If you're building your spiders using Scrapy, you can use whatever tool you prefer to handle JavaScript based pages on Scrapy Cloud. We recommend using Splash to that end, because it's already integrated with Scrapy Cloud. To setup a Splash instance for you, see Enabling Splash.
Check out this blog post for more information about Splash.
If you're using Portia, our visual scraping tool, you don't have to worry about JavaScript based pages, because Portia has builtin support for those.
Note: Portia is no longer available for new users. It has been disabled for all the new organisations from August 20, 2018 onward.