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JavaScript, Intermediate

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.

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Scope & functions, properly

9 min read · 2 exercises

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Scope & 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 read2 exercises
2 exercises
- 3 3 3 → 0 1 28 tests · beginner
- A function that returns a function8 tests · intermediate
Objects deeplyThe rest of the object/array toolbox — past what the beginner chapter covered.
8 min read4 exercises
4 exercises
Prototypes & OOPclass is real syntax now — but it's still prototypes underneath, every time.
7 min read4 exercises
4 exercises
Async, properlyPromises, done right — and the sequential-vs-parallel mistake almost everyone makes once.
10 min read9 exercises
9 exercises
Modules & toolingEverything that turns files full of JS into one thing a browser can run.
7 min read2 exercises
2 exercises
- Does this version satisfy a caret range?8 tests · intermediate
- Build a left-to-right pipe9 tests · intermediate
Regex & datesTwo toolboxes every real app ends up reaching for, and the traps that come with each.
5 min read2 exercises
2 exercises
- Pull every hashtag out of a post9 tests · intermediate
- Mask a card number down to the last 4 digits9 tests · intermediate
Browser APIs, in depthStorage, the URL bar, and the event system — past what the beginner DOM chapter reached.
5 min read2 exercises
2 exercises
- Read one query parameter from a URL10 tests · intermediate
- Build a query string from an object11 tests · intermediate
Modern JavaScript, ES2023 to ES2026What the language gained in the last few years, and how to tell whether you can use it yet.
7 min read2 exercises
2 exercises
- Escape text for a regular expression10 tests · intermediate
- Implement Promise.try10 tests · intermediate
Error handling & debuggingPast what the beginner errors chapter covered — error chains, lost rejections and real debugging tools.
6 min read2 exercises
2 exercises
- Chain a caught error into a new one8 tests · intermediate
- Update state without mutating it9 tests · intermediate
Real-time connectionsfetch answers one question at a time. This is what answers a stream of them.
5 min read2 exercises
2 exercises
- Exponential backoff, capped8 tests · intermediate
- Track reconnect attempts, and know when to give up8 tests · intermediate
Offline & storagelocalStorage is fine for a theme or a small cache. Past a few MB, it stops being enough.
7 min read2 exercises
2 exercises
- A cache-first lookup, the shape a service worker uses8 tests · intermediate
- Pick the right storage for the job9 tests · intermediate
The cheat pageNot another lesson — the night-before-the-interview skim of the other 38.
6 min read

Practice29 exercises

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