flatten.js

const test = [[1, [2, [3]]], 4];

const dig = (stack, object) =>
  Array.isArray(object) ? flatten(object, stack) : stack.concat(object);

const flatten = (input, output = []) => input.reduce(dig, output);

console.clear();
console.log(flatten(test)); // [1, 2, 3, 4]