A recursive flatten function for arbitrary nested arrays. Stack is the limit :-)

flatten.js

/**
 * Flatten arbitrary nested arrays.
 *
 * @arg the array to flatten
 * @return array with depth 1, empty array for non-arrays
 */
const flatten = (arrayOfArrays = []) => {
  if (arrayOfArrays.length === 0) {
    return []
  }
  
  const head = arrayOfArrays[0]
  const tail = flatten(arrayOfArrays.slice(1))
  
  if (Array.isArray(head)) {
    if (head.length > 0) {
      return [...flatten(head), ...tail]
    } else {
      return tail
    }
  } else {
    return [head, ...tail]
  }
}