[FEATURE]: Add Harshad numbers #1802

Hi! I noticed that the previous PR (#1801) was closed without merging. I’d love to take up this issue and implement the Harshad number checker for Hacktoberfest 2025. Could you please assign it to me?

I have worked on the Harshad number checker and created a pull request with the implementation and tests. Please consider my contribution for this issue.

function isHarshad(num) {
if (num <= 0) return false; // Harshad numbers are typically positive integers

// Convert number to string to iterate over digits, then sum them
const sumOfDigits = String(num)
    .split('')
    .reduce((sum, digit) => sum + parseInt(digit, 10), 0);

// Check divisibility
return num % sumOfDigits === 0;

}

// Test cases
console.log(isHarshad(18)); // true
console.log(isHarshad(19)); // false
console.log(isHarshad(1)); // true
console.log(isHarshad(10)); // true
console.log(isHarshad(12)); // true (1+2=3, 12/3=4)