Add is undefined tests and benchmarks

src/isUndefined.bench.ts

@@ -0,0 +1,26 @@
+import _isUndefined from 'lodash/isUndefined'
+import {bench, describe} from 'vitest'
+
+import {isUndefined} from './isUndefined'
+
+const testCases = [null, 0, '', false, {}, [], 'undefined', NaN]
+
+const ITERATIONS = 1000
+
+describe('isUndefined performance', () => {
+    bench('hidash', () => {
+        for (let i = 0; i < ITERATIONS; i++) {
+            testCases.forEach((testCase) => {
+                isUndefined(testCase)
+            })
+        }
+    })
+
+    bench('lodash', () => {
+        for (let i = 0; i < ITERATIONS; i++) {
+            testCases.forEach((testCase) => {
+                _isUndefined(testCase)
+            })
+        }
+    })
+})

src/isUndefined.test.ts

@@ -0,0 +1,51 @@
+import _isUndefined from 'lodash/isUndefined'
+import {expect, describe, it} from 'vitest'
+
+import {isUndefined} from './isUndefined'
+
+describe('isUndefined - direct behavior', () => {
+    it('returns true for undefined', () => {
+        expect(isUndefined(undefined)).toBe(true)
+    })
+
+    it('returns false for null', () => {
+        expect(isUndefined(null)).toBe(false)
+    })
+
+    it('returns false for 0', () => {
+        expect(isUndefined(0)).toBe(false)
+    })
+
+    it('returns false for empty string', () => {
+        expect(isUndefined('')).toBe(false)
+    })
+
+    it('returns false for false', () => {
+        expect(isUndefined(false)).toBe(false)
+    })
+
+    it('returns false for empty object', () => {
+        expect(isUndefined({})).toBe(false)
+    })
+
+    it('returns false for empty array', () => {
+        expect(isUndefined([])).toBe(false)
+    })
+
+    it('returns false for "undefined" string', () => {
+        expect(isUndefined('undefined')).toBe(false)
+    })
+
+    it('returns false for NaN', () => {
+        expect(isUndefined(NaN)).toBe(false)
+    })
+})
+
+describe('isUndefined - lodash parity', () => {
+    it('matches lodash for various defined values', () => {
+        const testCases = [null, 0, '', false, {}, [], 'undefined', NaN]
+        testCases.forEach((value) => {
+            expect(isUndefined(value)).toBe(_isUndefined(value))
+        })
+    })
+})

src/isUndefined.ts

@@ -1,3 +1,23 @@
+/**
+ * Checks whether the given value is strictly `undefined`.
+ *
+ * This function returns `true` only if the input is exactly `undefined`, and `false` for all other values,
+ * including `null`, `0`, `false`, empty strings, and other falsy values.
+ *
+ * It also serves as a TypeScript type guard, narrowing the type of the input to `undefined` when true.
+ *
+ * @param value - The value to check.
+ * @returns Whether the value is `undefined`.
+ *
+ * @example
+ * const a = undefined;
+ * const b = null;
+ * const c = 0;
+ *
+ * console.log(isUndefined(a)); // true
+ * console.log(isUndefined(b)); // false
+ * console.log(isUndefined(c)); // false
+ */
 export function isUndefined(value: unknown): value is undefined {
     return value === undefined
 }