feat: add narcissistic numbers algorithm

Maths/NarcissisticNumber.js

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+/**
+ * @function isNarcissisticNumber
+ * @description A narcissistic number (also known as an Armstrong number)
+ *   for a given number of digits n is a number that equals the sum of
+ *   its digits each raised to the power n. This function checks all
+ *   narcissistic numbers up to a given limit.
+ * @param {number} limit - A positive integer upper limit
+ * @returns {number[]} - Array of all narcissistic numbers up to limit
+ * @see https://en.wikipedia.org/wiki/Narcissistic_number
+ * @example getNarcissisticNumbers(500) => [1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407]
+ * @example getNarcissisticNumbers(10) => [1, 2, 3, 4, 5, 6, 7, 8, 9]
+ */
+const getNarcissisticNumbers = (limit) => {
+  if (!Number.isInteger(limit) || limit < 1) {
+    throw new TypeError('Input must be a positive integer')
+  }
+  const result = []
+  for (let n = 1; n <= limit; n++) {
+    const digits = String(n).split('')
+    const power = digits.length
+    const sum = digits.reduce((acc, d) => acc + Math.pow(Number(d), power), 0)
+    if (sum === n) result.push(n)
+  }
+  return result
+}
+
+export { getNarcissisticNumbers }

Maths/test/NarcissisticNumber.test.js

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+import { getNarcissisticNumbers } from '../NarcissisticNumber.js'
+
+describe('getNarcissisticNumbers', () => {
+  test('single digits 1-9 are all narcissistic', () => {
+    expect(getNarcissisticNumbers(9)).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9])
+  })
+
+  test('getNarcissisticNumbers(10) returns [1..9]', () => {
+    expect(getNarcissisticNumbers(10)).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9])
+  })
+
+  test('153 is narcissistic', () => {
+    const result = getNarcissisticNumbers(153)
+    expect(result).toContain(153)
+  })
+
+  test('370 is narcissistic', () => {
+    const result = getNarcissisticNumbers(370)
+    expect(result).toContain(370)
+  })
+
+  test('371 is narcissistic', () => {
+    const result = getNarcissisticNumbers(371)
+    expect(result).toContain(371)
+  })
+
+  test('407 is narcissistic', () => {
+    const result = getNarcissisticNumbers(407)
+    expect(result).toContain(407)
+  })
+
+  test('getNarcissisticNumbers(500) returns correct list', () => {
+    expect(getNarcissisticNumbers(500)).toEqual([
+      1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407
+    ])
+  })
+
+  test('100 is NOT narcissistic', () => {
+    const result = getNarcissisticNumbers(100)
+    expect(result).not.toContain(100)
+  })
+
+  test('throws TypeError for zero', () => {
+    expect(() => getNarcissisticNumbers(0)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => getNarcissisticNumbers(-5)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => getNarcissisticNumbers(9.5)).toThrow(TypeError)
+  })
+})