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The page does not have any JS error on page load.

What this tool does is, it’ll use a programmatic method to turn off JS files one-by-one, and detect if doing so result in any JS errors on the page.

This means that, the page has to be free from page errors in the first place. Otherwise, the tool can’t differentiate if the JS errors is introduced by disabling a JS file, or it’s already an existing JS error before anything is done.

View this video for a quick guide on how to check for existing JS error.

The page does not have any JS optimization turned on.

In WordPress’ context, plugins such as Autoptimize, Perfmatters, Asset Clean Up, Swift Performance and etc are frequently used to optimize the JavaScript of your website.

If optimization such as delayed loading or bundling is turned on, the tool cannot properly pickup the JS files needed to run the tests. So it’s essential that you turn off any JS optimization before running the tool. For WordPress, the most straightforward way is to deactivate any optimization plugins that are used to optimize JS on your site.

We need the JS file to take the most simple form like below, without any bundling or delayed loading. (Async or Defer is fine.)

<script type="text/javascript" src="your-js-file.js" async defer />

View this video for a quick guide on how to check if JS optimization is off.