Published: Apr 29, 2026
Polyfills and transforms let you use features that may not be Baseline—also known as Limited availability features. Baseline doesn't discourage you from using Limited availability features, but features which are either Baseline Newly or Widely available can often be used without them. The Legacy JavaScript audit lets you know where there are opportunities to improve performance by adopting features which are Baseline.
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 KiB of polyfills or transforms are detected for JavaScript features that are widely supported according to Baseline. To pass this insight, you'll likely need to configure your bundler toolchain to avoid certain pitfalls.
Use ES6 in production
ES6 is broadly supported in all browsers. If you're using a combination of a module bundler and a JavaScript transpiler—such as Babel and its @babel/preset-env preset. The vast majority of websites do not need to support older, ES5-only browsers.
A common tool used with bundlers and Babel is Browserslist, which accepts a variety of queries specified in natural language, which are then translated to a list of targeted browser environments. For example, you can target Baseline Widely available features with this Browserslist query anywhere a Browserslist config can be found:
baseline widely available
Baseline Widely available is a suggested default for new web projects. As always, however, you should evaluate the browsers used to access your website. One such tool for this is the Google Analytics Baseline Checker.