Offload Heavy Compute to a Web Worker
Move a 200ms image processing loop into a worker so the main thread stays responsive and the UI does not jank.
Task
Apply const worker = new Worker('./processor.js'); worker.postMessage(imageData); 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 const worker = new Worker('./processor.js'); worker.postMessage(imageData);, scoring 72 on mobile.
Prerequisites
- A Lighthouse or WebPageTest baseline measurement
- Access to the build configuration
Steps
1. Configure const worker = new Worker('./processor.js'); worker.postMessage(imageData); in the project configuration file with the recommended defaults.
const worker = new Worker('./processor.js'); worker.postMessage(imageData);
Expected output: The configuration is accepted and const worker = new Worker('./processor.js'); worker.postMessage(imageData); runs without errors on the first invocation.
2. Run the check locally to verify const worker = new Worker('./processor.js'); worker.postMessage(imageData); 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 const worker = new Worker('./processor.js'); worker.postMessage(imageData); blocks merge on failure.
- name: Run add web worker offload
run: pnpm test
Expected output: Pull requests with failing const worker = new Worker('./processor.js'); worker.postMessage(imageData); 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. Move a 200ms image processing loop into a worker so the main thread stays responsive and the UI does not jank. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run const worker = new Worker(‘./processor.js’); worker.postMessage(imageData); against the current codebase and inspect the output before wiring it into CI.
const worker = new Worker('./processor.js'); worker.postMessage(imageData);Verify the improvement
Once the gate is in place, make failures actionable. A failing const worker = new Worker(‘./processor.js’); worker.postMessage(imageData); 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.