Arbitrary flattens a nested array.

flatten.js

/**
 * 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) {
  /**
   * Checks if the given array has any nested array.
   * @param {array} array Array to check if it is flat.
   */
  const isFlat = (array) => array.every(x => Array.isArray(x) === false)
  array = array.reduce((acc, x) => acc.concat(x), [])
  return isFlat(array) ? array : flatten(array)
}