From afe39c0301f9d979d76725ef741025e48162649a Mon Sep 17 00:00:00 2001
From: sarthakgupta1907-cell <sarthakgupta1907@gmail.com>
Date: Sat, 7 Nov 2026 02:18:12 +0530
Subject: [PATCH] feat: add Harshad number algorithm

---
 Maths/HarshadNumber.js           | 23 ++++++++++++++
 Maths/test/HarshadNumber.test.js | 51 ++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 Maths/HarshadNumber.js
 create mode 100644 Maths/test/HarshadNumber.test.js

diff --git a/Maths/HarshadNumber.js b/Maths/HarshadNumber.js
new file mode 100644
index 0000000000..15d589a70b
--- /dev/null
+++ b/Maths/HarshadNumber.js
@@ -0,0 +1,23 @@
+/**
+ * @function isHarshadNumber
+ * @description A Harshad number (also called Niven number) is an integer
+ *   that is divisible by the sum of its digits.
+ *   For example, 18 is a Harshad number because 1+8=9 and 18/9=2.
+ * @param {number} n - A positive integer
+ * @returns {boolean} - True if n is a Harshad number, false otherwise
+ * @see https://en.wikipedia.org/wiki/Harshad_number
+ * @example isHarshadNumber(18) => true
+ * @example isHarshadNumber(21) => true
+ * @example isHarshadNumber(19) => false
+ */
+const isHarshadNumber = (n) => {
+  if (!Number.isInteger(n) || n < 1) {
+    throw new TypeError('Input must be a positive integer')
+  }
+  const digitSum = String(n)
+    .split('')
+    .reduce((sum, d) => sum + Number(d), 0)
+  return n % digitSum === 0
+}
+
+export { isHarshadNumber }
diff --git a/Maths/test/HarshadNumber.test.js b/Maths/test/HarshadNumber.test.js
new file mode 100644
index 0000000000..ea3100cb83
--- /dev/null
+++ b/Maths/test/HarshadNumber.test.js
@@ -0,0 +1,51 @@
+import { isHarshadNumber } from '../HarshadNumber.js'
+
+describe('isHarshadNumber', () => {
+  test('1 is a Harshad number', () => {
+    expect(isHarshadNumber(1)).toBe(true)
+  })
+
+  test('18 is a Harshad number (1+8=9, 18/9=2)', () => {
+    expect(isHarshadNumber(18)).toBe(true)
+  })
+
+  test('21 is a Harshad number (2+1=3, 21/3=7)', () => {
+    expect(isHarshadNumber(21)).toBe(true)
+  })
+
+  test('12 is a Harshad number (1+2=3, 12/3=4)', () => {
+    expect(isHarshadNumber(12)).toBe(true)
+  })
+
+  test('100 is a Harshad number (1+0+0=1, 100/1=100)', () => {
+    expect(isHarshadNumber(100)).toBe(true)
+  })
+
+  test('111 is a Harshad number (1+1+1=3, 111/3=37)', () => {
+    expect(isHarshadNumber(111)).toBe(true)
+  })
+
+  test('19 is NOT a Harshad number (1+9=10, 19/10 is not integer)', () => {
+    expect(isHarshadNumber(19)).toBe(false)
+  })
+
+  test('11 is NOT a Harshad number', () => {
+    expect(isHarshadNumber(11)).toBe(false)
+  })
+
+  test('23 is NOT a Harshad number', () => {
+    expect(isHarshadNumber(23)).toBe(false)
+  })
+
+  test('throws TypeError for zero', () => {
+    expect(() => isHarshadNumber(0)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => isHarshadNumber(-18)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => isHarshadNumber(18.5)).toThrow(TypeError)
+  })
+})