If you use JS objects instead of map you can use JSON.stringify method to convert the object to a string and then store it to file system. Imagine something like this:

const database = {"user1": {"some": "data"}, "user2": {"some": "data"}};
const serializedDatabase = JSON.stringify(database);
fs.writeFileSync("./data/database.json", serializedDatabase);

You can run this code inside setInterval loop to make it run every 30 seconds:

setInterval(() => {
    const serializedDatabase = JSON.stringify(database);
    fs.writeFileSync("./data/database.json", serializedDatabase);
}, 30 * 1000);

Then, if you want to read the data once you start you application, just require the file:

const database = require("./data/database.json");

You will get your data in memory. Some points:

• If you do it this way you can lose data. If your server crashes 20 seconds after last save you are going to lose every data stored to memory in the last 20 seconds.
• Method writeFile should be probably used instead of writeFileSync to make it non-blocking.
• In general, storing data to file system by hand is hard and tricky. Using a real database us usually a better call.
• If you don't have any "big" database such as MySQL or MongoDB, you can use sqlite. It stores all the data to a single file. It's quite easy to use and if you want to migrate it somewhere else you just copy one file.