http://www.freecodecamp.com/minsooshin 's solution for Bonfire: Steamroller

bonfire-steamroller.js

// Bonfire: Steamroller
// Author: @minsooshin
// Challenge: http://www.freecodecamp.com/challenges/bonfire-steamroller
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function steamroller(arr) {
  // I'm a steamroller, baby
  
  function flatten(array, newArr) {
    if (newArr === undefined) newArr = [];
  
    if (Array.isArray(array)) {
      array.forEach(function(index) {
        flatten(index, newArr);
      });
    } else {
      newArr.push(array);
    }
    return newArr;
  }
  
  return flatten(arr);
}

steamroller([[["a"]], [["b"]]]);   //=> ["a", "b"]
steamroller([1, [2], [3, [[4]]]]); //=> [1, 2, 3, 4]
steamroller([1, [], [3, [[4]]]]);  //=> [1, 3, 4]
steamroller([1, {}, [3, [[4]]]]);  //=> [1, {}, 3, 4]