Why Upgrading to Webpack 5 Breaks process.env and How to Fix It

Friendly Reminder

The article focuses on the break‑change that webpack 5 no longer includes nodejs polyfills after upgrading from webpack 4. If you already know this, you can skip reading unless you want to skim.

Encounter

It started on a Thursday evening when a colleague reported a white screen in production. The upgrade to

[email protected]

seemed stable, but the devtool showed

Uncaught ReferenceError: process is not defined

.

Discovering Anomaly

Rolling back the release removed the error, indicating the issue only appears in the production build. The problematic line was:

<code>// Application entry page
XXX.init({
  ...,
  environment: process.env.PUBLISH_ENV,
});
</code>

The

process.env.PUBLISH_ENV

variable was undefined, causing the crash.

Further inspection revealed the code is guarded by:

<code>if (process.env.GM_BUILD_ENV === 'prod') {
  // ...
  XXX.init({
    ...,
    environment: process.env.PUBLISH_ENV,
  });
  // ...
}
</code>

Thus the logic only runs in production.

process.env.PUBLISH_ENV

is missing, leading to the crash.

process.env.GM_BUILD_ENV === 'prod'

ensures only production hits the code.

Rolling back to

master

works, so the bug originates from the feature branch.

Approaching the Truth

Inspecting the webpack

DefinePlugin

configuration showed no definition for

PUBLISH_ENV

. The variable had never existed in the repository for at least six months.

Two possibilities: either

process.env.PUBLISH_ENV

has always been undefined, or the recent upgrade removed it.

Testing on

master

printed

undefined

without crashing, suggesting a polyfill added

process

in the old build.

Searching the codebase revealed a file that defines

process

as a polyfill. Removing that definition reproduces the error.

Encounter Again

Comparing branches showed the

package.json

webpack version changed from 4 to 5, introducing the break‑change: webpack 5 no longer bundles nodejs polyfills.

Webpack 4 automatically provided

process

, but the new branch lacked a proper

DefinePlugin

entry for

PUBLISH_ENV

, causing the production build to fail.

The fix is either to add the missing definition via

DefinePlugin

or to use

node-polyfill-webpack-plugin

to re‑introduce the polyfills.

That concludes the investigation; you can now finish your day.