### arrayGcd

Calculates the greatest common denominator (gcd) of an array of numbers.

Use `Array.reduce()` and the `gcd` formula (uses recursion) to calculate the greatest common denominator of an array of numbers.

```js
const arrayGcd = arr => {
  const gcd = (x, y) => (!y ? x : gcd(y, x % y));
  return arr.reduce((a, b) => gcd(a, b));
};
```

```js
arrayGcd([1, 2, 3, 4, 5]); // 1
arrayGcd([4, 8, 12]); // 4
```