Legacy JavaScript
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Published: Oct 8, 2025
Polyfills and transforms enable older browsers to use new JavaScript features. However, many aren't necessary for modern browsers.
Consider modifying your JavaScript build process to not transpile Baseline features, unless you know you must support older browsers.
How to pass this insight
This insight fails if more than 5 KB of polyfills or transforms are detected for JavaScript features that are widely supported according to Baseline.
The vast majority of websites do not need to support older, ES5-only browsers. To pass this insight, configure your build process or dependencies to produce more modern JavaScript code. If your site really needs to support older browsers, you should ignore this insight.
Additional references
Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
Last updated 2025-10-08 UTC.
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