My solution to the problem at http://reaktor.fi/ura/fast_track/

fast_track-1.js

function (input) {
    return [].reduce.call(input, function (mem, n, i) {
        return mem[i % 5] = n, mem.push(Math.max(mem.pop(), eval(mem.join("*")))), mem;
    }, [0, 0, 0, 0, 0, 0]).pop();
}

fast_track-2.js

function (input) {
    return [].reduce.call(input, function (max, n, i, seq) {
        return Math.max(max, eval([].slice.call(seq, i, i + 5).join("*")));
    }, 0);
}

fast_track-2b.js

function (input) {
    return _.reduce(input, function (max, n, i, seq) {
        return Math.max(max, eval(_.head(_.tail(seq, i), 5).join("*")));
    }, 0);
}

fast_track-3.js

function (input) {
    return _.max(_.map(input, function (n, i, seq) {
        return eval(_.head(_.tail(seq, i), 5).join("*"));
    }));
}