JS: Symmetric Difference of 2 Arrays

function diff(arr1, arr2) {
  return arr1.filter(function(val){
      return arr2.indexOf(val) < 0;
    }).concat(
    arr2.filter(function(val){
      return arr1.indexOf(val) < 0;
    })
  );
}

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