Hello,
Unfortunately, we do not have a sample project for a custom action written in JavaScript.
We do, however, have an article which explains how you can
Get and Set properties in VBScript and Javascript.
Additionally, please have a look on why
not to use VBScript and JavaScript.
If you want, you can let me know what you want to achieve and I may be able to help you in that matter.
Hope it helps.
Kind regards,
Catalin