Great work! Here's another way to eliminate the method but keep the button and click binding:
<button @click="task.completed = !task.completed">Toggle</button>
Here's the fiddle:
https://jsfiddle.net/harps116/2hmu4yyh/2/
This technique is pretty handy for toggle scenarios. Of course you could abstract the logic into a method for reuse and have a global toggle utility method which might be useful.
Thank you. I think that this "demo app" idea of a Task List or Todo List is a good way to learn. I think I will create one for myself with a database backend as a way to learn how to put everything together.