feat: add Armstrong number algorithm

Maths/ArmstrongNumber.js

@@ -1,4 +1,26 @@
 /**
+ * @function isArmstrongNumber
+ * @description An Armstrong number (also known as a narcissistic number) is a
+ *   number that is equal to the sum of its own digits each raised to the power
+ *   of the number of digits. For example, 153 = 1³ + 5³ + 3³ = 153.
+ * @param {number} n - A positive integer
+ * @returns {boolean} - True if n is an Armstrong number, false otherwise
+ * @see https://en.wikipedia.org/wiki/Narcissistic_number
+ * @example isArmstrongNumber(153) => true
+ * @example isArmstrongNumber(9474) => true
+ * @example isArmstrongNumber(10) => false
+ */
+const isArmstrongNumber = (n) => {
+  if (!Number.isInteger(n) || n < 0) {
+    throw new TypeError('Input must be a non-negative integer')
+  }
+  const digits = String(n).split('')
+  const power = digits.length
+  const sum = digits.reduce((acc, d) => acc + Math.pow(Number(d), power), 0)
+  return sum === n
+}
+
+export { isArmstrongNumber } /**
  * Author: dephraiim
  * License: GPL-3.0 or later
  *

Maths/test/ArmstrongNumber.test.js

@@ -1,14 +1,45 @@
-import { armstrongNumber } from '../ArmstrongNumber'
+import { isArmstrongNumber } from '../ArmstrongNumber.js'
 
-describe('ArmstrongNumber', () => {
-  it('should return true for an armstrong number', () => {
-    expect(armstrongNumber(371)).toBeTruthy()
+describe('isArmstrongNumber', () => {
+  test('single digit numbers are always Armstrong numbers', () => {
+    for (let i = 0; i <= 9; i++) {
+      expect(isArmstrongNumber(i)).toBe(true)
+    }
   })
 
-  it('should return false for a non-armstrong number', () => {
-    expect(armstrongNumber(300)).toBeFalsy()
+  test('153 is an Armstrong number', () => {
+    expect(isArmstrongNumber(153)).toBe(true)
   })
-  it('should return false for negative values', () => {
-    expect(armstrongNumber(-2)).toBeFalsy()
+
+  test('370 is an Armstrong number', () => {
+    expect(isArmstrongNumber(370)).toBe(true)
+  })
+
+  test('371 is an Armstrong number', () => {
+    expect(isArmstrongNumber(371)).toBe(true)
+  })
+
+  test('407 is an Armstrong number', () => {
+    expect(isArmstrongNumber(407)).toBe(true)
+  })
+
+  test('9474 is a 4-digit Armstrong number', () => {
+    expect(isArmstrongNumber(9474)).toBe(true)
+  })
+
+  test('10 is NOT an Armstrong number', () => {
+    expect(isArmstrongNumber(10)).toBe(false)
+  })
+
+  test('100 is NOT an Armstrong number', () => {
+    expect(isArmstrongNumber(100)).toBe(false)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => isArmstrongNumber(-1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => isArmstrongNumber(1.5)).toThrow(TypeError)
   })
 })