Learning new concepts and having a refresher of JavaScript, many like
- Variable
- Loops
- Arrays
- Functions
- Highers Order Array Functions
- Maps & WeakMaps
- Sets & WeakSets
- Objects
- Constructor Functions
- Date & Time
- JavaScript Objects
- JSON, and JSON mehtods(.stringify and .parse)
- Recursion
- Prototypes (Flags, Descriptors, Getters and Setters)
- Inheritance
- OOP
- ES6 Classes
- Class constructor
- Super - super()
- Classs checking - instanceof
- Mixins
and more...
All these were gotten from the JavaScript.info and Traversy Media which covers alot of basic to advanced concepts of JavaScript.