/**
 * Flattens an array of arbitrarily nested arrays into a flat array.
 * E.g. [[1,2,[3]],4] -> [1,2,3,4].
 * @param {array} array Array to be flatten.
 */
function flatten(array) {
  array = array.reduce((acc, x) => acc.concat(x), []);
  return array.every(x => Array.isArray(x) === false) === true
    ? array
    : flatten(array)
}