# Commands

In this chapter, we're going to learn how to make commands.

## Making a command

A command is an execution unit that can be called by menu item, keyboard shortcuts or any parts of applications via API. All menu items such as **File > Open...**, **Edit > Copy** have corresponding commands. So you must make a command first before to add a menu item. As shown in [Getting Started](/developing-extensions/getting-started.md), we will make a simple command showing a message in alert dialog.

First you have to defined an unique ID for a command. Typical command ID have the form `<group>:<function>` where `<group>` is the group name of commands and `<function>` is the function name of the command. An extension may have a set of commands, so typically the group name is the extension name. For example, the ID of the command showing a message of HelloWorld extension is `helloworld:show-message`.

And then, we will define a handler function to be executed when the command is called.

```javascript
function handleShowMessage() {
  window.alert('Hello, world!')
}
```

Finally, we need to register this command to application by calling `app.commands.register` method. The first parameter is the ID of the command and the second is the handler function and the third (optional) is the display name of the command (Shown in [Command Palette](/user-guide/user-interface.md#command-palette)).

```javascript
app.commands.register('helloworld:show-message', handleShowMessage, 'Show Message')
```

## Calling a command

Now we have a new `helloworld:show-message` command. It can be called manually as follow:

```javascript
app.commands.execute('helloworld:show-message')
```

All functionalities of StarUML are defined as commands so that you can call without defining duplicated functionality.

```javascript
var ids = Object.keys(app.commands.commands)
console.log(ids); // you can see all available command IDs
app.commands.execute('project:open') // execute `project:open` command
```

## Passing parameters

You can pass one or more parameters to the command. We will fix the handle function so that it can receive a parameter (two or more parameters are possible).

```javascript
function handleShowMessage(message) {
  if (message) {
    window.alert(message)
  } else {
    window.alert('Hello, world!')
  }
}
```

Then, you can pass a string to the parameter as follow:

```javascript
app.commands.execute('helloworld:show-message', 'New Message')
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.staruml.io/developing-extensions/commands.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.