arrayDiff = (arr1, arr2) => {
    const a = arr1 || [];
    const b = arr2 || [];

    return a
        .filter(x => !elContains(b, x))
        .concat(b.filter(x => !elContains(a, x)));
}
Symmetric difference between 2 arrays
@param {any[]} arr1
@param {any[]} arr2