From e2a4ed6a05ee3279fe2916b64aa278026dfbaefa Mon Sep 17 00:00:00 2001
From: Igor Kamyshev <igor@kamyshev.me>
Date: Fri, 12 Aug 2022 13:21:17 +0700
Subject: [PATCH] Introduce `empty` operator (#220)

* feat(empty): add operator

* Update src/empty/readme.md

Co-authored-by: Sergey Sova <5620073+sergeysova@users.noreply.github.com>

* Update src/empty/index.ts

Co-authored-by: Sergey Sova <5620073+sergeysova@users.noreply.github.com>

* Update src/empty/index.ts

* chore(empty): fixes after review

Co-authored-by: Sergey Sova <5620073+sergeysova@users.noreply.github.com>
---
 src/babel-plugin-factories.json |  1 +
 src/empty/empty.fork.test.ts    | 28 ++++++++++++++++++++++++++++
 src/empty/empty.test.ts         | 24 ++++++++++++++++++++++++
 src/empty/index.ts              |  5 +++++
 src/empty/readme.md             |  3 +++
 5 files changed, 61 insertions(+)
 create mode 100644 src/empty/empty.fork.test.ts
 create mode 100644 src/empty/empty.test.ts
 create mode 100644 src/empty/index.ts
 create mode 100644 src/empty/readme.md

diff --git a/src/babel-plugin-factories.json b/src/babel-plugin-factories.json
index c633189f..e0b5aa2c 100644
--- a/src/babel-plugin-factories.json
+++ b/src/babel-plugin-factories.json
@@ -8,6 +8,7 @@
     "patronum/delay",
     "patronum/either",
     "patronum/equals",
+    "patronum/empty",
     "patronum/every",
     "patronum/format",
     "patronum/in-flight",
diff --git a/src/empty/empty.fork.test.ts b/src/empty/empty.fork.test.ts
new file mode 100644
index 00000000..95dbab25
--- /dev/null
+++ b/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);
+});
diff --git a/src/empty/empty.test.ts b/src/empty/empty.test.ts
new file mode 100644
index 00000000..f5ee62d7
--- /dev/null
+++ b/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);
+});
diff --git a/src/empty/index.ts b/src/empty/index.ts
new file mode 100644
index 00000000..d7aa6ae7
--- /dev/null
+++ b/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);
+}
diff --git a/src/empty/readme.md b/src/empty/readme.md
new file mode 100644
index 00000000..0ef6f467
--- /dev/null
+++ b/src/empty/readme.md
@@ -0,0 +1,3 @@
+# empty (experimental)
+
+> Since 1.10.0