const permute = (input = [], permutation = []) => {
    if (input.length === 0) return [permutation]; // this will be one of the result

    // choose each number in a loop
    return input.reduce((allPermutations, current) => {
        // reduce the input by removing the current element
        // as we'll fix it by putting it in `permutation` array
        const rest = input.filter(n => n != current);
        return [
            ...allPermutations,
            // fixing our choice in the 2nd arg
            // by concatenationg current with permutation
            ...permute(rest, [...permutation, current])
        ];
    }, []);
}