What is a JS crawl?
Many websites today have dynamic content generated by Javscript. Most search engine crawlers are only able to index html content since indexing JS pages are costly and difficult.
Usually JS websites are built on the following frameworks. You can use builtwith.com to know if your site is built on these frameworks.
You can turn this off if you do not want to take screenshots of pages to show in search results. Here is a snapshot of search results that have JS crawl screenshots.