ServiceWorker.extendableMessageEventThe event delivered to the worker's message / messageerror handlers. Unlike a plain Dom_html.messageEvent it extends extendableEvent, so the handler can call waitUntil to keep the worker alive until asynchronous processing of the message finishes. The type parameter 'a is the type of the data payload.
inherit extendableEventThe message source: a client, a serviceWorker or a MessageChannel.messagePort; null when there is none.