Flatten a nested array

flatten.js

const flatten = (arr) => {
  return Array.isArray(arr) ? [].concat(...arr.map(flatten)) : arr;
}

let ar = [[1,2,3], [4, [5, 6]], [[7, [8, [9]]]]]; // [1, 2, 3, 4, 5, 6, 7, 8, 9]