diff.js

// Bonfire: Diff Two Arrays
// Author: @codebubb
// Challenge: http://www.freecodecamp.com/challenges/bonfire-diff-two-arrays?solution=function%20diff(arr1%2C%20arr2)%20%7B%0A%20%20var%20newArr%20%3D%20arr1.concat(arr2)%3B%0A%20%20return%20newArr.filter(function(elem%2C%20index)%7B%0A%20%20%20%20%20return%20newArr.lastIndexOf(elem)%20%3D%3D%3D%20index%20%26%26%20newArr.indexOf(elem)%20%3D%3D%3D%20index%3B%0A%20%20%7D)%3B%0A%7D
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function diff(arr1, arr2) {
  var newArr = arr1.concat(arr2); // Maybe this could be incorporated inside the filter function? I don't know how!
  return newArr.filter(function(elem, index){
     return newArr.lastIndexOf(elem) === index && newArr.indexOf(elem) === index;
  });
}