Introduce empty operator (#220)

* feat(empty): add operator

* Update src/empty/readme.md

Co-authored-by: Sergey Sova <[email protected]>

* Update src/empty/index.ts

Co-authored-by: Sergey Sova <[email protected]>

* Update src/empty/index.ts

* chore(empty): fixes after review

Co-authored-by: Sergey Sova <[email protected]>

src/babel-plugin-factories.json

@@ -8,6 +8,7 @@
     "patronum/delay",
     "patronum/either",
     "patronum/equals",
+    "patronum/empty",
     "patronum/every",
     "patronum/format",
     "patronum/in-flight",

src/empty/empty.fork.test.ts

@@ -0,0 +1,28 @@
+import { allSettled, createEvent, createStore, fork } from 'effector';
+import { empty } from './index';
+
+test('boolean', async () => {
+  const makeFalse = createEvent();
+  const $a = createStore<boolean | null>(null).on(makeFalse, (_) => false);
+  const $result = empty($a);
+
+  const scope = fork();
+
+  expect(scope.getState($result)).toBe(true);
+
+  await allSettled(makeFalse, { scope });
+  expect(scope.getState($result)).toBe(false);
+});
+
+test('numbers', async () => {
+  const increment = createEvent();
+  const $a = createStore<number | null>(null).on(increment, (a) => (a ?? -1) + 1);
+  const $result = empty($a);
+
+  const scope = fork();
+
+  expect(scope.getState($result)).toBe(true);
+
+  await allSettled(increment, { scope });
+  expect(scope.getState($result)).toBe(false);
+});

src/empty/empty.test.ts

@@ -0,0 +1,24 @@
+import { createEvent, createStore } from 'effector';
+import { empty } from './index';
+
+test('boolean', () => {
+  const makeFalse = createEvent();
+  const $a = createStore<boolean | null>(null).on(makeFalse, (_) => false);
+  const $result = empty($a);
+
+  expect($result.getState()).toBe(true);
+
+  makeFalse();
+  expect($result.getState()).toBe(false);
+});
+
+test('numbers', () => {
+  const increment = createEvent();
+  const $a = createStore<number | null>(null).on(increment, (a) => (a ?? -1) + 1);
+  const $result = empty($a);
+
+  expect($result.getState()).toBe(true);
+
+  increment();
+  expect($result.getState()).toBe(false);
+});

src/empty/index.ts

@@ -0,0 +1,5 @@
+import { Store } from 'effector';
+
+export function empty<A>(a: Store<A | null>): Store<boolean> {
+  return a.map((value) => value === null);
+}

src/empty/readme.md

@@ -0,0 +1,3 @@
+# empty (experimental)
+
+> Since 1.10.0