Unused JavaScript: Audit Your App’s Bundle

Unused JavaScript is code the browser downloaded but did not execute during the observed session. Use that observation to investigate your app’s delivery cost, while checking the journeys that may need the code before you remove it.

AI-built apps can accumulate libraries and components as features change. A useful bundle review asks what visitors receive on each route, which resources dominate the transfer and which functionality actually needs them.

Separate transfer size from execution coverage

Transfer size describes what the visitor downloads. Coverage describes which parts of a resource were used during the recording. These are different measurements, and neither by itself tells you the complete user experience.

Chrome’s Coverage documentation explains how to record JavaScript and CSS use while interacting with a page. A recording that stops after the initial load cannot tell you whether a later interaction uses an apparently unused section.

Record the route, viewport, cache state and actions you performed. Those conditions make the result comparable with another release and help explain a change in the figures.

Review the largest resource first

Choose a slow or unnecessarily heavy public page and inspect the largest JavaScript resources. Identify the packages and components behind them using your build tooling and source code.

Consider a hypothetical landing page that downloads a charting library used only on an account dashboard. Investigate whether a shared import pulls the library into the landing-page route. The repair might be a route boundary or a deferred import rather than deleting the chart feature.

For CSS, check whether the resource belongs to a current page, a responsive state or a component not exercised in your recording. The unused percentage is a starting point for inspection, not an automatic deletion list.

Exercise the features before drawing conclusions

Repeat coverage while opening navigation, submitting a form, using a dialog and moving through the page’s important states. On a product route, include the interaction that makes the page useful.

Compare a fresh load with the relevant journey. If a large library activates only after a user chooses an optional feature, review whether loading it later would preserve the experience. Test loading indicators and failures as part of that change.

Do not optimize a public landing page by accidentally breaking the authenticated app. Identify which route owns the feature and test both routes after moving imports or dependencies.

Request a focused bundle repair

Identify the largest browser JavaScript resources on this route and map them to source imports. Explain which features need them. Propose a small change to delay or remove unnecessary delivery, preserving existing behavior. Include a comparison under the same cache and interaction conditions and tests for the affected feature.

Review the dependency diff as well as the component diff. A replacement package can change behavior, licensing, browser compatibility or error handling; a smaller file is not the only acceptance criterion.

Deploy a preview build and repeat the same measurements. Keep the before and after transfer values, observed interactions and any user-experience changes in the repair record. Avoid claiming a speed increase from unused-code reduction alone.

Fit the measurement into your audit routine

VibeShield’s Bundle & Tech tool reports browser resource and coverage observations alongside technology information. The features page describes how it fits into verified account scans.

This scope is different from a complete Lighthouse or real-user performance assessment. A resource reduction may help, but user-perceived responsiveness also needs its own measurement.

Use the deployment monitoring workflow to retain comparable release records. Keep security, accessibility and bundle findings separate enough that one improved score does not hide a regression in another area.

Common questions

Should I delete every line reported as unused?

No. The report reflects the recorded session. Exercise relevant interactions and map the code to its intended feature before changing it.

Does a smaller bundle prove the page is faster?

It establishes a smaller measured resource under the tested conditions. Verify loading and interaction performance separately, using a comparable test setup.