function foldl(f, acc, [x, ...xs]) {
    return typeof x === 'undefined' ? acc : foldl(f, f(acc, x), xs);
}

foldl((x, y) => x * y, 1, [2, 3, 4]);
// => 24

function map(f, [...xs]) {
    return foldl((x, y) => x.concat([f(y)]), [], xs);
}

map(x => x * 2, [1, 2, 3, 4]);
// => [ 2, 4, 6, 8 ]

function filter(predicate, [...xs]) {
    return foldl((x, y) => predicate(y) ? x.concat(y) : x, [], xs);
}

filter(x => x % 2 === 0, [1, 2, 3, 4, 5]);
// => [ 2, 4 ]

const head = ([x, ..._]) => x;

head([1, 2, 3]);
// => 1

const tail = ([_, ...xs]) => xs;

tail([1, 2, 3]);
// => [ 2, 3 ]