Does QB64pe already have a function like IFF ?

[bplus]

I ask because IFF typically takes a numeric expression that evaluates to "True" or "False" numerically -1 or 1 and 0 for false.

That's incorrect.

The first parameter in IFF is an expression that results in an integer number (or any number, really; the number will get treated as an integer).

The expression in the first parameter of IFF could be as simple as a literal number, or a numeric constant, or a numeric variable.   Or it could be a function that returns a number.  Or it could be any kind of complex expression that results in a number.

So why are you saying IFF(string, ...   ? 

It should be IFF(integer, ...

I quote you: "This IFF in BAM will only work with primitive types (string, integer, floating point)."