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Measured: pnpm cut three projects' node_modules from 132 MB to 42 MB and blocked a phantom import npm let through. The real difference is what that single architectural choice compounds into at scale.

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.

Redis is fast because of specific design decisions — a reactor-pattern event loop, copy-on-write snapshots, compact in-memory encodings — not just because it's in-memory.

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.

Kafka is a distributed commit log, not a message queue. Segments on disk, the ISR, the KRaft controller quorum and zero-copy fetches — and how each one explains a config option you've had to set.

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.