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feat: add Harshad number algorithm #1985

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feat: add Harshad number algorithm #1985

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23 changes: 23 additions & 0 deletions Maths/HarshadNumber.js
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Original file line number Diff line number Diff line change
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/**
* @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 }
51 changes: 51 additions & 0 deletions Maths/test/HarshadNumber.test.js
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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)
})
})
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