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

bonfire-steamroller.js

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

//Flatten a nested array. You must account for varying levels of nesting.

function steamroller(arr) {
  // I'm a steamroller, baby
    var flattenedArray = [];

  // Create function that adds an element if it is not an array.
  // If it is an array, then loops through it and uses recursion on that array.
  var flatten = function(arg) {
    if (!Array.isArray(arg)) {
      flattenedArray.push(arg);
    } else {
      for (var a in arg) {
        flatten(arg[a]);
      }
    }
  };

  // Call the function for each element in the array
  arr.forEach(flatten);
  return flattenedArray;
}

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