---
title: on
card_id: 26982
modified: yes
---

# on

```
on [ph:messageName] [[ph:parameterList]]
    [ph:statements]
end [ph:messageName]
```

The `on` keyword defines a new message handler of the specified name.

The optional `[ph:parameterList]` lets the handler receive values sent along with a message. HyperCard assigns each value to a parameter variable in the `[ph:parameterList]`.

## Demo Script

```
<b>on</b> mouseUp
  flashTheButton "bkgnd btn id 57",2
<b>end</b> mouseUp

<b>on</b> flashTheButton whichBtn, howMany
  repeat for (howMany * 2) times
    set the hilite of whichBtn to not the hilite of whichBtn
  end repeat
<b>end</b> flashTheButton
```

## Related Topics

* [exit](/HyperTalkReference/keywords/exit)
* [Handling messages](/HyperTalkReference/hypertalkbasics/Handling-messages)
* [param](/HyperTalkReference/functions/param)
* [paramCount](/HyperTalkReference/functions/paramCount)
* [params](/HyperTalkReference/functions/params)
* [pass](/HyperTalkReference/keywords/pass)
* [result](/HyperTalkReference/functions/result)
* [return](/HyperTalkReference/keywords/return)
* [send](/HyperTalkReference/keywords/send)
* [target](/HyperTalkReference/functions/target)
* [The message-passing order](/HyperTalkReference/hypertalkbasics/The-message-passing-order)
* [Using parameter variables](/HyperTalkReference/hypertalkbasics/Using-parameter-variables)
* [What are messages?](/HyperTalkReference/hypertalkbasics/What-are-messages)
* [Writing message handlers](/HyperTalkReference/hypertalkbasics/Writing-message-handlers)