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Lodash's curry is 10 lines. Build curry, partial, compose, and pipe from scratch in JavaScript — then use them to write clean data pipelines without a single nested callback.

Open a modal 10 times without cleanup and you've got 10 stacked timers silently draining memory. Here's what this looks like in a PR and how senior devs catch it.

Cypress runs inside the browser; Playwright runs outside it. That one architectural split explains the debugging experience, the WebKit story, and why parallelism costs money on one and nothing on the other.

An Observable is just a function. Build one from scratch in about 120 lines of JavaScript and finally understand why unsubscribing stops work, how operators chain, and what 'lazy' actually means.

Redux's core is under 60 lines of JavaScript. Build createStore, combineReducers and a middleware system with logger and thunk from scratch, then test the parts that are easy to get quietly wrong.

Floating promises — async calls nobody awaits — don't crash immediately. They fail silently, corrupt state, and get cut off by serverless runtimes. Here's what to look for in code review.

TypeScript is the default for serious JavaScript work now — but the real question is when the type overhead pays off, and which parts of your codebase genuinely don't need it.

Most useMemo and useCallback calls in the wild accomplish nothing. Learn exactly when each hook works, why one unstable prop breaks the whole memoization chain, and when to just delete it.

Here's a scenario I see in almost every codebase I review. A developer writes an async function that reads some state, does a bit of work, then writes back a result. In tests, it's fine. In development, it's fine.