Topic · 12 lessons in 10 series

Async and concurrency

The event loop, promises and async/await, threads and workers, and the race conditions that appear when things happen at the same time.

JavaScript CoreWhole series →

First-Class Functions, Callbacks and Event ListenersFunctions as values: passing them as arguments, returning them from other functions, sync vs async callbacks stepped side by side, and attaching and removing event listeners without leaking memory.14 min
Async JS and the Event LoopCall stack, Web APIs, callback queue, microtask queue and the event loop that ties them together, stepped tick by tick — plus async/await, where rendering fits, starvation, and why setTimeout(0) is not 0.20 min

System DesignWhole series →

Processes, Threads & Concurrency for Backend EngineersMost backend work is waiting. How one server juggles thousands of requests — processes, threads, event loops and lightweight threads — and the race conditions, deadlocks and pool limits that appear the moment it does.12 min

The Browser ContractWhole series →

Browser Rendering and the Event Loop in PracticeParse, style, layout, paint and composite. Also layout thrashing, and where tasks, microtasks and requestAnimationFrame fit.18 min

Backend in DepthWhole series →

Node.js Performance Basics: Event Loop Blocking, Streams, WorkersWhy one heavy CPU job can stop every request, plus streams for large data and worker threads.15 min

Scenario DebuggingWhole series →

Duplicate Requests, Race Conditions and Stale ResponsesStrict Mode running effects twice, double-click submits, responses that arrive in the wrong order, AbortController, removing duplicate requests, and idempotency keys on the server.15 min

Senior Frontend DepthWhole series →

Concurrent React, Suspense and Server Components in Next.jsTransitions, Suspense boundaries, streaming SSR, hydration errors and the line between server and client components.20 min

Full-Stack QuestionsWhole series →

Async JavaScript, Promises and the Event LoopOrder puzzles in the browser and Node, promise rules, combinators, concurrency limits, cancellation, race conditions and async memory leaks.36 min

Design Patterns & LLDWhole series →

Design a Parking LotBest-fit and nearest-spot strategies, hourly pricing with a daily cap, and a double-booking race fixed with compare-and-set.28 min

JavaScript Part 2Whole series →

Promises and async/await From the Ground UpPromise states, microtask order, a promise built by hand, async/await, the combinators and AbortController.30 min

Node.js From ZeroWhole series →

More topics

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