plugin.html

<plugin>
	<!-- this Plugin does not have any HTML content -->
	<script>

	// Windy API modules are imported via '@windy/nameOfModule'
	import map from '@windy/map'

	/*
	This part of code will be executed just once, since
	plugin remains 'mounted' in a page even after	closing
	*/
	console.log('I am being mounted')

	let popup = null

	//	this.onopen method is called when your plugin is being opened
	this.onopen = () => {

		const center = map.getCenter()

		/*
			this.ononpen method can be called repeatedly (without your plugin
			being closed before), so make sure, that you will not to subscribe
			to any listener twice
		*/
		if( popup ) {

			popup.setLatLng(center)

		} else {

			popup = L.popup()
            		.setLatLng(center)
            		.setContent("Hello World")
            		.openOn( map );
		}
	}

	/*
		this.onclose method is called when your plugin is being closed

		Unsubscribe from all your listeners, and remove all your
		stuff from a map

	*/
	this.onclose = () => {
		if( popup ) {
			map.removeLayer( popup )
			popup = null
		}
	}

	</script>
</plugin>