# Controllers

In Frappe.js you can extend the metadata class as well as the document class for a particular DocType.

You can write event handlers in controllers, by declaring a `.js` file in the `models/doctype/` folder along with the model file.

## Naming

1. The name of the controller class must be the slugged name of the DocType (example `todo`)
2. The name of the `meta` class must be the name of the controller class prefixed by `meta_` (example `meta_todo`)

To add a standard handler, you must bind all handlers in `setup` method.

## Document Controller

You can bind events to the controller that will be called when an action is completed on a document or its property.

The document controller represents a single record and is subclassed from the `frappe.document.Document` class

```js
const frappe = require('frappe-core');

// extend the document and add event handlers
class todo extends frappe.document.Document {
	setup() {
		this.add_handler('validate');
	}
	validate() {
		if (!this.status) {
			this.status = 'Open';
		}
	}
}
```

## Metadata Controller

The `meta` class contains actions that are done on a group of objects and a document represents a single object. So properties and actions related to the group will be part of the `meta` class.

```js
// extend the meta class
class todo_meta extends frappe.meta.Meta {
	setup_meta() {
		Object.assign(this, require('./todo.json'));
		this.name = 'ToDo';
		this.list_options.fields = ['name', 'subject', 'status', 'description'];
	}

	get_row_html(data) {
		return `<a href="#edit/todo/${data.name}">${data.subject}</a>`;
	}

}
```


### Controller Events

Standard events on which you can bind handlers are

- `before_insert`
- `before_update`
- `validate` (called before any write)
- `after_insert`,
- `after_update` (called after any write)
- `before_submit`
- `after_submit`
- `before_cancel`
- `after_cancel`
- `before_delete`
- `after_delete`