Flatten array

flattenArray.js

const array = [1, [2, [3]], 4, []];

const flatter = (array) => {
  let flatArray = [];
  array.forEach((item, index) => {
    if (Array.isArray(item)) {
      flatArray = flatArray.concat(flatter(array[index]));
    } else {
      flatArray.push(item);
    }
  });

  return flatArray;
};

console.log(flatter(array));