How much JavaScript do you already have?

2. B1

   Setup & mental modelThe layer everybody skips — and it shows in interviews.
4. B2

   Execution contextWhat JavaScript does with your code before it runs a single line of it.
6. B3

   var, 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.
8. B4

   ScopeWhich 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.
10. B5

    One thread, one stackWhy JavaScript can only do one thing at a time — and how it still does ten.
12. B6

    Inside the browserFrom a script tag to pixels on screen — and where your code sits in that.
14. B7

    Types & valuesEight types, one conversion table, and the equality check that never stops mattering.
16. B8

    Strings, numbers & MathThe everyday toolbox every other chapter quietly assumes you already have.
18. B9

    Operators & flowEvery operator here has an interview question hiding behind it.
20. B10

    FunctionsA function is a value first, a block of code second.
22. B11

    Objects & arraysThe two shapes almost everything you build is made of.
24. B12

    The this keywordNot where it was written — who called it.
26. B13

    ClosuresThe function that walked out with its birthplace still attached.
28. B14

    DOM & eventsThe DOM is a live tree of objects — and JS can poke every branch of it.
30. B15

    Callbacks, then promisesHanding work to someone else, and getting the answer back later.
32. B16

    Errors & toolsThe last lesson before the beginner project — reading what the engine is trying to tell you.
34. B17

    Guided project: build a to-do appEverything from the beginner chapters, in one small thing that works.

Checkpointyou can build a to-do list app that survives a refresh, without copying a tutorial.

2. I1

   Scope & functions, properlyThe corners of how a scope is built, new versus bind, and what you build once functions are values: currying, composition, recursion.
4. I2

   Objects deeplyThe rest of the object/array toolbox — past what the beginner chapter covered.
6. I3

   Prototypes & OOPclass is real syntax now — but it's still prototypes underneath, every time.
8. I4

   Async, properlyPromises, done right — and the sequential-vs-parallel mistake almost everyone makes once.
10. I5

    Modules & toolingEverything that turns files full of JS into one thing a browser can run.
12. I6

    Regex & datesTwo toolboxes every real app ends up reaching for, and the traps that come with each.
14. I10

    Browser APIs, in depthStorage, the URL bar, and the event system — past what the beginner DOM chapter reached.
16. I11

    Modern JavaScript, ES2023 to ES2026What the language gained in the last few years, and how to tell whether you can use it yet.
18. I7

    Error handling & debuggingPast what the beginner errors chapter covered — error chains, lost rejections and real debugging tools.
20. I8

    Real-time connectionsfetch answers one question at a time. This is what answers a stream of them.
22. I9

    Offline & storagelocalStorage is fine for a theme or a small cache. Past a few MB, it stops being enough.

Checkpointyou can explain prototypes and promise ordering to someone else, run independent requests in parallel, and cancel or retry one without guessing.

2. A1

   Engine & memoryWhat V8 is actually doing while your code just runs.
4. A2

   Advanced asyncPast promises: pausable functions, streams, and running real work concurrently.
6. A3

   MetaprogrammingCode that changes how ordinary-looking code behaves.
8. A4

   Types & dataWhat happens once a number, a string, or a file gets big or exotic enough to need its own type.
10. A5

    Patterns & architectureShapes that show up again and again once code has to scale past one file.
12. A6

    PerformanceMaking a page feel fast is a different skill than making code run fast.
14. A7

    SecurityThe mistakes that turn into a real incident, not just a bug.
16. A8

    Ecosystem & professionalHow the tools around JS actually work, and where the language itself comes from.
18. A9

    TestingNot optional at this level — and this whole site's own test suite is the example.
20. A10

    Browser observabilityThe bug is on somebody else's device, on a network you cannot reproduce.
22. A11

    JavaScript interview bankThirty-four questions, split by the level they are asked at, with the answer that lands.
24. A12

    Build it yourselfThe six things a senior loop asks you to build from scratch — because you can, not because you should ship them.

Checkpointyou can find a memory leak from a heap snapshot, explain why a function deoptimized, and defend an architectural choice with tradeoffs.