Learning JavaScript

A test-first course for core JavaScript. Inspired by learn_ruby, but rebuilt for JavaScript concepts and run with Vitest.

Each lab is a folder with:

• index.html — instructions (open it in a browser),
• a *.test.js file — the tests you must make pass,
• a source .js file — function stubs for you to complete.

You write code until the tests go green. That's the whole game.

Setup

You need Node.js 18+ (any recent LTS is fine).

cd learning-javascript
npm install

Running the tests

Watch mode — re-runs automatically every time you save. This is what you want open while you work:

npm test

Run a single lab:

npx vitest 00_hello

Run the whole suite once and exit (no watch, good for a final check or CI):

npm run test:run

How to use the course

1. Open the course home page in a browser:

   open index.html      # macOS
   # or just double-click index.html

2. Start with 00 Hello and go in order. Each lab builds on the last.

3. For each lab:

   • Read the instructions in its index.html.
   • Open the lab's source file (e.g. 00_hello/hello.js).
   • Start the tests (npm test) and watch them fail.
   • Read the failure message — it tells you exactly what's expected.
   • Write the smallest code that makes the next test pass.
   • Repeat until the lab is all green, then move on.

Some labs (Timers, Promises) are asynchronous. The tests use Vitest's fake timers where helpful, so they run instantly — no real waiting.

Course outline

#	Lab	Concepts
00	Hello	function syntax, arguments, return values
01	Math	arithmetic, precedence, division, modulo, **
02	Control Flow	if/else, ternary, switch, truthiness
03	Loops & Recursion	for, while, recursion
04	Objects	building objects, iterating keys/values
05	Object References	shared references, copying, accidental mutation
06	Array Transform	map, filter, forEach, reduce
07	Array Mutation	sort/toSorted, push, pop, splice, slice
08	Strings	split, join, slicing, case
09	Regex	matching, capturing groups, replacing
10	Higher-Order Functions	functions as values, closures, compose
11	Scope	function vs block scope, var/let/const
12	Gotchas	== vs ===, number + string, NaN, mutation
13	Dates	the Date object, diffs, formatting
14	Timers	setTimeout, setInterval
15	Promises	async/await, rejecting, Promise.all/race/allSettled
16	Fetching Data	fetch, JSON, transforming / filtering / paginating API data
17	Writing Tests	you write the tests and the code: matchers, edge cases, Arrange-Act-Assert

Tips

• When stuck, read the test file. It is the precise spec.
• console.log freely — Vitest shows your output.
• When in doubt, try it in the Node REPL: run node and experiment.
• Google the error message. Seriously.