retention.js

const Redis = require("ioredis");
const redis = new Redis();

app.get("/retention", async (req, res) => {
    const userId = req.query.userId;
    const timestamp = Date.now();
    const bitmapKey = `user_login:${userId}`;
    const offset = Math.floor(timestamp / (24 * 60 * 60 * 1000));

    const retention = await redis.bitcount(bitmapKey, 0, offset);
    res.send(`User retention: ${retention}`);
});