i've tried everything.

my vercel.json:
{ "version": 2, "rewrites": [{ "source": "/(.*)", "destination": "/" }] }

my index.js:
const express = require("express");
const app = express();

app.get("/", (req, res) => res.send("Express on Vercel"));

app.listen(3000, () => console.log("Server ready on port 3000."));

module.exports = app;

in the .vercel.app page it shows the code for index.js.