euclidGCD.js

function gcd(a, b) {
  if (a < 0 || b < 0) throw Error ('positive numbers only');
  
  if (a === 0 || b === 0)
    return Math.max(a, b);
  else if (a >= b)
    return gcd(a % b, b);
  else
    return gcd(a, b % a);
}

console.log(gcd(15, 10));