Flatten Array ES6

flatten.js

/**
 * Flattens deeply nested array into simple array
 * @param  {Array}  arr   - input array of numbers
 * @param  {Array}  [acc] - accumulator to store resulting array during recursion calls
 * @return {Array}
 */
const flatten = (arr, acc = []) => {
  const isArray = Array.isArray(arr);

  // safe check for input
  if (!isArray) return arr;
  
  // Returns accumulator when there is no input items
  if (isArray && !arr.length) return acc;

  // get head and tail of input array
  const head = arr[0];
  const tail = arr.slice(1);

  // recursion call when input item is an array
  if (Array.isArray(head)) {
    return flatten(head.concat(tail), acc);
  }

  // store head into accumulator and process tail recursively
  return flatten(tail, acc.concat(head));
}

console.log(flatten([[1,2,[3]],4]));