Evaluate Expression as String in Extension

[TKendrick20]

I am building a listbox extension and in the ,js file I'd like to evaluate an expression with something like app.Evaluate(*string*).

Specifically, I want to get the COUNT of the values in a field before a selection is made in the listbox.

Is this possible?

[Stefan_Walther]

Can you pls explain a bit more in detail what you are trying to achieve, maybe with some code samples?

[TKendrick20]

Before I call this line in my listbox extension:

self.backendApi.selectValues(dim, [value], true);

I want to first check the calculation of an expression which, in layman's terms, says "If selecting the value that you just clicked on would make another expression < 1, do not allow the selection."

If (expression < 1)

     Make the selection

else

     Do not

The question is, how can I calculate the result of this expression on the fly?