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

bonfire-sorted-union.js

// Bonfire: Sorted Union
// Author: @minsooshin
// Challenge: http://www.freecodecamp.com/challenges/bonfire-sorted-union
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function unite(arr1, arr2, arr3) {
  var args = Array.prototype.slice.call(arguments);
  arr1 = args.reduce(function(old, cur) {
    var obj = {}, diff;
    for (var i = 0; i < old.length; i++) {
      obj[old[i]] = true;
    }
    diff = cur.filter(function(val) {
      return !obj[val];
    });
    return old.concat(diff);
  });
  return arr1;
}

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