Hi,
you will run into trouble if you try to get these values from two separate fields, as the order will be messed up, if either “slug” or “name” is empty.
One way to go about this is to create a function field in your Categories table and reference that instead.

Here is the formula:
'[{"category":"' & {Name} & '","slug":"' & {Slug} & '"}]'
In you script you will probably have to use the JSON.parse() method, as it will be interpreted as a string and not an object.