Difference between global.process.env and process.env

[mikhail-fedosenko]

Summary

How is that possible that running a production build from Next.js I have the following output on a dynamically rendered page?

process.env === global.process.env: true

process === global.process: true

process.env.NEXT_PUBLIC_TEST === global.process.env.NEXT_PUBLIC_TEST: no

process.env.NEXT_PUBLIC_TEST: from env file

global.process.env.NEXT_PUBLIC_TEST: from_proc

Scripts in the package.json:

    "build": "next build",
    "start": "NEXT_PUBLIC_TEST=from_proc next start"

.env file:

NEXT_PUBLIC_TEST=from env file

So, I'm wondering how is that possible that process.env === global.process.env is true, while at least some values inside env are different??

Why is that nuance not documented?

Additional information

Next.js 15 App Router

Example

https://github.com/mikhail-fedosenko/ask-about-env-vars

[icyJoseph]

This one is a bit tricky to understand, but basically the Webpack DefinePlugin is used to replace NEXT_PUBLIC_ prefixes env vars with their value during build time, into the source code. This depends on the exact process.env.NEXT_PUBLIC_ declaration. It is a bit of magic, coming from the ability to statically analyze this declaration.

As you can image, this is rather sensitive, and something like global.process.env.NEXT_PUBLIC_ falls out and is considered to be dynamic access.

See this documentation from that DefinePlugin plugin:

• https://webpack.js.org/plugins/define-plugin/

Something like this they document over at that plugin:

new webpack.DefinePlugin({
  'typeof window': JSON.stringify('object'),
  'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV), 
  'process.env.NEXT_PUBLIC_FOO': JSON.stringify(process.env.NEXT_PUBLIC_FOO), // this bit@
});

I can never find where this is done in Next.js but I'll do a bit of search - anyway, you can imagine that something like this is done to replace process.env.NEXT_PUBLIC_

I think it is here, that the expressions to replace with the DefinePlugin are defined:

next.js/packages/next/src/lib/static-env.ts

Lines 17 to 28 in 531e938

	export function getNextPublicEnvironmentVariables() {
	const defineEnv: Record<string, string | undefined> = {}
	for (const key in process.env) {
	if (key.startsWith('NEXT_PUBLIC_')) {
	const value = process.env[key]
	if (value != null) {
	defineEnv[`process.env.${key}`] = value
	}
	}
	}
	return defineEnv
	}

,

I checked if Turbopack builds also do this, and yeah - the Turbopack replacement is also expecting strictly process.env.NEXT_PUBLIC_