How to make permission to open pages for user-agent on Node js?

Question

Node.js

I have an application to serve static html files to node js using express

app.get("/app/:type", async(req, res) => {
    if (!req.query.host || !req.query.key) return res.end("error");

    if (["1", "2", "3"].indexOf(req.params.type) === -1) res.end(":)")

    if (!fs.existsSync("./appfile/" + req.params.type + ".txt")) res.end("error")

    var file = await readFile("./appfile/" + req.params.type + ".txt");

    res.end(file);
  });

How can I make sure that only the user agents I specified have access to these files?
Suppose I am making a script for parsing my files, and I must specify exactly the agent that I have above in the script, and the one who does not know which one to insert cannot access the pages above.