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`setTimeout(fn, 0)` **does not run the function immediately**, as it might seem!
It puts the `fn` callback into a **queue to run after the current code**, that is, **asynchronously**, on the next iteration of the event loop.

---

## What happens

```javascript
setTimeout(() => console.log('timeout'), 0);
console.log('main');
```

Output:

```javascript
main
timeout
```

Even though the delay is 0, JS **first finishes the whole current stack** (everything synchronous),
and only then runs the callbacks from the timer queue (callback queue).

---

## Why this happens

JavaScript is **single-threaded**.
All asynchronous operations (`setTimeout`, `fetch`, `Promise`) go into the event queue
and run **only after the call stack becomes empty**.

---

## What `setTimeout(fn, 0)` is used for

Deferring code execution to:

- wait for the current operations to finish;
- avoid blocking the interface;
- run code "after everything else";
- let the browser update the DOM before execution.

Example:

```javascript
console.log('A');
setTimeout(() => console.log('B'), 0);
console.log('C');
```

Output:

```javascript
A
C
B
```

---

## Summary

| What it does | Puts the callback in the queue for the next event loop iteration |
|---|---|
| Delay time | At least 0 ms, but actually a bit later |
| Execution | After all synchronous code |
| Used for | Deferred code, synchronization, avoiding UI blocking |

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