steamroller.js

function steamroller(arr) {
  // I'm a steamroller, baby
  function flatReduce(collection) {
    return collection.reduce((acc, curr) => {
      return acc.concat(Array.isArray(curr) ? flatReduce(curr) : curr);
    }, []);
  }
  return flatReduce(arr);
}

steamroller([1, [2], [3, [[4]]]]);