Tree Shake Unused Exports with Side Effects Flag

Mark the package with sideEffects false so the bundler drops unused exports and the bundle shrinks by 30 percent.

~15 minperformancebundlingtree-shaking

Task

Apply "sideEffects": false // in package.json to reduce a measured bottleneck that shows up in the performance budget or Lighthouse audit.

When to use this

Lighthouse reports the page fails the performance budget because of "sideEffects": false // in package.json, scoring 72 on mobile.

Prerequisites

A Lighthouse or WebPageTest baseline measurement
Access to the build configuration

Steps

1. Configure "sideEffects": false // in package.json in the project configuration file with the recommended defaults.

"sideEffects": false  // in package.json

Expected output: The configuration is accepted and "sideEffects": false // in package.json runs without errors on the first invocation.

2. Run the check locally to verify "sideEffects": false // in package.json produces the expected result before pushing.

pnpm test

Expected output: Local run matches CI behavior; no surprises after push.

3. Add the gate to the CI pipeline so "sideEffects": false // in package.json blocks merge on failure.

- name: Run add tree shake unused exports
  run: pnpm test

Expected output: Pull requests with failing "sideEffects": false // in package.json are blocked from merging.

Troubleshooting

Problem

The check passes locally but fails in CI

Solution

Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.

Problem

Configuration is rejected with a schema error

Solution

Check the tool version matches the config schema; older versions may not support newer fields.

Limitations

The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
Configuration drift is possible if someone edits settings outside version control.

Measure before optimizing

Start by measuring the problem. Mark the package with sideEffects false so the bundler drops unused exports and the bundle shrinks by 30 percent. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run “sideEffects”: false // in package.json against the current codebase and inspect the output before wiring it into CI.

"sideEffects": false  // in package.json

Verify the improvement

Once the gate is in place, make failures actionable. A failing “sideEffects”: false // in package.json should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.