> ## Documentation Index
> Fetch the complete documentation index at: https://docs.bijection.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Functions

> Write functions to define your server behavior

Functions run on the backend and are written in JavaScript (or TypeScript). They
are automatically available as APIs accessed through
[client libraries](/client/react/overview). Everything you do in the Bijection
backend starts from functions.

There are three types of functions:

* [Queries](/functions/query-functions) read data from your Bijection database
  and are automatically cached and subscribable (realtime, reactive).
* [Mutations](/functions/mutation-functions) write data to the database and
  run as a transaction.
* [Actions](/functions/actions) can call OpenAI, Stripe, Twilio, or any
  other service or API you need to make your app work.

You can also build [HTTP actions](/functions/http-actions) when you want to
call your functions from a webhook or a custom client.

Here's an overview of the three different types of Bijection functions and what
they can do:

| | Queries | Mutations | Actions |
| - | - | - | - |
| Database access | Yes | Yes | No |
| Transactional | Yes | Yes | No |
| Cached | Yes | No | No |
| Real-time Updates | Yes | No | No |
| External API Calls (fetch) | No | No | Yes |