nextjs – next build with NODE_ENV=development

UPDATE 2022-11-23:

See how-to-set-environment-variables-from-within-package-json.

In short:
"start:dev": "NODE_ENV=development next start"

You might need cross-env (on Windows ? I don’t know):
"start:dev": "cross-env NODE_ENV=development next start"

More specific documentation is now available (and maybe requirements also have changed, or maybe not):

environment-variable-load-order:

… Note: The allowed values for NODE_ENV are production, development and test.

non-standard-node-env:

… only permitting three (3) values:

• production: When your application is built with next build
• development: When your application is ran with next dev
• test: When your application is being tested (e.g. jest)

ORIGINAL ANSWER:

See issue #9123, (Oct 18, 2019) :

NODE_ENV is a reserved environment variable that cannot be changed.
The only valid values are production, development, and test.

If you need your app behavior to change in different production
environments, please use a different variable like APP_ENV.

And issues #17032 (Sep 12, 2020):

process.env.NODE_ENV only has 2 possible values development and
production. If this is not set to that value you’ll run into all kinds
of library edge cases (especially in node_modules) where you get
severely de-optimized results. E.g. if you run a performance test
you’ll get significantly worse results if process.env.NODE_ENV is not
set to production