# JavaScript

Runs your code on the person's device: open a link, copy text to the clipboard, share, close the app, write variables. When the app reaches the block, it runs the code and goes on through the "Done" or "Error" exit.

![JavaScript block settings](https://support.watbot.org/_media/en/02-mini-apps/_images/js-block.webp?v=1791781230)

## Settings

- **"Code"** — the body of an async function with the `app` object. Up to 20,000 characters.
- **"Time limit, seconds"** — from 1 to 60, 10 by default.
- **"Result variable"** — what `return` returned.
- **"Error variable"** — the error text.
- **"What the code may change"** — the "Change only what is listed" switch and lists of allowed variables and tags. On for a new block.

What the code can access in the `app` object, how the restriction works and a code sample are on the [Code in the app](https://support.watbot.org/mini-apps/code#the-javascript-block) page.

## Exits

- **"Done"** — the code finished.
- **"Error"** — the code threw an error or ran out of time.

## Specifics

- The code runs on the device, so a savvy person can fake its result. Do not turn the restriction off if access, discounts or mailings depend on the variables and tags.
- `{{ name }}` placeholders do not work in the code: read values with `app.variables.get()`.