Edit in JSFiddle

function diff(arr1, arr2) {
  var newArr = arr1.filter(function(elem) {
    return arr2.indexOf(elem) === -1;
    //returns an empty array since arr1 has the same elements of arr2
  }).concat(arr2.filter(function(elem) {
    return arr1.indexOf(elem) === -1;
    //returns array that has elements that equal false and then concates that value to newArray
  }));
  return newArr;
}
diff([1, 2, 3, 5], [1, 2, 3, 4, 5]); //Outputs [4];