Edit in JSFiddle

function diff(arr1, arr2) {
  var newArr = arr1.concat(arr2);
  console.log(newArr);
  //Outputs [1, 2, 3, 5, 1, 2, 3, 4, 5]
  var filteredArr = newArr.filter(function(elem, pos) {
    return newArr.indexOf(elem) === pos;
  })
  console.log(filteredArr);
  //Outputs [1, 2, 3, 5, 4]
}
diff([1, 2, 3, 5], [1, 2, 3, 4, 5]);