The event loop, single-threaded and still fast
By the end of today you can predict the order in which asynchronous code runs, explain why a single thread can serve many requests, and say exactly what blocking the loop does to everyone else.
YesterdayOn Day 17 you learned the call stack. Today you learn what happens around it when work has to wait.
TomorrowTomorrow you write against this model directly with promises and async and await.
Why this matters
This is the model behind Node, browsers, and every streaming response you handle from an LLM on Day 72. Not understanding it produces code that appears to work and collapses under load.
- The event loop
- Stack and queues in JS
- Microtasks and macrotasks
- Blocking the loop
Learn it
75 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
4 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
40 minWrite a script mixing five synchronous logs, two setTimeout callbacks with zero delay, and two resolved promise callbacks. Predict the exact output order in a comment before running it. Then write a function that blocks for three seconds with a busy loop, run it alongside a timer, and observe what happens to the timer.
Recall it
25 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01The loop only takes queued work when the call stack is empty
- 02Microtasks drain fully before the next macrotask runs
- 03A zero delay means as soon as possible, not now
- 04Blocking the loop stops everyone, not just the request that did it
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.