Which part don’t you understand?? The ES6 feature used in the return statement of the function diffArray is called array destructuring the spread operator, you can read up more on it on google.
And the filter array method is also quite easy to understand, you can read more about it on MDN or in dozens of articles.
If you have a question about some particular part of the code that you don’t understand or some thing that confuses use it would help a lot if you were able to articulate that so we don’t have to guess Thanks!
It is returning an array that contains all the elements in the array returned by the diff function. The spread operator ... copy the elements one by one, so you get a copy of the two arrays. In this case it is the same as writing diff(arr1, arr2).concat(diff(arr2, arr1))