http://www.freecodecamp.com/narshe1412 's solution for Bonfire:Steamroller

Steamroller.js

function steamroller(arr) {
  // I'm a steamroller, baby
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? steamroller(toFlatten) : toFlatten); //ternary
  }, []);
  
  /*
  Uses the same concat flatten example on the MDN, but with recursion to further flatten
    ex:
    var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
      return a.concat(b);
      }, []);
   In this case b is checked if is still an array, then steamroller is called again if it is.
   Otherwise it's just returned
  */
}

References:
http://stackoverflow.com/questions/10865025/merge-flatten-an-array-of-arrays-in-javascript
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat