Javascript - Compare two arrays and return a new array with any items not found in both of the original arrays.

differt.js

function diff(arr1, arr2) {
  return newArr = arr1.filter(function(val){
    return arr2.indexOf(val) === -1;
  }).concat(arr2.filter(function(val){
    return arr1.indexOf(val) === -1;
  }));
}

diff(['diorite', 'andesite', 'grass', 'dirt', 'pink wool', 'dead shrub'], ['diorite', 'andesite', 'grass', 'dirt', 'dead shrub']);