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Simulator JavaScript Event Loop

Pelajari cara kerja asinkron JavaScript: jalankan contoh kode dengan setTimeout, Promise, async/await, dan lihat bagaimana event loop memproses antrean tugas secara visual.

Status alatBerjalan di browser Anda
Pratinjau

Script (in source order)

Execution order

Step 1console.log('1')Sync (console.log)
Step 2console.log('4')Sync (console.log)
Step 3Promise.resolve().then(() => log('3'))Microtask (Promise.then)
Step 4setTimeout(() => log('2'), 0)Macrotask (setTimeout)

Cara kerja

  1. Masukkan data Anda

    Isi nilai yang diperlukan, tempel teks Anda, atau unggah file yang ingin diproses.

  2. Lihat hasil seketika

    Sebagian besar alat diperbarui secara langsung saat Anda mengetik; beberapa menggunakan satu tombol tindakan.

  3. Gunakan hasil Anda

    Salin, unduh, atau bagikan hasil yang dihasilkan — Anda selalu memiliki kendali penuh atas data Anda.

Privasi dan pemrosesan data — Berjalan secara lokal di browser Anda. Alat ini tidak mengunggah data Anda.

Apa itu Simulator JavaScript Event Loop?

Build a small script out of synchronous code, microtasks, and macrotasks to see the exact order the JavaScript event loop would run it in.

PAHAMI CARA KERJA ALAT

How the JavaScript event loop simulator works

The event loop's rules

  • All synchronous code runs first, top to bottom.
  • Once the synchronous code finishes, the entire microtask queue (Promise .then callbacks, queueMicrotask) drains before anything else runs.
  • Then the next macrotask (setTimeout, setInterval) runs, immediately followed by draining the microtask queue again — and this repeats for each macrotask.

How to use it

  • Add steps and mark each as sync, microtask, or macrotask.
  • Give a macrotask a delay in milliseconds — it affects the order relative to other macrotasks.
  • Read the computed execution order on the right.

Common gotchas

A `setTimeout(fn, 0)` never runs before a `Promise.resolve().then(fn)`, even with a 0ms delay — every microtask always drains before the next macrotask starts. This surprises most people the first time they see it.

Classic ordering example

sync, timeout, promise, sync

Synchronous logs run first, then the promise microtask, then the timeout macrotask.

log(1); setTimeout(2); Promise.then(3); log(4) -> 1, 4, 3, 2

Script simulation notes

The steps you add are labels for a local ordering model; the tool does not execute arbitrary JavaScript code.

Pertanyaan yang Sering Diajukan (FAQ)

Why does the Promise callback run before setTimeout(fn, 0)?

Because microtasks always fully drain before the event loop picks up the next macrotask, regardless of the macrotask's delay — a 0ms timeout is not "immediate", it's just the shortest possible macrotask delay.

Does this cover async/await?

Not directly as its own step type, but async/await is built on promises under the hood — an `await` suspends and resumes as a microtask, the same as a `.then()` callback would.

Is this ordering guaranteed across all browsers?

Yes — the relative ordering of "microtasks fully drain between macrotasks" is part of the HTML/ECMAScript specification, not an implementation detail, so every spec-compliant JavaScript engine follows it.

How are macrotasks with the same delay ordered?

The simulator sorts macrotasks by delay and keeps source order for ties, matching the stable ordering used in the model.

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