# include_once

## Function

The BT file is introduced and executed at runtime, but the same real file is only executed once within the current execution context.

## Syntax

```bt
include_once(path)
include_once path
```

## Parameters

| Parameters | Type | Required | Default value | Description |
| ------ | ------ | ------ | ------ | ------ |
| path | String | Yes | None | The path of the BT file to be imported. |

## Return value

| Type | Description |
| ------ | ------ |
| Any/Empty | Returns the return value of the imported file when executed for the first time; returns Empty when the same real file is imported repeatedly. |

## Example

```bt
fs('once-demo.bt').write('counter++')

counter = 0
include_once 'once-demo.bt'
include_once './once-demo.bt'

// Output: 1
echo(counter)
```

## Notes

- Repeated judgment uses the normalized real file path instead of the original string path.
- The once state belongs only to the current execution context; the state is re-created with every request to the Web and every call to the desktop `bt.call()`.
- include_once shares the file compilation cache, but does not cache variable scopes, return values, or execution status.
- Bracketless syntax only supports statement form; please use `include_once(path)` when you need to assign the return value to a variable.