Need list of all possible category values and associated IDs for scripting

I am attempting to use a custom field to set the "filedAgainst (aka category)" value.  The filedAgainst values will start with the value in the custom field  [if category.startsWith(customFieldValue)].  That is, they are already setup that way. 

I need to use the "calculated value" javascript approach, not java.  I cannot find docs on how to retrieve all possible literal values of the category so that I can to a match between the custom field and the category value in order to set the correct "ID" for the category on return.

Any thoughts?