flattenArray.js

// [1] => 1
// [1, [2,3,4]] => [1,2,3,4]
// [1, [2,3, [4,5]]] => [1,2,3,4,5]

const flatten = (arr) =>
  arr.reduce((acc, next) =>
    acc.concat(Array.isArray(next) ? flatten(next) : next), []);

const data = [1, [2, 3, [4, 5]]];
flatten(data);