I recently completed the Headless JSS concept of Sitecore with Sitecore Omni. JSS is a software development kit and is one of the products of Sitecore Omni which can be used to create JavaScript applications, for headless omnichannel customer experience.
It's a complete package for Sitecore JSS developers. It explains all the concepts for starting with JSS. It explains concepts like code-first approach, maintaining app directories, creating a route in App, JSS App templates, rendering components, working with Mock Data, and many other concepts in detail.
I will recommend learning Sitecore JSS from this collection as it contains most of the things needed for starting with JSS.
To make yourself comfortable with JSS, you can complete and earn the Sitecore JSS certification by going through the collection "JavaScript Services (JSS) Collection" from https://learning.sitecore.com/.
Happy Learning 🙂