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:

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

The process.env.PUBLISH_ENV variable was undefined, causing the crash.

Further inspection revealed the code is guarded by:

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

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.