JavaScript

Javascript has been used primarily for programming the web browser and also used on the server-side with Node. Main uses include adding interactive behavior to web pages, Creating web and mobile apps, Building web servers and developing server applications, game development, etc. 

This key competency includes Collections, Iterators, generators, Factory Function, Async Generator, Micro stack Queue and many others.

Key Competencies:

1. Collections - Built-in data structures in JavaScript that allow storing and manipulating collections of data. Learn the usage of collections.
2. Iterators, Generators - Iterators are objects that allow iterating over collections of data, one element at a time. Generators are functions that can be paused and resumed, allowing lazy and efficient iteration over data. Knowledge and usage of iterators and generators.
3. Factory Function - Functions that create and return objects without the need for the 'new' keyword.
4. Async Generator - Generators that allow for asynchronous iteration over data, using promises.
5. Micro stack Queue - A queue data structure that allows adding and removing items in a first-in-first-out (FIFO) order, used in the event loop to manage the execution of asynchronous tasks.