LRU.js

const DEFAULT_MAX_ITEMS = 1000;

function LRU(size) {
    const items = {};

    let max_items = DEFAULT_MAX_ITEMS;
    if (size != null) {
        max_items = size;
    }

    function contains(key) {
        return Object.keys(items).indexOf(key) > -1;
    }

    function prune() {
        if (Object.keys(items).length > max_items) {
            setTimeout(function() { // do this on next tick to not hold up the put
                const oldest_key = Object.keys(items).reduce(function(oldest, nextKey) {
                    if (oldest == null) {
                        return nextKey;
                    }
                    if (items[oldest].accesstime > items[nextKey].accesstime) {
                        return nextKey;
                    }
                    return oldest;
                }, null);

                if (oldest_key != null && contains(oldest_key)) {
                    delete items[oldest_key];
                }
                prune(); // recurse to ensure we prune right back
            });
        }
    }

    function put(key, data) {
        if (!contains(key)) {
            items[key] = {};
        }
        items[key].data = data;
        items[key].accesstime = Date.now();
        prune();
    }

    function get(key) {
        if (!contains(key)) {
            return null;
        }
        items[key].accesstime = Date.now();
        return items[key].data;
    }

    function remove(key) {
        if (!contains(key)) {
            return;
        }
        delete items[key];
    }

    function purge() {
        Object.keys(items).forEach(remove);
    }

    return {
        get,
        put,
        remove,
        purge
    };
}

module.exports = LRU;