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

bonfire-symmetric-difference.js

// Bonfire: Symmetric Difference
// Author: @nirajkrz
// Challenge: http://www.freecodecamp.com/challenges/bonfire-symmetric-difference
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function sym() {

  // Convert the argument object into a proper array
  var args = Array.prototype.slice.call(arguments);

  // Return the symmetric difference of 2 arrays
  var getDiff = function(arr1, arr2) {

    // Returns items in arr1 that don't exist in arr2
    function filterFunction(arr1, arr2) {
      return arr1.filter(function(item) {
        return arr2.indexOf(item) === -1;
      });
    }

    // Run filter function on each array against the other then get unique values
    return filterFunction(arr1, arr2)
      .concat(filterFunction(arr2, arr1))
      .filter(function(item, idx, arr) {
        // Keep any items that are unique - the index of the current item === index of the first occurrence in the array
        return arr.indexOf(item) === idx;
      });
  };

  // Reduce all arguments getting the difference of them
  return args.reduce(getDiff, []);
}

sym([1, 2, 3], [5, 2, 1, 4]);