> For the complete documentation index, see [llms.txt](https://archivedocs.stackstate.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://archivedocs.stackstate.com/5.1/develop/developer-guides/custom-functions/functions.md).

# StackState functions

## Overview

Functions in StackState are predefined scripts that transform input into an output. They're called by StackState on-demand. For example, when a component changes state, new telemetry flows in or a user triggers an action. Advanced users can develop their own functions to customize StackState. These functions can then be exported and [packaged with a custom StackPack](/5.1/develop/developer-guides/stackpack/develop_stackpacks.md).

## Function types

In StackState, different function types complete different tasks. Depending on the function type, it may be possible to specify [asynchronous or synchronous execution](#asynchronous-and-synchronous-execution) when creating a custom function. Some default functions are implemented as [native functions](#native-functions).

| Function type                                                                                                          | Synchronous execution | Asynchronous execution | Native functions |
| ---------------------------------------------------------------------------------------------------------------------- | :-------------------: | :--------------------: | ---------------- |
| [Propagation functions](/5.1/develop/developer-guides/custom-functions/propagation-functions.md#propagation-functions) |           ✅           |            ✅           | ✅                |
| [Event handler functions](/5.1/develop/developer-guides/custom-functions/event-handler-functions.md)                   |           ✅           |            ✅           | -                |
| [Component actions](/5.1/develop/developer-guides/custom-functions/component-actions.md)                               |           -           |            ✅           | -                |
| [Monitor functions](/5.1/develop/developer-guides/custom-functions/monitor-functions.md)                               |           -           |            ✅           | -                |
| [Check functions](/5.1/develop/developer-guides/custom-functions/check-functions.md)                                   |           ✅           |            -           | -                |
| [Component mapping functions](/5.1/develop/developer-guides/custom-functions/mapping-functions.md)                     |           ✅           |            -           | -                |
| [ID extractor functions](/5.1/develop/developer-guides/custom-functions/id-extractor-functions.md)                     |           ✅           |            -           | -                |
| [Relation mapping functions](/5.1/develop/developer-guides/custom-functions/mapping-functions.md)                      |           ✅           |            -           | -                |

## Asynchronous and synchronous execution

Functions in StackState run with either synchronous or asynchronous execution. For some functions it's possible to choose the execution type.

* **Asynchronous execution** - functions have access to all StackState [Script APIs](/5.1/develop/reference/scripting.md). Selecting asynchronous execution also makes it possible for more functions to run in parallel.
* **Synchronous execution** - functions don't have access to the StackState script APIs. If the function offers the possibility to be run with either asynchronous or synchronous execution, it's recommended to use asynchronous execution.

## Native functions

To improve performance, some default StackState functions have been implemented as native functions. It isn't possible to view or edit the script body of a native function in the StackState UI. It isn't possible to create a custom native function.