Returns the number of items in the dataset that match its filter criteria.
Calling getTotalCount() on a write-only dataset causes an error.
Note:
A dataset needs to load its data before you call its getTotalCount() function.
Usually a dataset finishes loading a short time after the page it is on finishes
loading. So if you call getTotalCount() inside the page’s onReady()
event handler, the dataset might not be ready yet.
To call getTotalCount() as soon as possible after a page loads, use the dataset's
onReady() function inside the page’s onReady()
event handler to ensure that both the page and the dataset have finished loading.