });
+  });
+
+  describe('source emits some value(s)', () => {
+    it('calls the callback function with last emitted value', async () => {
+      const callback = jest.fn();
+      await lastValueFrom(from([1, 2]).pipe(finalizeWithLatest(callback)));
+      expect(callback).toBeCalledWith(2);
+    });
+  });
+});