webpack 5 "process is not defined" - Fix Removed Node Polyfills

By Daniel Zoghalchali·Latchkey

webpack 4 auto-polyfilled Node globals like process and Buffer for the browser. webpack 5 removed that, so code (or a dependency) referencing process now throws at runtime, and the build no longer auto-injects the shim.

What this error means

After moving to webpack 5, the app throws Uncaught ReferenceError: process is not defined (or Buffer/global), or the build errors that a Node core module needs a fallback. webpack 4 had no such issue.

Module not found: Error: Can't resolve 'process/browser'
# or at runtime:
Uncaught ReferenceError: process is not defined
BREAKING CHANGE: webpack < 5 used to include polyfills for node.js core modules by default.

Diagnose it: is it resolution, transform, or memory?

Bundler failures in CI fall into three families and the error text often points at the wrong one. A module that resolves on your machine and not on the runner is nearly always case sensitivity or a missing optional dependency; a transform error is a config or version mismatch; and an unexplained kill with no stack is the out-of-memory reaper, not a build error at all.

# 1. resolution: does the file exist with EXACTLY that case?
git ls-files | grep -i "the/imported/path"

# 2. transform: what versions is CI actually resolving?
npm ls webpack vite rollup esbuild typescript 2>/dev/null | head -20

# 3. memory: was it killed rather than failed?
#    exit 137 = SIGKILL (OOM). Nothing in the bundler log will explain it.
node --max-old-space-size=4096 node_modules/.bin/vite build

Common causes

webpack 5 dropped automatic Node polyfills

Code or a dependency reads process.env, Buffer, etc., expecting the browser polyfill webpack 4 injected automatically. webpack 5 ships none by default.

No ProvidePlugin/DefinePlugin shim configured

Without explicitly defining process.env or providing process/Buffer, the reference is undefined in the bundle.

How to fix it

Provide the polyfill and define process.env

Inject process/Buffer with ProvidePlugin and supply env values with DefinePlugin.

// webpack.config.js
const webpack = require('webpack')
plugins: [
  new webpack.ProvidePlugin({ process: 'process/browser', Buffer: ['buffer', 'Buffer'] }),
  new webpack.DefinePlugin({ 'process.env.NODE_ENV': JSON.stringify('production') }),
],
resolve: { fallback: { process: require.resolve('process/browser'), buffer: require.resolve('buffer') } },

Stop relying on Node globals in browser code

1. Replace process.env.X reads with bundler-provided env (e.g. DefinePlugin or import.meta.env).
2. Avoid Buffer/global in browser bundles; use Web APIs instead.
3. Add resolve.fallback only for the specific core modules a dependency truly needs.

Make the build reproducible before you debug it

• Pin the Node major in setup-node and in engines. A bundler that resolves native bindings will pick a different prebuilt binary across majors.
• Delete node_modules locally and reinstall from the lockfile before concluding the runner is at fault; most "works locally" reports are stale local state.
• Set CI=true locally to reproduce. Several toolchains change behaviour under it, including treating warnings as errors.
• Exit code 137 is an out-of-memory kill. Raise --max-old-space-size or move to a larger runner rather than searching the bundler config.

How to prevent it

• Audit for process/Buffer/global usage before upgrading to webpack 5.
• Inject env via DefinePlugin rather than expecting an ambient process.
• Add only the specific resolve.fallback polyfills your deps require.

Frequently asked questions

Related guides

References

• Node.js documentation
• npm CLI documentation
• GitHub Actions documentation