FreeCodeCamp Steamroller Sample Solution

function steamrollArray(arr) {
  // I'm a steamroller, baby
  var flattened = arr.reduce(
    function(a, b) {
      return a.concat(Array.isArray(b) ? steamrollArray(b) : b);
    },[]);
  return flattened;
}

steamrollArray([[["a"]], [["b"]]]);