const flatten = (array) => {
  // Concat nested arrays recursively
  const f = (arr, acc) => {
    if(arr.length > 0) {
      const first = arr[0];

      if(Array.isArray(first)) {
        // Item is an array, start new call
        acc = acc.concat(f(first, []));
      } else {
        // Add iem to accumulator
        acc.push(first);
      }

      // Continue with next item
      return f(arr.slice(1), acc);
    } else {
      // Nothing to do, return accumulator
      return acc;
    }
  };

  // Start
  return f(array, []);
};

// console.log('Result:',flatten([1, [2, 3], 4, [5, [6, [7, 8]]], 9, 0]));

module.exports = flatten;