Flatten array of arrays (multidimensional) in ES6

flatten.js

var arr = [[1,2,[3]],4,[5,6,[7],8,[9,[10,11,12,[13]]]]];

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

flatten(arr);