Avoid serving legacy JavaScript to modern browsers

Heads up! This is a developer-level document. You need to be one or hire one to implement this optimization. It may require changes to code in your theme or a plugin.

This means your site is using old JavaScript rules for browsers that don’t need them, and this should be avoided. 

WP Rocket cannot fix this recommendation. It requires the developer to make code changes, but since its impact may not be noticeable for the visitor experience you could safely ignore it.

What can I do then?

To identify plugins that are serving legacy JS, run the PageSpeed test while bypassing WP Rocket, by adding the ?nowprocket query to your URL (i.e: http://www.yourdomain.com/?nowprocket) .

Expand the recommendation to find which specific files are causing the warning:

Mouseover each one to reveal the full path so you can identify the theme/plugin.

Send the following request to the respective plugin/theme/external service support team:

"Polyfills and transforms enable legacy browsers to use new JavaScript features. However, many aren't necessary for modern browsers. For your bundled JavaScript, adopt a modern script deployment strategy using module/nomodule feature detection to reduce the amount of code shipped to modern browsers, while retaining support for legacy browsers." 

More information

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