flatArray.js

const arrayToFlat = [[1, 2, [3]], 4];
const arrayToFlat2 = [[1, 2, [3, [4]]], 5];
const arrayToFlat3 = [[1, 2, [3, [4, [5, 6, [7]]]]], 8];

const flatArray = arr => {
  return arr.reduce(
    (acc, curr) =>
      Array.isArray(curr) ? acc.concat(flatArray(curr)) : acc.concat(curr),
    []
  );
};

console.log(flatArray(arrayToFlat));
console.log(flatArray(arrayToFlat2));
console.log(flatArray(arrayToFlat3));