# set_timeout

## Function

Registers a one-time deferred callback and returns a `Timer` object immediately. After the callback expires, it will be executed once by the single-threaded event loop of the VM to which it belongs, and the callback return value will be ignored.

## Syntax

```bt
timer = set_timeout(fn, delay_ms)
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| fn | Fn | Yes | None | Delayed execution of the BT function. |
| delay_ms | Int | No | 0 | Delay in milliseconds; negative numbers are treated as 0. |

## Return value

| Type | Description |
| ------ | ------ |
| Timer | Timer handle, you can call `cancel()` to cancel the callback that has not yet been triggered. |

## Example

```bt
fired = false

timer = set_timeout(fn() {
    fired = true
}, 100)

// Output: false
print fired
```

## Notes

- `set_timeout()` will not execute the callback synchronously; when `delay_ms` is 0, it will be delivered to the event queue as soon as possible.
- CLI scripts with an active timeout will remain in progress until the callback execution completes or is cancelled.
- A timer cannot be created in the Web request context, and the VM will be released after the request ends.
- For the method of canceling the timer, see [Timer.cancel](/en/docs/system/timer).