flatten an arbitrarily nested array

flatten_neo.js

const flatten = (arr) => {
  const _helper = (arr) => arr.reduce(
    (acc, itm) => Array.isArray(itm) ? [...acc, ..._helper(itm)] : [...acc, itm], []);
  
  return _helper(arr);
}

// upgrade to old version https://gist.github.com/kra3/f802061437e3efe062ab2d4c047fe7c0
// still, I advocate to use underscore, lodash or ramda ;) they are more battle tested

console.log(flatten([[1,2,[3]],4]));
console.log(flatten([1, [2, [3, 4]]]).length === 4);