Flatten Array

function flatten(array) {
  const flattered = [];
  array.forEach(item => {
    if (Array.isArray(item)) {
      flattered.push(...flatten(item));
    } else {
      flattered.push(item);
    }
  })
  return flattered;
}

function flatten2(array) {
  return array.reduce((acc, item) => {
    Array.isArray(item) ? acc.push(...flatten(item)) : acc.push(item);
    return acc;
  }, []);
}

const testArray = [[[1,2,[3,5,[554,5,[1, []],6]]],4]];
console.log('forEach', flatten(testArray));
console.log('reduce', flatten2(testArray));