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];