You will learn how to write the reducer for the counter application in a test driven development way, as well as the conventions in implementing reducers.
For historical reasons JS function declarations have weird semantics. "function" is not block-scoped (that it, behaves like "var" rather than like ES6 "let" or "const"), it is hoisted (a very weird thing to cloud beginners' heads with), it has "this" context which is frustrating to explain when you're just teaching some library's API, etc.
Figured out through later episodes it is loaded through as a script tag on the HTML src="https://wzrd.in/standalone/expect@latest". Loving the series so far, same architecture as ELM but with JS!