JavaScript

Start at the bottom of the ladder — what the engine does with your code before line 1 runs, why one thread is enough, and then types, values, operators and functions.

1. B1Setup & mental modelThe layer everybody skips — and it shows in interviews.8 min · 1 exercise
2. B2Execution contextWhat JavaScript does with your code before it runs a single line of it.11 min · 2 exercises
3. B3var, let and constThree keywords, four questions they answer differently — where a name lives, when you can touch it, whether it can change, and whether you can declare it twice.14 min · 2 exercises
4. B4ScopeWhich names a line of code can see, why that is decided by where you wrote it, and how JavaScript finds a name — for reading and for writing.15 min · 2 exercises
5. B5One thread, one stackWhy JavaScript can only do one thing at a time — and how it still does ten.10 min · 2 exercises
6. B6Inside the browserFrom a script tag to pixels on screen — and where your code sits in that.8 min · 2 exercises
7. B7Types & valuesEight types, one conversion table, and the equality check that never stops mattering.9 min · 3 exercises
8. B8Strings, numbers & MathThe everyday toolbox every other chapter quietly assumes you already have.5 min · 3 exercises
9. B9Operators & flowEvery operator here has an interview question hiding behind it.11 min · 1 exercise
10. B10FunctionsA function is a value first, a block of code second.7 min · 2 exercises
11. B11Objects & arraysThe two shapes almost everything you build is made of.8 min · 2 exercises
12. B12The this keywordNot where it was written — who called it.6 min · 2 exercises
13. B13ClosuresThe function that walked out with its birthplace still attached.8 min · 2 exercises
14. B14DOM & eventsThe DOM is a live tree of objects — and JS can poke every branch of it.7 min · 2 exercises
15. B15Callbacks, then promisesHanding work to someone else, and getting the answer back later.9 min · 3 exercises
16. B16Errors & toolsThe last lesson before the beginner project — reading what the engine is trying to tell you.6 min · 2 exercises
17. B17Guided project: build a to-do appEverything from the beginner chapters, in one small thing that works.5 min · 2 exercises
18. ★The cheat pageNot another lesson — the night-before-the-interview skim of the other 38.6 min

You know the syntax. Now learn the machinery: objects and prototypes, async that holds up on a real network, modules, and the browser APIs every real app reaches for.

1. I1Scope & functions, properlyThe corners of how a scope is built, new versus bind, and what you build once functions are values: currying, composition, recursion.9 min · 2 exercises
2. I2Objects deeplyThe rest of the object/array toolbox — past what the beginner chapter covered.8 min · 4 exercises
3. I3Prototypes & OOPclass is real syntax now — but it's still prototypes underneath, every time.7 min · 4 exercises
4. I4Async, properlyPromises, done right — and the sequential-vs-parallel mistake almost everyone makes once.10 min · 9 exercises
5. I5Modules & toolingEverything that turns files full of JS into one thing a browser can run.7 min · 2 exercises
6. I6Regex & datesTwo toolboxes every real app ends up reaching for, and the traps that come with each.5 min · 2 exercises
7. I7Error handling & debuggingPast what the beginner errors chapter covered — error chains, lost rejections and real debugging tools.6 min · 2 exercises
8. I8Real-time connectionsfetch answers one question at a time. This is what answers a stream of them.5 min · 2 exercises
9. I9Offline & storagelocalStorage is fine for a theme or a small cache. Past a few MB, it stops being enough.7 min · 2 exercises
10. I10Browser APIs, in depthStorage, the URL bar, and the event system — past what the beginner DOM chapter reached.5 min · 2 exercises
11. I11Modern JavaScript, ES2023 to ES2026What the language gained in the last few years, and how to tell whether you can use it yet.7 min · 2 exercises
12. ★The cheat pageNot another lesson — the night-before-the-interview skim of the other 38.6 min

Memory, generators, Proxy, performance, security and testing — and how V8 actually stores your objects. The layers almost nobody can explain out loud.

1. A1Engine & memoryWhat V8 is actually doing while your code just runs.10 min · 2 exercises
2. A2Advanced asyncPast promises: pausable functions, streams, and running real work concurrently.9 min · 2 exercises
3. A3MetaprogrammingCode that changes how ordinary-looking code behaves.10 min · 2 exercises
4. A4Types & dataWhat happens once a number, a string, or a file gets big or exotic enough to need its own type.5 min · 2 exercises
5. A5Patterns & architectureShapes that show up again and again once code has to scale past one file.7 min · 2 exercises
6. A6PerformanceMaking a page feel fast is a different skill than making code run fast.9 min · 2 exercises
7. A7SecurityThe mistakes that turn into a real incident, not just a bug.14 min · 4 exercises
8. A8Ecosystem & professionalHow the tools around JS actually work, and where the language itself comes from.8 min · 2 exercises
9. A9TestingNot optional at this level — and this whole site's own test suite is the example.8 min · 2 exercises
10. A10Browser observabilityThe bug is on somebody else's device, on a network you cannot reproduce.5 min · 2 exercises
11. A11JavaScript interview bankThirty-four questions, split by the level they are asked at, with the answer that lands.14 min
12. A12Build it yourselfThe six things a senior loop asks you to build from scratch — because you can, not because you should ship them.7 min · 9 exercises
13. ★The cheat pageNot another lesson — the night-before-the-interview skim of the other 38.6 min