Promises, async, await, and real concurrency
By the end of today you can write asynchronous code that runs independent work at the same time rather than one after another, and you can spot the sequential await that quietly triples a response time.
YesterdayOn Day 27 you learned that queued work waits for an empty stack. Today you write against that model deliberately.
TomorrowTomorrow you add types, so the shapes flowing through this code stop being guesses.
Why this matters
Awaiting three calls in sequence when they could have run together is the most common performance bug in modern application code, and it looks completely correct.
- Promises
- Async and await
- Concurrent or sequential
- Async error handling
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
45 minWrite three functions that each resolve after one second. Call them sequentially with await and time the total. Then call them with Promise.all and time it again. Then build a realistic case where the third genuinely depends on the first, and write the fastest correct version. Finally, remove one await and observe where the failure appears.
Recall it
20 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
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Rate it
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Mastery tracking
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Recap
- 01Await suspends a function; it does not block the thread
- 02Independent work awaited in sequence costs the sum instead of the maximum
- 03Promise.all fails fast; allSettled reports everything
- 04A forgotten await fails far from where you made the mistake
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.