Node.js: cache file and directory reads by wrapping `fs`

fs-cache.js

import fs from "fs";
import moment from "moment";

const cache = new Map();

function put(key, data) {
    cache.set(key.toLowerCase(), {
        expires: moment.utc().add(moment.duration(1, "h")),
        value: data
    });
}

function get(key) {
    const result = cache.get(key.toLowerCase());

    if (!result || result.expires.isBefore(moment.utc())) {
        return null;
    }

    return result.value;
}

function wrap(op) {
    return function(...args) {
        const passthrough = args.slice(0, args.length - 1);
        const callback = args[args.length - 1];
        const key = `${op}:${passthrough.join("📎")}`;
        const cached = get(key);

        if (cached) {
            return callback(null, cached);
        }

        fs[op].apply(
            fs,
            passthrough.concat((err, data) => {
                if (err) {
                    return callback(err, data);
                }

                put(key, data);
                return callback(err, data);
            })
        );
    };
}

const wrapper = {
    readFile: wrap("readFile"),
    readdir: wrap("readdir")
};

export default wrapper;

I assume trying to be clever and use ":paperclip:" as the join separator will cause problems on some weird edge-case systems, ":" is probably fine.