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Visualize JS call stack, microtasks queue, macrotasks queue, and Web APIs in real time.
JavaScript is single-threaded. The event loop coordinates the Call Stack, Microtask Queue (Promises), and Macrotask Queue (setTimeout/events) to enable non-blocking asynchronous execution.
Write custom async JavaScript code or pick preset scenarios (Promises, setTimeouts, async/await).
Click Step or Play to watch stack frames get pushed/popped and queues drained in real-time animations.
Everything you need to know about this developer tool
Microtasks (Promise callbacks, queueMicrotask) run immediately after the current Call Stack empties before the browser paints. Macrotasks (setTimeout, setInterval, I/O) run on subsequent event loop ticks.
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iframe sandbox with no access to local storage, documents, or cookies.