process.env.NODE_ENV

We recommend the following.

In development:

• Don't set process.env.NODE_ENV, or set it to one of the following values: ['development', 'dev', '', undefined].
• If you don't use Vite's development server, then add Vite's development middleware to your server (as shown in API > renderPage() > Usage).

In production (and staging):

• Set process.env.NODE_ENV to a value different than ['development', 'dev', '', undefined], for example 'production' or 'staging'.
• Don't add Vite's server middleware to your server: it isn't needed in production and would unnecessarily bloat your production server.

process.env.NODE_ENV is a convention to tell libraries whether your app is running in production or development:

• Node.js Docs - The difference between development and production
• Stack Overflow - What is NODE_ENV and how to use it in Express?

While the convention was introduced by Node.js, it's used by libraries regardless whether you use Node.js or not. That's why we recommend to set process.env.NODE_ENV in Edge environments as well.

Vite-plugin-ssr shows a warning if you don't respect the recommendation above:

[vite-plugin-ssr][Warning] Vite's development server was instantiated while the environment
is set to be a production environment by `process.env.NODE_ENV === 'production'` which is
contradictory.

[vite-plugin-ssr][Warning] Vite's development server wasn't instantiated while the environment
is set to be a development environment by `process.env.NODE_ENV === undefined` which is
contradictory.