We'll create an exchange that filters out mutations that are executed while the user is offline and map them to an error result. When the user tries to add or remove a bookmark when offline, show an informative alert message.
Resources: Authoring exchanges: https://formidable.com/open-source/urql/docs/advanced/authoring-exchanges/ Wonka: https://github.com/kitten/wonka
Checkpoint: Add an offline mutation exchange