I added node and express to my project with dotenv to hide the API KEY I am using. Everything works fine but I cannot see how to pass the process.env.API_KEY to be used app.js file.

index.js

const express = require('express');
const app = express();
require('dotenv').config();

console.log(process.env.API_KEY);

app.listen(3000, () => console.log('Starting server: http://localhost:3000'));
app.use(express.static('public'));

app.js

document.addEventListener('readystatechange', (event) => {
    if (event.target.readyState === 'complete') {
        initApp();
    }
});

const initApp = () => {...}