Redditor u/keveridge put together a nice guide on how to use JavaScript when creating Shortcuts. JavaScript lets you perform complex actions that would be hard to do with regular shortcut actions.
We make use of the Safari web browser, running within the shortcut, in order to execute the JavaScript. To do so, we:
make an HTML file that contains our code and provides an output;
pass the contents of the file to Safari as a Data URL;
use Get Contents of Web Page to render the page provide the output to the shortcut.
Check It Out: How to Use JavaScript When Creating Shortcuts