Here is a collection of free Javascript ES6 courses:
https://medium.freecodecamp.org/want-to-learn-es6-take-this-free-23-part-course-and-become-a-javascript-ninja-55002db1ff74
https://scrimba.com/g/gintrotoes6
Category: Polymer
Polymer 3: How to customize starter kit application
2019/04/14: New strategy on creating apps with litElement + RXFIRE + Redux
Below are the links for sample application using React, but it can be a nice source of information.
https://medium.com/quick-code/adding-authentication-to-react-redux-firebase-app-f0efcb1c519a
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