reduce.js

const newStats = days.map( day => ({ stats: day.statistics.breakdown.byTurnover }) ).reduce(function (pastDay, currentDay) {
    const nextStats = currentDay.stats.map(function(stat) {
        const oldSold = pastDay.stats.find(function (old) {
            return old.name === stat.name;
        });

        const newSold = stat.sold + oldSold.sold;
        stat.sold = newSold;
        return stat;
    });

    return {
        stats: nextStats,
    };
});