added: [N4S] isNull rule. (#404)

packages/n4s/docs/rules.md

@@ -32,6 +32,8 @@ Enforce rules are functions that allow you to test your data against different c
 - [isNotNumber](#isnotnumber)
 - [isNaN](#isNaN)
 - [isNotNaN](#isNotNaN)
+- [isNull](#isnull)
+- [isNotNull](#isnotnull)
 - [isString](#isstring)
 - [isNotString](#isnotstring)
 - [isUndefined](#isundefined)
@@ -886,6 +888,45 @@ enforce('A' / 'B').isNaN();
 // throws
 ```
 
+
+## isNull
+
+### Description
+
+Enforces that a specified value is `null`.
+
+### Usage examples:
+
+```js
+enforce(null).isNull();
+// passes
+```
+
+```js
+enforce(undefined).isNull();
+enforce(true).isNull();
+// throws
+```
+
+## isNotNull
+
+### Description
+
+Reverse implementation of `isNull`. Checks that a value is not null.
+
+### Usage examples:
+
+```js
+enforce("hello").isNull();
+enforce(200).isNull();
+// passes
+```
+
+```js
+enforce(null).isNull();
+// throws
+```
+
 ## isString
 
 ### Description

packages/n4s/src/rules/index.js

@@ -7,6 +7,7 @@ import isArray from './isArray';
 import isEmpty from './isEmpty';
 import isEven from './isEven';
 import isNaN from './isNaN';
+import isNull from './isNull';
 import isNumber from './isNumber';
 import isNumeric from './isNumeric';
 import isOdd from './isOdd';
@@ -32,6 +33,7 @@ const rules = {
   isEmpty,
   isEven,
   isNaN,
+  isNull,
   isNumber,
   isNumeric,
   isOdd,

packages/n4s/src/rules/isNull/index.js

@@ -0,0 +1,7 @@
+function isNull(value) {
+  return value === null;
+}
+
+isNull.negativeForm = 'isNotNull';
+
+export default isNull;

packages/n4s/src/rules/isNull/spec.js

@@ -0,0 +1,29 @@
+import isNull from '.';
+
+describe('Tests isNull rule', () => {
+  it('Should return true for `null` value', () => {
+    expect(isNull(null)).toBe(true);
+  });
+
+  it.each([
+    undefined,
+    NaN,
+    false,
+    true,
+    Object,
+    Array(0),
+    '',
+    ' ',
+    0,
+    1,
+    '0',
+    '1',
+    () => {},
+  ])('Should return false for %s value', v => {
+    expect(isNull(v)).toBe(false);
+  });
+
+  it('Should expose negativeForm property', () => {
+    expect(isNull.negativeForm).toBe('isNotNull');
+  });
+});

packages/n4s/src/rules/spec.js

@@ -25,6 +25,7 @@ const negativeRules = [
   'notInside',
   'notMatches',
   'numberNotEquals',
+  'isNotNull',
 ];
 
 const positiveRules = [
@@ -56,6 +57,7 @@ const positiveRules = [
   'numberEquals',
   'shorterThan',
   'shorterThanOrEquals',
+  'isNull',
 ];
 
 const allRules = [...new Set([...positiveRules, ...negativeRules])];