feat: add isFalsy

Ref #66

src/index.d.ts

@@ -633,6 +633,12 @@ declare namespace RamdaAdjunct {
          */
         isTruthy(val: any): boolean;
 
+        /**
+         * A falsy value is a value that translates to false when evaluated in a Boolean context.
+         * Falsy values are `false`, `0`, `""`, `null`, `undefined`, and `NaN`.
+         */
+        isFalsy(val: any): boolean;
+
          /**
          * Identity type.
          */

src/index.js

@@ -60,6 +60,7 @@ export { default as isNotPair } from './isNotPair';
 export { default as isThenable } from './isThenable';
 export { default as isPromise } from './isPromise';
 export { default as isTruthy } from './isTruthy';
+export { default as isFalsy } from './isFalsy';
 // Function
 export { default as stubUndefined } from './stubUndefined';
 export { default as stubNull } from './stubNull';

src/isFalsy.js

@@ -0,0 +1,29 @@
+import { complement } from 'ramda';
+
+import isTruthy from './isTruthy';
+
+
+/**
+ * A falsy value is a value that translates to false when evaluated in a Boolean context.
+ * Falsy values are `false`, `0`, `""`, `null`, `undefined`, and `NaN`.
+ *
+ * @func isFalsy
+ * @memberOf RA
+ * @since {@link https://char0n.github.io/ramda-adjunct/2.2.0|v2.2..0}
+ * @category Type
+ * @sig * -> Boolean
+ * @param {*} val The value to test
+ * @return {Boolean}
+ * @see {@link https://developer.mozilla.org/en-US/docs/Glossary/Falsy|falsy}, {@link RA.isTruthy|isTruthy}
+ * @example
+ *
+ * RA.isFalsy(false); // => true
+ * RA.isFalsy(0); // => true
+ * RA.isFalsy(''); // => true
+ * RA.isFalsy(null); // => true
+ * RA.isFalsy(undefined); // => true
+ * RA.isFalsy(NaN); // => true
+ */
+const isFalsy = complement(isTruthy);
+
+export default isFalsy;

src/isTruthy.js

@@ -13,7 +13,7 @@ import { pipe, equals } from 'ramda';
  * @sig * -> Boolean
  * @param {*} val The value to test
  * @return {Boolean}
- * @see {@link RA.isFalsy|isFalsy}
+ * @see {@link https://developer.mozilla.org/en-US/docs/Glossary/Truthy|truthy}, {@link RA.isFalsy|isFalsy}
  * @example
  *
  * RA.isTruthy({}); // => true

test/isFalsy.js

@@ -0,0 +1,31 @@
+import * as RA from '../src/index';
+import eq from './shared/eq';
+import Symbol from './shared/Symbol';
+import args from './shared/arguments';
+
+
+describe('isFalsy', function() {
+  it('tests a value for `falsy`', function() {
+    eq(RA.isFalsy(false), true);
+    eq(RA.isFalsy(0), true);
+    eq(RA.isFalsy(''), true);
+    eq(RA.isFalsy(null), true);
+    eq(RA.isFalsy(undefined), true);
+    eq(RA.isFalsy(NaN), true);
+
+    eq(RA.isFalsy('abc'), false);
+    eq(RA.isFalsy(Object('abc')), false);
+    eq(RA.isFalsy(args), false);
+    eq(RA.isFalsy([1, 2, 3]), false);
+    eq(RA.isFalsy(new Date()), false);
+    eq(RA.isFalsy(new Error()), false);
+    eq(RA.isFalsy(Array.prototype.slice), false);
+    eq(RA.isFalsy({ 0: 1, length: 1 }), false);
+    eq(RA.isFalsy(1), false);
+    eq(RA.isFalsy(/x/), false);
+    eq(RA.isFalsy(Symbol), RA.isUndefined(Symbol));
+    eq(RA.isFalsy({}), false);
+    eq(RA.isFalsy([]), false);
+    eq(RA.isFalsy(Infinity), false);
+  });
+});