Apologies in advance for the newbie-level question. According to the Sitecore JSS site ( https://jss.sitecore.com/features ), Sitecore JSS allows you to "preview all your items on your website without publishing them first." That sounds like I can preview an item before publishing but I cannot preview all of the items in the context of a page like I can normally in Sitecore. Is that correct?
When using JSS, depending on your mode, you can still have full editor functionality in Sitecore.
Check out integrated mode: