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AsyncScope

Write modern JavaScript or TypeScript and watch how it executes.

How AsyncScope works

How to use it

1. Pick an example or write your own JavaScript or TypeScript.
2. Press Run. It records the execution and pauses at the first step, so you do not miss the beginning.
3. Press Resume to play, or Next to step through the call stack, queues and event loop one step at a time.

Shortcuts: Ctrl/Cmd+Enter Run,Space Resume/Pause, →/← step forward/back.

What you are watching

Call Stack — the functions currently running, topmost last.Call stack on MDN
Event Loop — picks the next task only when the stack is empty, after draining microtasks.Event loop on MDN
Microtask Queue — promise callbacks and queueMicrotask; always drains before the next task.Microtask guide on MDN
Task Queue — timer callbacks waiting for their turn.Event loop on MDN
Browser APIs — pending timers, with their remaining virtual time.
Console — what your code prints, in order.console on MDN
Timeline — every step as a clickable list; click any step to jump to it.

Supported JavaScript

Everything runs in an isolated worker in this browser. These work:

Variables with let andconst, functions andarrow functions.
setTimeout /clearTimeout andsetInterval /clearInterval.
Promise,Promise.all,Promise.race,.then / .catch.
async functions,await andtry / catch around awaited promises.
queueMicrotask andconsole.log / warn / error.

Not supported (by design)

The sandbox keeps execution deterministic and safe, so these are rejected before running:

Network: fetch, XMLHttpRequest, WebSocket.
DOM and browser chrome: document, window, storage, dialogs.
Modules: import / export and dynamic import().
Generators (function*, yield) andfor await...of loops.
Node.js APIs such as require and process, evaland dynamic code.

Limits to keep in mind

Timing is virtual and deterministic — it teaches ordering, not real performance.
Traces are capped at 2000 events; endless loops and runaway intervals are stopped.
TypeScript types are erased before execution. There is no type checking in the browser; a wrong annotation can disappear and still run.
Your code never leaves this browser: no account, no server, nothing is uploaded.

All links in this help open MDN Web Docs in a new tab.

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