Resources
Here's a list of resources/courses I've used and recommend. They're divided into sections for convenience.
I'm writing this as a reference for myself but if this helps you? Even better!
Frontend
React Documentation
The new React docs have been my go-to recommendation for OGs and new devs alike. They're much easier to understand than the old docs.
Epic React by Kent C. Dodds
This is a great beginner/intermediate course that shows different patterns that can be used in react apps, as well as how to properly test them.
Learn X in Y Minutes
Used this to learn AngularJS quickly while building out a feature on our legacy web application.
Backend
MongoDB University
This is possibly the best resource for learning about MongoDB.
SQL - The Complete Developer's Guide (MySQL, PostgreSQL)
While there's no MongoDB University for SQL this was my first video resource for it and it gave me a better grasp of the fundamentals.
MySQL For Developers
This is an amazing (and free!) course by PlanetScale, with Aaron Francis as the instructor. It covers great techniques for working with MySQL and will make you a better MySQL user.
Ruby on Rails
As a developer that mainly writes JavaScript, here are some resources that helped me learn Rails.
The official Rails docs/guides are probably the best resources for learning rails out there.
This is my goto resource for working with graphql on a rails application.
A great template for some of the common conventions. (It's not to be treated as scripture.)
This is my go to resource for all advice and examples for tests in rails applications.
General
MDN References for HTML, CSS, JavaScript
MDN is simply the best resource for staying up date with new web features without digging into the specs.
The Modern JavaScript Tutorial
This free online book is a great in-depth resource on JavaScript. You can even take the exercises after each lesson as part of your interview prep.