YOUR CHALLENGE
JAVASCRIPT / MAKE ASYNC MAKE SENSE
Stop guessing.
Understand async.
Learn promises, await results, handle failures, and see why the order of your code is not always the order of its output.
ONE THREAD. DIFFERENT QUEUES.
sync: start queue: promise reaction queue: timer callback sync: end → promise → timer
Predict. Run. Explain. Repeat.
Real JavaScript. Local data. Code runs in an isolated, disposable browser worker with network access blocked and a three-second limit. No account or server execution. The visual experiments use their own fixed operations, separate from your code.
Custom runs use this input. Challenge checks use the fixed cases above. JSON is limited to 20,000 characters. Ctrl/⌘ + Enter runs custom input. Large result and log previews are abbreviated; downloaded run reports contain those same bounded previews.
A DETERMINISTIC TIMING MODEL
When will the work finish?
Compare dependent, sequential work with independent concurrent work. These are simulated durations, not real network requests. Real timers and servers are less predictable.
PREDICT BEFORE YOU REVEAL
Become an order detective.
Choose a snippet, type the log labels in order separated by commas, then check your reasoning. These browser scenarios assume no other queued work.
Turn practice into working habits.
A practical async checklist
- Decide which tasks depend on earlier results.
- Await or return every promise you need to observe.
- Choose all, allSettled, any or race intentionally.
- Handle HTTP status and parsing errors separately.
- Bound retries and prevent stale UI updates.
- Remember: racing a timeout does not cancel the operation.
Tests check observable results, not which syntax you used. Passing three fixtures does not prove general correctness. Network examples are local fixtures; no real requests leave this lab.
Your saved workspace
Drafts, input, notes and passing checks save in this browser when storage is available. Editing code removes its pass. Results are session-only; download a run report to keep them.