feat: check if number is a harshad number

Author: ridge-kimani

Maths/isHarshad.js

@@ -0,0 +1,24 @@
+/**
+ * Wikipedia: https://en.wikipedia.org/wiki/Harshad_number
+ * A Harshad number (or Niven number) is a positive integer that is divisible by the sum of its own digits.
+ *
+ * The function checks if a number is a Harshad (Niven) number.
+ * @param {any} n - The number to check
+ * @returns {boolean} - true if Harshad, false otherwise
+ * @example isHarshad(2025) // true
+ */
+export const isHarshad = (n) => {
+  const num = Number(n)
+
+  if (num <= 0 || !Number.isInteger(num)) return false
+
+  let sum = 0,
+    temp = n
+
+  while (temp > 0) {
+    sum += temp % 10
+    temp = Math.floor(temp / 10)
+  }
+
+  return n % sum === 0
+}

Maths/test/isHarshad.test.js

@@ -0,0 +1,31 @@
+import { isHarshad } from '../isHarshad.js'
+
+describe('Harshad number tests', () => {
+  test('Basic Harshad numbers', () => {
+    expect(isHarshad(18)).toBe(true)
+    expect(isHarshad(21)).toBe(true)
+    expect(isHarshad(12)).toBe(true)
+    expect(isHarshad(6804)).toBe(true)
+    expect(isHarshad('18')).toBe(true)
+    expect(isHarshad(2025)).toBe(true)
+  })
+
+  test('Non-Harshad numbers', () => {
+    expect(isHarshad(19)).toBe(false)
+    expect(isHarshad(25)).toBe(false)
+    expect(isHarshad(97)).toBe(false)
+  })
+
+  test('Edge cases', () => {
+    expect(isHarshad(0)).toBe(false)
+    expect(isHarshad(-18)).toBe(false)
+    expect(isHarshad(1)).toBe(true)
+    expect(isHarshad(10)).toBe(true)
+  })
+
+  test('Input validation', () => {
+    expect(isHarshad(18.5)).toBe(false)
+    expect(isHarshad(null)).toBe(false)
+    expect(isHarshad(undefined)).toBe(false)
+  })
+})