# preact

The `preact` challenge runs a small JavaScript program in the browser.
It checks that the client can run JavaScript through [Preact](https://www.npmjs.com/package/preact), a lightweight framework similar to React.

The browser calculates a SHA-256 response and waits before it submits the result.
Each unit of `difficulty` adds 125 milliseconds to the browser timer.
The server requires at least 80 milliseconds per unit before it accepts the result.

## Configuration

Add a browser rule to the `bots` list:

```yaml
bots:
  - name: generic-browser
    expression: userAgent.matches("Mozilla|Opera")
    action: CHALLENGE
    challenge:
      algorithm: preact
      difficulty: 1
```

The expression matches a User-Agent that contains `Mozilla` or `Opera`.
See [policies](/docs/policies.md) for rule order and actions.
See [expressions](/docs/configuration/expressions.md) for other match conditions.