gistfile1.js

function filter (predicate, list) {
    return list.map(function (a) {
        return predicate(a) ? [a] : null;
    }).reduce(function (prev, current) {
        return current ? prev.concat(current) : prev;
    }, []);
}

function filter2 (predicate, list) {
    return list.reduce(function (prev, current) {
        return predicate(current) ? prev.concat(current) : prev;
    }, []);
}

function quicksort (list) {
    if (list.length == 0) {
        return [];
    }

    var x = list[0],
        xs = list.slice(1);

    return quicksort(filter2(function (a) { return a <= x; }, xs))
        .concat(x)
        .concat(
            quicksort(filter2(function (a) { return a > x; }, xs)));
}

console.log(
    filter(function (a) { return a < 5; }, [2,4,1,5,8,3,5])
);