how to treat empty parameters in function calls?

Question

When you call a function like x = myfunc(a,b,,d) what happens with the 3rd parameter? is it empty? null? nothing?

I'm having issues with my function, say,

function myfunc(p1, p2, p3, p4)  
  if p3 <> "" then whatever  
end function

gives me a dreaded type mismatch

P.S. I'm trying to replace a COM object with vbscript functions, and those empty parameter calls were done to the COM objects which didn't have a problem with those, but vbscript doesn't like them. I can't change the calls, only the functions so I need to treat the empty parameter somehow, just don't know how (tried isnull with no luck, also isempty no luck, and is nothing gives me an object required error)

Answer 1

The data type of missed parameters is ERROR.
I wrote an example, try the following:

Function IsMissing(p)
    IsMissing = (VarType(p) = vbError)
End Function

Function Myfunc(p1, p2, p3, p4)
    If IsMissing(p1) Then Response.Write "p1 is missing "
    If IsMissing(p2) Then Response.Write "p2 is missing "
    If IsMissing(p3) Then Response.Write "p3 is missing "
    If IsMissing(p4) Then Response.Write "p4 is missing "
End Function

str = myfunc(, , , 1)
'str = myfunc(1, 2, , ) 'causes error

Notice: The last parameter of function / sub cannot be blank because of syntax error.

Answer 2

In the spirit of teaching a man to fish:

The TypeName function will reveal to you what type it is.

MsgBox TypeName(p3) ' Returns string: "Error"

Instead of if p3 <> "" you can test for omitted arguments like this:

    If TypeName(p3) = "Error" Then MsgBox "Whatever!"

or, preferably,

    If VarType(p) = vbError Then MsgBox "Whatever!"
    ' vbError is a built-in constant whose value is 10.

But really, for best practice, your program design should avoid omitting arguments. VBScript isn't really made to deal with "optional" arguments.