Suggest an edit

Improve this article

Refine the answer for “Async/sync callback”. Your changes go to moderation before they’re published.

Approval required

Content

What you’re changing

Title (EN)

Short answer (EN)

Shown above the full answer for quick recall.

Answer (EN)

### Synchronous callback

> Runs **immediately**, during the call of the main function.
> That is - **in the same call stack** (before the current function finishes).

Example:

```javascript
function process(callback) {
  console.log('Before the callback');
  callback(); // runs immediately
  console.log('After the callback');
}

process(() => console.log('Synchronous callback'));
```

Output:

```javascript
Before the callback
Synchronous callback
After the callback
```

Everything runs **in order**, with no delays.

---

### Asynchronous callback

> Called **later**, after the current code finishes -
> for example, via a timer, after a request, an event, and so on.
> That is - **on the next turn of the event loop**.

Example:

```javascript
function processAsync(callback) {
  console.log('Before the callback');
  setTimeout(callback, 0); // runs later
  console.log('After the callback');
}

processAsync(() => console.log('Asynchronous callback'));
```

Output:

```javascript
Before the callback
After the callback
Asynchronous callback
```

The callback does not run **right away**, but after the call stack becomes free.

---

**In short:**

| Type | When it runs | Example |
|---|---|---|
| **Synchronous** | Immediately, during the call | `array.map(fn)` |
| **Asynchronous** | Later, after the main code | `setTimeout(fn, 0)`, `fetch().then(fn)` |

Markdown · drag & drop images · ⌘B / ⌘I shortcuts188 words

For the reviewer

Note to the moderator (optional)

Visible only to the moderator. Helps review go faster.