algorithm: Hexagonal number

Maths/HexagonalNumber.js

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+/*
+ * Author: Akshay Dubey (https://github.com/itsAkshayDubey)
+ * Hexagonal Number: https://en.wikipedia.org/wiki/Hexagonal_number
+ * The nth hexagonal number hn is the number of distinct dots in a pattern of dots
+ * consisting of the outlines of regular hexagons with sides up to n dots, when the
+ * hexagons are overlaid so that they share one vertex.
+ */
+
+/**
+ * @function hexagonalNumber
+ * @description -> returns nth hexagonal number
+ * @param {Integer} number
+ * @returns {Integer} nth hexagonal number
+ */
+
+export const hexagonalNumber = (number) => {
+  if (number <= 0) {
+    throw new Error('Number must be greater than zero.')
+  }
+  return number * (2 * number - 1)
+}

Maths/test/HexagonalNumber.test.js

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+import { hexagonalNumber } from '../HexagonalNumber'
+
+const expectedValuesArray = [1, 6, 15, 28, 45, 66, 91, 120, 153, 190, 231, 276, 325, 378, 435, 496, 561, 630, 703, 780, 861, 946]
+
+describe('Testing hexagonalNumber', () => {
+  for (let i = 1; i <= 22; i++) {
+    it('Testing for number = ' + i + ', should return ' + expectedValuesArray[i], () => {
+      expect(hexagonalNumber(i)).toBe(expectedValuesArray[i - 1])
+    })
+  }
+
+  it('should throw error when supplied negative numbers', () => {
+    expect(() => { hexagonalNumber(-1) }).toThrow(Error)
+  })
+
+  it('should throw error when supplied zero', () => {
+    expect(() => { hexagonalNumber(0) }).toThrow(Error)
+  })
+})