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Async Visualizer - Free Online JavaScript Async Tool | IconVault

Watch async/await functions suspend and resume on animated lanes. Paste JavaScript and step through it. Free, runs in your browser.

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Async Visualizer

Watch async/await suspend and resume on animated lanes

5 of 5 free uses left

Presets

Your code

Subset: async functions, console.log, calls, return, await sleep(ms), await promise. No arguments yet.

5 of 5 free runs left. Stepping through a trace is always free.

Step 1 of 25

Top level calls main(): a new frame opens on its own lane.

virtual time: 0ms

Function lanes

main()runningconsole.log("main: start")

Source

 1async function fetchUser() {
 2  console.log("fetchUser: start");
 3  await sleep(800);
 4  console.log("fetchUser: got user");
 5  return "Ada";
 6}
 7async function fetchPosts() {
 8  console.log("fetchPosts: start");
 9  await sleep(500);
10  console.log("fetchPosts: got posts");
11  return "3 posts";
12}
13async function main() {
14  console.log("main: start");
15  const user = await fetchUser();
16  const posts = await fetchPosts();
17  console.log("main: done");
18}
19main();

Microtasks (continuations)

Empty

Timers (Web APIs)

Empty

Console

Nothing logged yet

Simplified model: await always yields through the microtask queue, even for already-settled promises, which is exactly what real engines do. Real scheduling also interleaves rendering and I/O.

About this tool

IconVault's Async Visualizer shows you how async/await really works: paste JavaScript, then watch each function suspend at an await and resume later on animated execution lanes. A tiny real interpreter runs your code step by step, so you see the exact order everything happens in.

It supports async functions, console.log, await sleep(ms) timers, function calls, and top-level calls, with play, pause, step forward, and step back controls. Built for students and interview prep. Free and runs fully in your browser.

Frequently asked questions

Popular searches

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