JavaScript.com - A free resource for learning and developing in JavaScript
DeepSource - Automated code reviews with static analysis.
Code Climate for Java - Code quality and test coverage for Java
Codacy - Automatically reviews code style, security, duplication, complexity, and coverage on every change while tracking code quality throughout your sprints.
JavaScript Operator Lookup - A full list of JavaScript operators with examples
The State of JavaScript 2018 - Discover the latest trends in the JavaScript ecosystem