This example creates an extender that allows an observable to be marked as required.
Instead of returning a new object, this extender simply adds additional sub-observables to the existing observable.
Since observables are functions, they can actually have their own properties.
However, when the view model is converted to JSON, the sub-observables will be dropped and we will simply be left with the value of our actual observable.
This is a nice way to add additional functionality that is only relevant for the UI and does not need to be sent back to the server.
Clear the prompts of data and the extend of the observable displays an error message
▶ShopNameCart: Household items
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