CODE EXAMPLE FOR JAVASCRIPT

nextjs process.env undefined

This envs just works in Server Side. To access this envs in Client Side,
you need declare in the next.config.js
This way:
module.exports = {
  reactStrictMode: true,
  env: {
    BASE_URL: process.env.BASE_URL,
  }
}
On Next +9, prefix the variables using
NEXT_PUBLIC_. This will expose them on the browser