Duplicated JavaScript

Published: Oct 8, 2025

Remove large, duplicate JavaScript modules from bundles to reduce unnecessary bytes consumed by network activity.

How to pass this insight

• For pages with multiple JavaScript bundles, code splits large dependencies needed by multiple bundles to a common bundle.
• Within individual JavaScript bundles, ensure there is only one version of large dependencies.

You can dive deeper into the JavaScript bundles on a page by clicking the View Treemap. This opens the page's bundles in the Lighthouse Treemap.

[画像:Lighthouse Treemap. The colored nodes represent duplicate modules found across multiple JavaScript bundles]

• In PageSpeed Insights and in the Lighthouse report, this button is in the Performance category below the metrics:

  [画像:Lighthouse View Treemap button]

• In the DevTools Performance Panel, this button is in the Duplicated JavaScript insight:

  [画像:DevTools Performance Panel View Treemap button]

Stack-specific guidance

Create an additional entry point for common, large dependencies and enable your JavaScript bundler's code splitting feature for the following:

• webpack
• rollup
• esbuild
• Parcel

Additional references

• Insight source code