function findHCF (x, y) {
  // If the input numbers are less than 1 return an error message.
  if (x < 1 || y < 1) {
    return 'Please enter values greater than zero.'
  }

  // If the input numbers are not integers return an error message.
  if (x !== Math.round(x) || y !== Math.round(y)) {
    return 'Please enter whole numbers.'
  }

  // Now apply Euclid's algorithm to the two numbers.
  while (Math.max(x, y) % Math.min(x, y) !== 0) {
    if (x > y) {
      x %= y
    } else {
      y %= x
    }
  }

  // When the while loop finishes the minimum of x and y is the HCF.
  return Math.min(x, y)
}

var num1 = 27;
var num2 = 36;

console.log('HCF of ' + num1 + ' and ' + num2 + ' is ' + findHCF(num1, num2));