const relativeComplement = function (array1, array2){
    array2 = array2.splice(0);
    return array1.filter((v,i,a) => {
        let index = array2.indexOf(v);
        if (index > -1) {
            array2.splice(index, 1);
            return false;
        }
        return !array2.includes(v);
    });
};