A set of method to flatten arrays

flatten.js

const makeFlat = (array) => {
  return array.reduce((all, current) => {
    return Array.isArray(current)
      ? all.concat(makeFlat(current))
      : all.push(current) && all;
  },[]);
};

expect(makeFlat([[1,2,[3]],4])).toEqual([1,2,3,4]);