Add (failing) tests for setting undefined as initial value in `deri…

…vedFrom` (#435)

* test(derived-from): `undefined` initial value

add tests for setting `undefined` as initial value in `derivedFrom`

* fix(derived-from): use 'in' operator to detect initialValue instead of comparing against undefined

---------

Co-authored-by: Chau Tran <[email protected]>

libs/ngxtension/derived-from/src/derived-from.spec.ts

@@ -104,6 +104,27 @@ describe(derivedFrom.name, () => {
 				expect(s()[0].toFixed(2)).toEqual('1.00'); // here we can call s()[0].toFixed(2) and will works!
 			});
 		}));
+		it(`for Observables that don't emit synchronously, you can pass undefined as options.initialValue to prevent error`, fakeAsync(() => {
+			TestBed.runInInjectionContext(() => {
+				const late = of(1).pipe(delay(1000)); // late emit after 1s
+				expect(() =>
+					derivedFrom(
+						[late],
+						map(([it]) => it),
+						{ initialValue: undefined },
+					),
+				).not.toThrow();
+				const s = derivedFrom(
+					[late],
+					map(([it]) => it),
+					{ initialValue: undefined },
+				);
+				expect(s()).toEqual(undefined);
+				tick(1000); // wait 1s for late emit
+				expect(s()).toEqual(1); // now we have the real value
+				expect(s().toFixed(2)).toEqual('1.00'); // here we can call s()[0].toFixed(2) and will works!
+			});
+		}));
 		it('value inside array', () => {
 			TestBed.runInInjectionContext(() => {
 				const value = new BehaviorSubject(1);
@@ -149,6 +170,28 @@ describe(derivedFrom.name, () => {
 				expect(s().value.toUpperCase()).toEqual('A'); // here we can call s().value.toUpperCase() and will works!
 			});
 		}));
+		it('with real async value, you can pass undefined as options.initialValue to prevent error', fakeAsync(() => {
+			TestBed.runInInjectionContext(() => {
+				const value = new Promise<string>((resolve) =>
+					setTimeout(resolve, 1000, 'a'),
+				); //Promise that emit 'a' after 1s
+				expect(() =>
+					derivedFrom(
+						{ value },
+						map(({ value }) => value),
+						{ initialValue: undefined },
+					),
+				).not.toThrow();
+				const s = derivedFrom(
+					{ value },
+					map(({ value }) => value),
+					{ initialValue: undefined },
+				);
+				expect(s()).toEqual(undefined);
+				tick(1000); // wait 1s for late emit of Promise
+				expect(s()).toEqual('a'); // after 1s we have the resolved value
+			});
+		}));
 		it('with a primitive string (that is Iterable), interally converted with from(iter) will emit single value last char (maybe not expected!? -> I suggest using of() for primitives/array)', () => {
 			TestBed.runInInjectionContext(() => {
 				const iter = 'abcd';

libs/ngxtension/derived-from/src/derived-from.ts

@@ -132,7 +132,7 @@ function _normalizeArgs<Input, Output>(
 		);
 	if (!hasOperator) args.splice(1, 0, identity);
 	const [sources, operator, options] = args;
-	const hasInitValue = options?.initialValue !== undefined;
+	const hasInitValue = !!options && 'initialValue' in options;
 	const normalizedSources = Object.entries(sources).reduce(
 		(acc, [keyOrIndex, source]) => {
 			if (isSignal(source)) {