Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flatten.js

const arrayFlatten = (arr) => {
	if (!Array.isArray(arr))return 'The input provided is not an array';
	return arr.reduce((x, y) => {
	if (Array.isArray(y)) return x.concat(arrayFlatten(y));
	return x.concat(y)
	}, [])
}