flatten [1, [2], 3, [4, 5, [6, 7, [8, 9, 10]]]] => [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

flatten.js

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

module.exports = {
    flatten: flatten
}

// let list = [1, [2], 3, [4, 5, [6, 7, [8, 9, 10]]]]
// console.log(flatten(list));