gistfile1.js

var Cache = function(n) {
    this.items = {};
    this.n = n;
};

Cache.prototype.set = function(id, item) {
    var items = this.items,
        n = this.n;

    items[id] = {
        item: item,
        time: Date.now()
    };

    if (items.length > n) {
        Object.keys(items).sort(function(a, b) {
            return items[a].time - items[b].time;
        }).slice(n).forEach(function(key) {
            delete items[key];
        });
    }
};