FCC - Bonfire: Sorted Union.js

function unite(arr1, arr2, arr3) {
  var unique = arr1.concat(arr2, arr3);
  console.log("arr:");
  console.log(unique);
  
  for (var i = 0; i != unique.length; ++i) {
    unique.reduce(function(isDuplicate, item, index, arr) {
        if (index != i && unique[i] === item) {
            delete unique[index];
        }
    }, 0);
  }
  var result = [];
  for (var key in unique)
    if (Boolean(unique[key]))
        result.push(unique[key]);
  return result;
}

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